Everything has a price. And the price of powerful rockets with nuclear propulsion is of course the dread horror of deadly atomic radiation. But the danger can be brought under control with appropriate counter-measures, and by treating the power plant with the respect it deserves. And the same measures will come in handy if your ship is an interplanetary warship that may be facing hostile nuclear warheads.
But it is important not to over-react. There is a lot of silly media hype about plutonium being "the most toxic substance known to man", there is a general agreement among experts in the field that this is false.
The characteristic blue glow you've seen in photographs of "swimming pool" reactors is called Cherenkov Radiation. If you the blue Cherenkov glow around an object IN THE AIR (not at the bottom of a swimming pool reactor), you'd better be viewing it through several inches of lead glass or you have already taken a lethal dose, it is far too late to do anything about it, you are already dead. This comes under the heading of "not treating radiation with the respect it deserves."
As a side note, Cherenkov Radiation is caused by radioactive particles exceeding the speed of light in the medium. The term "c" is not "the speed of light", it is the speed of light in a vacuum. The maximum speed of light is much slower in air and even slower in water. The practical upshot of this is that there is no Cherenkov Radiation in the vacuum of space, and to get the same level of glow seen in a swimming pool a radiation source in air will have to be much more radioactive.
The general term for dangerous unhealthy everybody-panic-now kind of radiation is "ionizing radiation." This is because the radiation is capable of ionizing atoms which compose the material being irradiated. Materials such as the poor crew's tender vulnerable pink bodies and internal organs. Non-ionizing radiation such as visible light and radio waves can be safely ignored (by which I mean a laser beam can chop you into bits but it won't give you cancer).
Ionizing the atoms composing the proteins of a living thing is much like using a machine gun to fill a running automobile engine full of bullets. Proteins are the tiny molecular machines that make cells work. If you ionize an atom of a given protein it either splits or crumples up into a tangled wad, rendering it useless. Destroy enough of the proteins and the cell dies. Destroy enough cells and you die.
Radiation also smashes DNA molecules like a jackhammer. This can kill the cell or turn it cancerous. No, it won't turn you into a mutant, but any future childen you have will be another matter.
The sun's ultraviolet light is a form of radiation that can give your skin a sunburn. Ionizing radiation is more penetrating, so it is capable of giving you a lethal "sunburn" on your internal organs.
Ionizing radiation comes in two types:
- electromagnetic radiation like gamma rays and x-rays
- particulate radiation like protons ("proton storm"), neutrons ("neutron radiation"), electrons ("beta particles"), alpha particles ("alpha particles"), and heavy primary nuclei ("HZE ions").
In the context of this website, you will be dealing with radiation in several areas.
- Radiation from the space environment
- Radiation from spacecraft nuclear propulsion and nuclear power plants
- Radiation from space combat weapons
Finally it is important to understand the subtle distinction between radiation and radioactivity. Radiation are the deadly rays and particles that kill you. An element is radioactive if that element emits the deadly rays and particles that kill you.
What's the practical difference?
If you say that the nuclear rocket engine is emitting radiation, this means it is emitting deady rays, which all nuclear rockets tend to do.
But if you say that the nuclear rocket engine is emitting radioactivity, this means that the reactor core has been breached, and it is spewing powdered nuclear reactor rods in the form of a lethal cloud of atomic fallout.
Astronauts traveling from planet to planet are exposed to the natural radiation of space. This is generally always particle radiation (but little or no neutrons), with zero gamma rays or x-rays. One usually only encounters gamma rays and x-rays from artificial sources, such as nuclear reactors and nuclear weapons (unless you are near a black hole or other extreme environment). Exposure time to natural space radiation is "chronic" (see below).
There are three sources of natural space radiation:
- GALACTIC COSMIC RAYS (cosmic rays or GCR)
- TRAPPED RADIATION (radiation belts)
- SOLAR ENERGETIC PARTICLES (solar proton storm, solar storm, solar proton event (SPE), proton storm)
Here is a good overivew of naturally occuring sources of space radiation.
The ionizing radiation in space is comprised of charged particles, uncharged particles, and high-energy electromagnetic radiation. The particles vary in size from electrons (beta rays) through protons (hydrogen nuclei) and helium atoms (alpha particles) to the heavier nuclei encountered in cosmic rays, e.g., HZE particles (High Z and Energy, where Z is the charge). They may have single charges, either positive (protons, p) or negative (electrons, e), multiple charges (alpha or HZE particles); or no charge, such as neutrons. The atomic nuclei of cosmic rays, HZE particles, are usually completely stripped of electrons and thus have a positive charge equal to their atomic number.
The ionizing electromagnetic radiation consists of x-rays and gamma-rays which differ from each other in their energy. By convention X-rays have a lower energy than the gamma-rays with the dividing line being at about 1Merv. In general, x-rays are produced either by the interaction of energetic electrons with inner shell electrons of heavier elements or through the bremsstrahlung or braking radiation mechanism when deflected by the Coulomb field of the atomic nuclei of the target material. Gamma-rays are usually products of the de-excitation of excited heavier elements.
Ionizing radiations vary greatly in energy. Electromagnetic radiations have energy quanta determined by their wavelength or frequency. The energy of particulate radiation depends on the mass and velocity of the particles. Figure 126.96.36.199.1-1 summarizes the main types of ionizing radiation including their charge, mass, and source.
Figure 188.8.131.52.1-1 Sources and Characteristics of Electromagnetic and Particulate Ionizing Radiations in Space. Name Nature of radiation Charge Mass Sources X-ray Electromagnetic 0 0
Primary: Solar corona, stars, galaxies, terrestrial atmosphere in auroral zone.
Secondary: Spacecraft structure in some parts of the radiation belts, in the auroral zone, and in interplanetary space following some solar flares
Gamma ray Electromagnetic 0 0 Stars, galaxies, unknown sporadic sources, and spacecraft atmosphere. Electron Particle -e 1me Radiation belts and auroral regions. Proton Particle +e 1840 me or 1 amu Galaxy cosmic rays, radiation belts, and solar flares. Neutron Particle 0 1841 me
Primary: Galactic cosmic ray atmosphere albedo neutrons.
Secondary: Galactic cosmic ray interaction with spacecraft structure.
Alpha particle (helium nucleus) Particle +2e 4 amu Galactic and solar. HZE particle (heavy primary nuclei) Particle => +3e => 6 amu Galactic and solar.
Galactic Cosmic Radiation is particle radiation coming from outside the solar system (some is from outside of the galaxy), from more or less the entire sky. Most of them come from supernovae, it is unclear what is the origin of the rest.
The intensity of the cosmic ray bombardment varies with the strength of the solar wind. The wind is not causing the cosmic ray flux, it is apparently providing some shielding from cosmic rays. At a distance of about 94 astronomical units from Sol the solar wind undergoes termination shock which forms the inner edge of the heliosphere. This provides the cosmic ray shielding, in an amount proportional to the current solar wind strength.
But even with maximum protection from solar wind, the cosmic ray flux never drops down to zero.
In a spacecraft or space station the crew is protected from galactic cosmic rays by hull armor. The armor has two layers: hydrogen-rich armor (e.g., paraffin) to stop particles and dense armor (e.g., tungsten) to stop electromagnetic radiation. It is vitally important that the hydrogen-rich armor is on the outside with the dense armor as an inner layer. This ensures that the cosmic rays are stopped before they hit the dense armor.
If the dense armor was on the outside so the cosmic rays hit it first, the charged particles in the GCR would create deadly "Bremsstrahlung" ("secondary") radiation and kill the crew. Basically you would have designed the spacecraft to be a huge x-ray machine with the crew at the emitter. That would be bad.
But in any event keep in mind that it is incredibly difficult to shield against GCRs.
NASA engineers fret about this because the transit time for a Mars mission with the currently available pathetically weak propulsion systems will expose the crew to more space radiation than is allowed. NASA's Curiosity space probe measured the radiation dosage inflicted by traveling through space to Mars, specifically from galactic cosmic rays and solar proton storms. The dosage was 1.84 millisieverts per day (0.00184 sieverts). However, keep in mind this was measured during the peak of Sol's 11-year activity cycle, when GCR flux is relatively low due to shielding from the heliosphere. Also keep in mind this is with zero radiation shielding.
Back in the 1950s the pulp scifi stories tried to scare the readers with references to the deadly belts of radiation found in space. In reality they are not instant death, but they are unhealthy to linger inside. They damage the electronics of satellites as well.
What happens is that planets with strong magnetic fields will created zones of radiation by trapping energetic particles from the solar wind. Sort of like a cosmic roach motel: "Radiation checks in, but they don't check out!" From the standpoint of life on the planet's surface, the magnetic field is a good thing. The radiation from the solar wind is either diverted or trapped in the radiation belts, in either case it is much better than hosing the planet's surface with deadly radiation. If the planetary life becomes intelligent enough to develop space flight, then it should be intelligent enough to invent radiation shielding.
Please note that the radiation in the belts never reach the planet itself.
In a spacecraft or space station the crew is protected from trapped radiation by storm cellars. The crew has to occupy the storm cellar as long as the ship is within the radiation belt. This is why ion drives are not favored for breaking out of low Terra orbit, because such low-thrust propulsion can take almost a year to climb out of Terra's Van Allen Radiation Belt.
There are two belts.
The Inner Belt starts at an altitude from 400 km to 1,200 km, depending on latitude, and ends at an altitude of about 6,000 km, with its most lethal area 3,500 km out (1.55 Terran radii). The South Atlantic Anomaly can potentially disrupt satellites in polar orbits, but usually does not pose a problem for manned spaceflights. Except for the ISS. The radiation is high-energy protons (400 MeV).
The Outer Belt ranges from 13,000 km to 60,000 km, with its most lethal area 27,000 km out (5.22 Terran radii). The Outer Belt is affected by solar winds, and is thus flattened to 59,500 km in the area directly between the Earth and the Sun, and extends to its maximum distance in the shadow of the Earth. The radiation is high-energy electrons (7 MeV).
A safe channel exists between the belts from 9,000 km to 11,000 km.
if you were in a ship with 2 g/cm2 of hull shielding, the maximum radiation intensity zone of the inner Van Allen belt would give you a radiation dose of 0.2 Grays per hour. Hull shielding of 25 g/cm2 would reduce that to 0.05 Grays/hr. The report cites a permissible limit of 2 Grays total for a mission. The Apollo lunar missions dealt with the belts using a trajectory that missed the inner belt and zipped through the outer belt as fast as possible.
Since Terra's rotational and magnetic axes do not intersect at Terra's Center (see diagram), there is a deadly spot in the inner belt called the South Atlantic Anomaly. The inner edge of the belt proper is usually 1,000 kilometers from Terra's surface, but the anomaly gets as close as 200 kilometers. Satellites and space stations need extra radiation shielding for when they periodically pass through the anomaly. The ISS has extra shielding for that reason. Astronauts have seen phosphene shooting lights in their eyeballs, laptops have crashed, control computers experience transient problems as they pass through the anomaly.
Also of note, the Starfish Prime nuclear test temporarily (for five years) made the radiation levels in the Van Allen belts much worse (crippled a third of all satellites in low Earth orbit). Yes, this means that somebody can easily intensify the radiation just by popping off a few fission bombs. Science fiction writers can easily imagine scenarios where this could be used to attack a planet or to defend a planet.
Since the Van Allen Belts will destroy expensive satellites as well, there have been proposals to drain the radiation out of the belt.
The planet Jupiter has radiation belts similar to Terra, except the radiation is thousands of times stronger. Io, Europa, and Ganymede are inside the radiation belt, Callisto is outside. Volcanic gas from Io makes things more complicated. In 1973 Pioneer 11 was surprised by radiation levels around Jupiter ten times greater than NASA had predicted. This is why Pioneer did not send back photos of the moon Io since the radiation belt had fried its imaging photo polarimeter. Work on the Voyager space probe came to a screeching halt as they frantically redesigned it to cope with the radiation, but still be assembled in time for the launch window.
According to Cairan:
|Io||36||13149||2 min||4 min||20 min||40 min||1.33h||2 h||2.67h||4 h||6.67h||33.33h|
|Europa||5||1972||13.33 min||26.66 min||2.22h||4.44h||8.89h||13.33h||17.78h||26.67h||44.44h||-|
The single year limit is 50 mSv, while the maximum 5-year cumulative exposure is 100 mSv (or 20 mSv per year). LD stands for Lethal Dose, LD x/y means "x" percent of individuals die within "y" days. LD 50/30 thus means half of people exposed at this level of radiation would die within 30 days.
For shielding purposes to limit exposure to below regulatory levels for 5-year periods, the number of halving thickness of shielding material stands as follows:
|Satellite|| Number of|
The sun (or any other star) occasionally belches out fast-moving clouds of deadly particle radiation (the technical term is Coronal Mass Ejection or CME). These are called a solar proton storm, solar storm, solar proton event (SPE), or proton storm. Planets lucky enough to have magnetic fields (like Terra) are protected from such storms. Planets without magnetic fields and spacecraft, are bombarded by the radiation. The International Space Station has an orbit low enough that it is protected by Terra's magnetic field, though it does periodically plough through the South Atlantic Anomaly.
The storm can be sped up or slowed down by interations with the solar wind and interplanetary magnetic field. They reach velocities from 20 to 3,200 km/s with an average speed of 489 km/s. This means the transit time from Sol to the mean radius of Terra's orbit will be from 13 hours to 86 days, with 3.5 days as the average
Solar storms typically have a duration from one to two days.
Please note that solar storms are blobs of radiation that are quite a bit larger than a planet, but smaller than the solar system. So, for instance, a storm that hits Terra might totally miss Mars or Ceres. Or force the crew of spacecraft heading to Luna to take shelter in the storm cellar, while astronauts near Mars will be safe.
In a spacecraft or space station outside of a planetary magnetic field the crew is protected from solar storms by storm cellars. The crew will have to occupy the storm cellar for several hours during the peak of the storm, maybe up to a couple of days.
Researchers have been looking into equipping spacecraft with artificial magnetic fields for protection, but so far this has proven difficult.
From NASA-STD-3000 Man-Systems Integration Standards. (ed note: Naturally any NASA document will be silent on nuclear weapons, so there is no mention of gamma-ray or x-ray shielding.)
If the spacecraft uses a nuclear propulsion system, or has a nuclear power reactor, these are also sources of both electromagnetic and particle radiation. The exposure time is "chronic." Typically the crew is protected by use of shadow shields.
In some propulsion systems, such as open-cycle gas core nuclear thermal rockets and Orion nuclear pulse rockets, the exhaust is radioactive. This means the shadow shield has to cover a broader arc.
There is a first order approximation here to calculate the radiation flux from a fission reactor or fission nuclear thermal propulsion system.
Fission fuel is radioactive. If the reactor core is breached the fuel can spread radioactive contamination. Also the neutron radiation emitted by the reactor in normal operation can cause neutron activation.
If the spacecraft is in a combat situation, it will be targeted by nuclear warheads and particle beam weapons. In these cases the exposure time is "acute."
Nuclear warheads emit both electromagnetic and particle radiation
Obviously particle beam weapons only emit particles. Having said that, if the particle beam hits a metal hull bremsstrahlung will create a flood of x-rays. This only happens if the particle in the beam are charged, it doesn't work with neutron beams.
Radiation is meaured with lots of different confusing units. To make it worse, each measurement has both a traditional obsolete deprecated unit and a new modern scientific metric unit. There are units for the amount of radiation emitted, units for the amount of radiation absorbed by an inert object, and units for the amount of radiation absorbed by a living being. And on top of that, metric units often have prefixes for various powers of 10: milli-, micro-, etc. There is a table of prefixes here.
Naturally, the United States Nuclear Regulatory Commission requires the use of the non-metric obsolete deprecated units curie, rad and rem as part of the Code of Federal Regulations 10CFR20. The United States hangs on like grim death to its stupid ramshackle non-decimal system of units, instead of adopting the metric system like the rest of the scientific and civilized world.
ACTIVITY: The amount of radiation emitted by a chunk of radioactive material is measured by the traditional obsolete unit the Curie or the new metric unit the Becquerel (Bq). One Curie is equal to the amount of radiation emitted by one gram of radium. One Becquerel is equal to one decay per second. 1 Curie equals 3.7 × 1010 Becquerels.
ABSORBED DOSE: The amount of radiation absorbed by an inert object is measured by the traditional obsolete unit the Rad or the new metric unit the Gray (Gy). One Rad is a dose of radiation causing 100 ergs of energy to be absorbed by one gram of matter. One Gray is 1 joule of radiation absorbed by one kilogram of matter. 1 Rad equals 0.01 Gray. An even more obsolete term is the Roentgen, currently it is defined as 1 Roentgen equals 0.0096 Gray.
MGy is the symbol for megagray, or 106 gray. mGy is the symbol for milligray, or 10−3 gray. cGy is the symbol for centigray, or 10-2 gray.
Sieverts are determined from Grays. The effects of Acute radiation exposure are figured by the exposure in Grays. The effects of Chronic radiation exposure are figured by the exposure in Sieverts. Radiation quality factors seem to mostly matter for chronic doses.
You see, as far as an inert lump of matter is concerned, all forms of radiation are pretty much the same. But when you get to living beings, different kinds of radiation do different levels of long-term chronic internal organ damage per joule. Some types of radiation are better at killing people than other. For example, if 1 Gray of gamma radiation does 1 unit of damage to a stricken crewperson, 1 Gray of alpha particles will do 20 units of damage to the crewperson.
What this boils down to is that each type of radiation has a quality factor Q. You look up the Q factor for the radiation in question, take the radiation dose in Rads, multiply by Q, and you will have the "dose equivalent" in Rems. Or take the dose in Grays, multiply by Q and you will have the dose equivalent in Sieverts. There is a list of Q factors here.
You will sometimes see radiation exposure expressed in units of mGy/a or mGy/yr. This stands for milliGrays per annum, where 1 milliGray = 0.001 Gray and 1 annum is 8760 hours = 365 days = 1 year. This appears when talking about the radiation exposure suffered by inanimate objects on an extraterrestrial planet. For instance a rover space probe on Mars will suffer 73 mGy/a from cosmic rays and proton storms, while a space probe on Terra is typically only exposed to 0.4 mGy/a from cosmic rays.
The corresponding measure for organic creature exposure is mSv/a or mSv/yr. This is of course milliSieverts per annum.
The size of the dose depends on two things: the intensity of the radiation, and the duration of exposure. Crewpersons who do not want to die a hideous radioactive death will do well to reduce the size of the dose. You reduce the intensity by getting as far away from the source of radiation that you can (allowing the inverse-square law to reduce the intensity) and trying to get some radiation shielding between you and the source of the radiation. You reduce the duration by performing the first two techniques as quickly as possible (i.e., don't just stand there with a stupid expression on your face, run for your life!). There is also a difference between "acute" and "chronic" exposure. An example of an acute exposure is being in the general neighborhood of a nuclear weapon when it goes boom: the exposure duration is measured in fractions of a second. An example of chronic exposure is the day-to-day job of a nuclear rocket engineer: exposure duration is measured in months. Obviously the only things you can do to reduce an acute dose is to always be inside plenty of shielding and only fight enemies who use tiny nuclear weapons.
To figure the dose in Grays, take the radiation from the "event." Calculate how many radiation joules managed to penetrate the shielding and intersect the cross section of a person and divide by the body mass of the unlucky crewperson.
Since I know you are impatient, I'm first going to give you the quick-and-dirty equations. I'll then give you how I derived them, to allow you to skip over it if you are not interested. The equations have build-in assumptions. Assume this is radiation from an exploding nuclear warhead. 80% of the energy is in the form of x-rays with an average energy of 10 keV. Each fissioning nuclei will produce 2 or 3 neutrons and about half will escape to become radiation. The neutrons will have an average Sievert quality factor of 10. The crewmember will have an average cross-section of 0.445 m2 and a mass of 68 kilograms. Finally assume no radiation shielding.
Gx = 1.78e9 * (Y / R2)
Gn = 7.2e8 * (Y / R2)
- Gx = person's acute radiation dosage from x-rays (Grays)
- Gn = person's acute radiation dosage from neutrons (Grays)
- Y = weapon yield (kiloton TNT)
- R = person's distance from weapon's detonation center (meters)
How was this derived? In a round about fashion. First you figure out the radiation flux in joules of radiation per square meter. You take the amount of joules in the detonation and divide them by the surface area of a sphere with radius R.
ssphere = 4 * π * R2
There are about 4.19e12 joules per kiloton of nuclear detonation, and 80% of that is x-rays. Putting it all together:
- Fx = joules / surfaceArea
- Fx = (Y * 4.19e12 * 0.8) / (4 * π * R2)
- Fx = 2.67e11 * (Y / R2)
As a general rule, figure an average person has a cross section of about 0.445 m2 and a mass of 68 kilos. How was the cross section calculated? Given a mass of 68 kilos (150 pounds), and a height of 168 centimeters (5 feet, 6 inches) the Body Surface Area Calculator says the surface area is 1.78 square meters. The average cross section will be approximately one quarter of the surface area, or 0.445 m2.
To obtain Grays, we take the radiation flux, multiply it by the cross section of the person (0.445), and divide by their mass (68).
- Gx = ((2.67e11 * 0.445) / 68) * (Y / R2)
- Gx = 1.78e9 * (Y / R2)
Figuring neutron radiation is a bit more involved. Each kiloton requires the fissioning of approximately 1.45e23 nuclei. Each fission produces 2 or 3 neutrons, with an average production of 2.5. About half (0.5) will escape the nuclear reaction to become radiation. The neutron flux is therefore:
- Fn = (Y * 1.45e23 * 2.5 * 0.5) / (4 * π * R2)
- Fn = 1.8e23 * (Y / R2)
Neutrons have a Sievert quality factor ranging from 2 to 20 for neutrons of energies 0.01 MeV to 2.0 MeV, with an average quality factor of 10. So according to the references I've found, in the absence of specific data, you can assume that a neutron flux of 2.5e11 neutrons per square meter is about 0.01 Sievert or 0.001 Grays. This means 1 Gray equals a neutron flux of 2.5e14 neutrons per square meter. So simply divide Fn by 2.5e14 to get Grays:
- Fn = (1.8e23 * (Y / R2)) / 2.5e14
- Fn = 7.2e8 * (Y / R2)
|Type of radiation||Quality|
|Gamma rays and bremsstrahlung||1|
|Beta particles, electrons, 1.0 MeV||1|
|Beta particles, 1.0 MeV||1|
|Neutrons, thermal energy||2.8|
|Neutrons, 0.0001 MeV||2.2|
|Neutrons, 0.005 MeV||2.4|
|Neutrons, 0.02 MeV||5|
|Neutrons, 0.5 MeV||10.2|
|Neutrons, 1.0 MeV||10.5|
|Neutrons, 10.0 MeV||6.4|
|Protons, greater than 100 MeV||1-2|
|Protons, 1.0 MeV||8.5|
|Protons, 0.1 MeV||10|
|Alpha particles (helium nuclei), 5 MeV||15|
|Alpha particles, 1 MeV||20|
|Galactic Cosmic Rays||1 to 30|
The important point is that acute radiation damage will heal. Chronic damage does not. Acute damage goes away with time, chronic damage gradually accumulates over a lifetime. On the other hand, acute exposure can cause death by radiation burns a few weeks after the exposure. Chronic never causes death from radiation burns, instead it might kill you with cancer decades after the exposure.
Acute is a sudden dose that occurs over a few seconds to minutes. Chronic is a dose that occurs over a few days to years. Acute radiation syndrome is damage due to raw energy which burns internal organs (the technical term for such direct tissue damage is "nonstochastic effects" or "deterministic effects"). Chronic radiation syndrome is damage to the cellular DNA, leading to cancer and genetic defects in the victims future offspring (the technical is, surprise-surprise, "stochastic effects"). Chronic doses also cause skin ulceration and blindness due to cataracts scarring.
For our purposes, acute doses happen due to reactor accidents, solar storms, nearby nuclear explosions, and hits by particle beam weapons. Chronic doses are due to the unavoidable radiation filtering through the reactor shielding, the normal background radiation in space, and prolonged stays in regions like the Van Allen radiation belts.
There is no nonstochastic effects from chronic radiation dosage because the body has time to repair the damage while the dose is absorbed. There is no time to repair with an acute dose. For example, if a person suffers a (chronic) dose of 4.5 Sieverts over one year, they will have a much higher chance of cancer for the rest of their life. If they suffer an acute dose of 4.5 Grays in a second, they have an LD50 dose and have a 50-50 chance of being dead within 6 weeks due to radiation burns.
Deterministic effects mean that X amount of acute radiation exposure will cause Y amount of tissue damage. Stochastic effects mean that X amount of chronic radiation exposure will raise your chance of getting cancer by Y percent.
Remember acute exposure is measured in Grays, chronic exposure is measured in Sieverts.
For acute doses, simply figure the exposure in Grays suffered by the crewperson, and refer to the Acute radiation syndrome chart below. When a person receives an acute dose, they suffer what is listed under "Immediate symptoms." Then they appear to get better, but this is only temporary. After the Latent phase time passes, the person will start to suffer what is listed under "Post-latent symptoms".
As a side note, when NASA is sterilizing containers of meat for astronaut supplies, the recommended dosage is 44,000 Grays to kill off all the bacteria. You can find dosages for other food sterilization here.
To figure chronic doses, one has to calculate the Dose Equivalent. Split the radiation into the x-ray/gamma-ray Grays and the neutron Grays. Multiply the Grays by the radiation type's Quality Factor to get the Equivalent Dose (in units called "Sieverts"). Gamma-rays have a quality factor of 1.0, neutrons have a quality factor ranging from 2 to 20 for neutrons of energies 0.01 MeV to 2.0 MeV (just use the average of 10). Add the two Sievert doses together and look it up in the Chronic radiation syndrome chart.
Doctors go further, calculating the Effective dose, which is a weighted average of the equivalent dose to each organ depending upon its radiosensitivity. But that's probably a bit too much detail for our purposes. Rocketeers will always keep close watch on their radiation dosimeters that measures their current chronic dose. There will also be a crewperson or officer who is assigned the job of logging and monitoring the chronic dose of every person on board. If a crewperson gets close to their maximum allowable dose, they may be restricted to the more shielded sections of the ship, or even grounded from shipboard duty until they recover.
Atomic rocketeers will also without fail have a package of potassium iodide tablets on their persons at all times. Why? If the reactor core is breached, the mildly radioactive fuel and the intensely radioactive fission fragments will be released into the atmosphere. While none of the fission fragment elements are particularly healthy, Iodine-131 is particularly nasty. This is because one's thyroid gland does its level best to soak up iodine, radioactive or not. Thyroid cancer or a hoarse voice from thyroid surgery might be common among atomic rocket old-timers. The tablets prevent this by filling up the thyroid first, before the Iodine-131 arrives. The instant the reactor breach alarm sounds, whip out your potassium iodide tablets and swallow one.
Radioactive contamination of an area is measured by a "swipe" or "smear" survey. A small piece of absorbent paper is rubbed over a 100 square centimeter area in a S-shaped pattern. A radiation detector then is used on the paper to measure the disintegrations per minute (dpm). Depending on the standard used, an area is considered "contaminated" if the dpm is above 100 - 500.
Note, in your research you may run across the terms "rem" and "rad." These are sort of obsolete terms. One Sievert equals 100 rems. One Gray equals 100 rads. LD50 is the radiation dose that is expected to kill 50% of an exposed population.
|Dose (Grays)||Immediate symptoms||Latent phase||Post-latent symptoms||Prognosis|
|0 - 0.5||No obvious effect||None||No obvious effect, except, possibly, minor blood changes and anorexia.||Certain survival|
|0.5 - 1.0||Vomiting and nausea for about 1 day in 10 to 20% of exposed personnel. Fatigue, but no serious disability.||days to weeks||In this dose range no obvious sickness occurs. Detectable changes in blood cells begin to occur at 0.25 Gy, but occur consistently only above 0.50 Gy. These changes involve fluctuations in the overall white blood cell count (with drops in lymphocytes), drops in platelet counts, and less severe drops in red blood cell counts. These changes set in over a period of days and may require months to disappear. They are detectable only by lab tests. At 0.50 Gy atrophy of lymph glands becomes noticeable. Impairment to the immune system could increase the susceptibility to disease. Depression of sperm production becomes noticeable at 0.20 Gy, an exposure of 0.80 Gy has a 50% chance of causing temporary sterility in males. At 0.75 Gy there is a 10% chance of nausea.||Almost certain survival|
|1.0 - 2.0||Mild acute symptoms occur in this range. Symptoms begin to appear at 1 Gy, and become common at 2 Gy. Typical effects are mild to moderate nausea (50% probability at 2 Gy) , with occasional vomiting, setting in within 3-6 hours after exposure, and lasting several hours to a day. This will be followed by other symptoms of radiation sickness in up to 50% of personnel.||10 - 14 days||Tissues primarily affected are the hematopoietic (blood forming) tissues, sperm forming tissues are also vulnerable. Blood changes set in and increase steadily during the latency period as blood cells die naturally and are not replaced. A reduction of approximately 50% in lymphocytes and neutrophils will occur. There is a 10% chance of temporary hair loss. Mild clinical symptoms return in 10-14 days. These symptoms include loss of appetite (50% probability at 1.5 Gy), malaise, and fatigue (50% probability at 2 Gy), and last up to 4 weeks. Recovery from other injuries is impaired and there is enhanced risk of infection. Temporary male sterility is universal. The higher the dosage in this range, the more likely the effects, the faster symptoms appear, the shorter the latency period, and the longer the duration of illness.||Fatality rate is about 10%|
|2.0 - 3.5||Nausea becomes universal, the incidence of vomiting reaches 50% at 2.8 Gy and 100% at 3 Gy. Nausea and possible vomiting starting 1 to 6 hours after irradiation and lasting up to 2 days. This will be followed by other symptoms of radiation sickness, e.g., loss of appetite, diarrhea, minor hemorrhage||7 - 14 days||Illness becomes increasingly severe, and significant mortality sets in. Hematopoietic tissues are still the major affected organ system. When symptoms recur, the may include epilation (hair loss, 50% probability at 3 Gy), malaise, fatigue, diarrhea (50% prob. at 3.5 Gy), and hemorrhage (uncontrolled bleeding) of the mouth, subcutaneous tissue and kidney (50% prob. at 4 Gy). Suppression of white blood cells is severe, susceptibility to infection becomes serious. At 3 Gy the mortality rate without medical treatment becomes substantial (about 10%). The possibility of permanent sterility in females begins to appear. Recovery takes 1 to 3 months.||Fatality rate 35% to 40%|
|3.5 - 5.5||Nausea and vomiting within half an hour, lasting up to 2 days. This will be followed by other symptoms of radiation sickness, e.g., fever, hemorrhage, diarrhea, emaciation.||7 - 14 days||Hair loss, internal bleeding, severe bone marrow damage with high risk of bleeding and infection. Hemopoietic Syndrome. Mortality rises steeply in this dose range, from around 50% at 4.5 Gy (LD50) to 90% at 6 Gy (unless heroic medical intervention takes place). Hematopoietic tissues remain the major affected organ system. The symptoms listed for 2.0-3.5 Gy increase in prevalence and severity, reaching 100% occurrence at 6 Gy. When death occurs, it is usually 2-12 weeks after exposure and results from infection and hemorrhage. Recovery takes several months to a year, blood cell counts may take even longer to return to normal. Female sterility becomes probable. Survivors convalescent for about 6 months.||Fatality rate 50% within 6 weeks|
|5.5 - 7.5||Severe nausea and vomiting within 15 - 30 minutes, lasting up to 2 days, followed by severe symptoms of radiation sickness, as above.||5 - 10 days||Hair loss, internal bleeding, severe bone marrow damage leading to complete failure of blood system, high risk of infection, moderate gastrointestinal damage. Gastrointestinal Syndrome. Survival depends on stringent medical intervention. Bone marrow is nearly or completely destroyed, requiring marrow transfusions. Gastrointestinal tissues are increasingly affected. The final phase lasts 1 to 4 weeks, ending in death from infection and internal bleeding. Recovery, if it occurs, takes years and may never be complete. Survivors convalescent for about 6 months.||Death probable within 3 weeks|
|7.5 - 10||Excruciating nausea and vomiting within 5 - 15 minutes, lasting for several days||5 - 7 days||Hair loss, internal bleeding, severe bone marrow damage leading to complete failure of blood system, high risk of infection, severe gastrointestinal damage.||Death almost certain within 3 weeks. Complete recovery impossible.|
|10 - 20||Immediate nausea occurs due to direct activation of the chemoreceptive nausea center in the brain. The onset time 5 minutes.||5 - 7 days||Very high exposures can sufficient metabolic disruption to cause immediate symptoms. Above 10 Gy rapid cell death in the gastrointestinal system causes severe diarrhea, intestinal bleeding, and loss of fluids, and disturbance of electrolyte balance. These effects can cause death within hours of onset from circulatory collapse. Following an initial bout of severe nausea and weakness, a period of apparent well-being lasting a few hours to a few days may follow (called the "walking ghost" phase). This is followed by the terminal phase which lasts 5 - 12 days. In rapid succession prostration, diarrhea, anorexia, and fever follow. Death is certain, often preceded by delirium and coma. Therapy is only to relieve suffering.||Certain death in one week or less.|
|20 - 80||Immediate disorientation and coma will result, onset is within seconds to minutes.||None||CNS Syndrome. Metabolic disruption is severe enough to interfere with the nervous system. Convulsions occur which may be controlled with sedation. Victim may linger for up to 48 hours before dying.||Certain death|
|> 80||Coma||None||The U.S. military assumes that 80 Gy of fast neutron radiation (from a neutron bomb) will immediately and permanently incapacitate a soldier. Lethal within 24 hours due to damage to central nervous system.||Certain death|
|Criteria||General Public||Occupational Workers||Astronauts|
|30-day limit||0.0004 Sieverts (0.4 milli-Sieverts)||0.004 Sieverts||1.5 Sieverts|
|annual limit||Adult: 0.05 Sieverts|
minor: 0.005 Sieverts
Colonist: 0.02 Sieverts
Pregnant colonist: 0.0066 Gray
|0.05 Sieverts||3 Sieverts|
|Male career limit||N/A||2 + 0.075 x (age - 30) Sieverts||4 Sieverts|
|Female career limit||N/A||2 + 0.075 x (age - 38) Sieverts||4 Sieverts|
|accident limit||0.25 Sieverts||1 Sievert||N/A|
|acute limit||N/A||N/A||0.1 Sieverts|
One study suggested for a pregnant woman it is 0.005 Sievert total for the duration of the pregnancy. Another later study suggested an annual limit of 0.0066 Grays, using Grays instead of Sieverts because nobody knew the Q factor for embryonic tissue and obviously nobody was about to try experimentation to find the answer.
Colorful Chronic Terms
Writer Allen Steele uses the following terms:
- Singed: receiving a radiation dose that put one close to a chronic limit
- Cooked: receiving a radiation dose that put one over a chronic limit, could be career-ending
- Fried: receiving a radiation dose that put over the lethal LD50 limit, could be life-ending
An amusing unit of radiation exposure is the Banana equivalent dose. It provides some perspective, and can be used to calm down scientifically illiterate people who go hysterical when they hear the "R" word.
As it turns out, ordinary bananas are very slightly radioactive due to their potassium-40 content. Under this scale eating one banana exposes you to 0.1 μSv or 0.0000001 Sievert.
Living within 50 miles of a nuclear power plant for one year will give you a dose of half a banana. Living within 50 miles of a coal power plant for one year will give you a dose of three bananas (a pound of coal contains only small traces of radioactive elements, but such plants typically burn 4 million tons of coal every year).
Living in a stone, brick, or concrete building for a year will expose you to a dose of 700 bananas. The average dose from the Three Mile Island accident to someone living within 10 miles is 800 bananas. One mammogram is 30,000 bananas. A chest CT scan is 58,000 bananas.
If you spend one hour at the site of Chernobyl nuclear disaster in the year 2010 you will receive a dose of 60,000 bananas.
Half-life applies to many things, but in our area of interest, it determines how long it takes hideously radioactive elements to decay into safe non-radioactive elements. Scientists use half-life instead of full-life because  we want to know the rate and  full-life is typically freaking huge.
Half-life is also useful for figuring out how long you'll get useful power out of an RTG.
So, say there is a slug of Strontium-90 that is emitting about 10 sieverts of radiation. We will say that "safe" means radiation at a level about equal to background radiation emitted by the ground, about 0.21 millisieverts (0.00021 sieverts). How long will it take to for the strontium-90 to decay to a safe level?
Assume that strontium-90 decays directly into a non-radioactive isotope (it doesn't, but let's not complicate things). Assume that if the amount of strontium-90 is reduced by half due to decay, the amount of radiation will also be reduced by half. The fraction of an element undecayed after n half-lives is 1/2n.
After playing around with numbers, I found that 16 half lives will have an undecayed fraction of 1/216 = 1/65,536 = 0.000015. This means 10 sieverts will become 0.00015 sieverts (below 0.00021 sv background) after 16 half lives.
Strontium-90 has a half life of 28.8 years. Sixteen half-lives is 16 * 28.8 = 460.8 years.
Now, in reality you'd have to figure the radioactive decay products, figure their radiation level, and figure their decay time.
An atom of a radioactive element decays by emitting radiation. So as a general rule, the shorter an isotope's half-life, the more intensely radioactive it is. Specifically, the activity of a lump of isotope (in Becquerels) is:
Abq = N * (ln / t½)
Abq = (radio)activity (Becquerels)
N = number of atoms in the lump of isotope
ln = Natural logarithm of 2, about 0.69315...
t½ = half-life of the isotope (seconds)
|Uranium 235||7.13×108 years|
|Uranium 233||160,000 years|
|Plutonium 239||24,100 years|
|Curium 245||8,500 years|
|Plutonium 238||87.7 years|
|Polonium 210||138.376 days|
The question arises "how many atoms are in a gram?" The answer was told to you in chemistry class, when your eyes glazed over as the professor talked about "molar mass" and the "Avogadro constant". Avogadro constant is about 6.02214179×1023 mol-1. This means if you made a pile of 6.02214179×1023 Uranium-235 atoms it would weigh exactly 235 grams. A pile of that number (one "mole") of Plutonium-239 would weigh exactly 239 grams.
The point is, you can use this to convert between atomic mass units and grams. Basically you divide Avogadro constant by the atomic mass of the element to find the number of atoms of that element in one gram. So Strontium-90 contains 6.02214179×1023 / 90 = about 6.69126865×1021 atoms per gram.
The radiation flux depends upon the energy per atomic decay. These are generally listed in terms of MeV or megaelectron volts. 1 MeV = 1.6021773×10-13 joules. For instance, strontium-90 undergoes beta-decay, at 0.546 MeV per decay. Multiply this by the Becquerels to get the total radiation flux in joules.
The total flux can then be used to calculate the dosage inflicted on anybody unfortunate enough to be exposed to the lump.
Note that Doc Smith to the contrary, chelating decontamination doesn't quite work this way.
While human beings and other living things suffer harm from nuclear radiation, inanimate objects are not fond of it either. Charged particle radiation can be deflected from your ship by magnetic fields, but vexing neutron radiation is uncharged. It hits whatever is in the way, you cannot divert it.
This is why the magnetic nozzles of fusion engines are an open lattice full of holes: to reduce the surface area of nozzle that can be hit by neutrons. Meaning if from the viewpoint at the center of the reaction the sky is 10% lattice and 90% holes, only 10% of the neutron radiation is damaging the engine.
Neutron Activation happens when an ordinary harmless atomic nucleus swallows a low-energy neutron from the radiation flux of, say, a nuclear thermal propulsion system. This changes the nucleus from a stable isotope into an unstable isotope, and all unstable isotopes are radioactive and emit gamma-rays. Or the neutron can actually split the atom, with much the same results but now including bonus fission fragments and neutrons in the induced radioactivity.
Translation: the steel girders and everything else too close to the reactor will eventually become radioactive in and of themselves, glowing with dangerous gamma-rays. Your nuclear engine structure will gradually be transformed into low-level radioactive waste. This makes it treacherous for the crew to leave the spacecraft, and drastically lowers the re-sale value. This is a good reason to make your spacecraft modular, so you can detach the nuclear engine and swap it for one that doesn't glow in the dark.
Low-activation elements are chemical elements that are resistant to neutron activation. These include tungsten, tantalum, vanadium, and chromium. Use these to construct nuclear reactors and nuclear rocket engines.
High-activation elements can easily be transmuted into glowing radioactive chunks of death by trival amounts of neutrons. These include nickel, copper, aluminium, molybdenum, cobalt and niobium. As a matter of fact, cobalt has been suggested as a component of a salted bomb. Avoid these when making your nuclear engines.
There is a nice list of common elements that can be transmuted into radioactive isotopes here, along with the half-life of said radionuclides.
Neutron activation is a good thing in a breeder reactor or a medical isotope generator, but very bad anywhere else. Heavy neutron radiation is usually never found naturally, it is found unnaturally in nuclear reactors, nuclear explosions and other artificial things made by intelligent creatures.
The gamma dose rates arising from the neutron activation of a test article (modular concept) (the Lockheed nuclear shuttle concept) has been calculated at axial detector locations at Station 46 and at a second point 20 feet below Station 46 for times of 50, 100, 200, and 500 hours following an irradiation history consisting of nine 400 second irradiation periods followed by four 1800 second irradiation periods with a 30 day interval between each irradiation period. These gamma dose rates in units of milli-tissue rads/hr (1 milliRad equals 0.00024 Gray) are as follows:
Times after Final Irradiation 50 hr 100 hr 200 hr 500 hr At Station 46 1600.0 mTRad/hr 151.0 mTRad/hr 25.5 mTRad/hr 17.0 mTRad/hr At 20 feet below Station 46 60.6 mTRad/hr 5.53 mTRad/hr 0.86 mTRad/hr 0.605 mTRad/hr
From these data it appears that it is safe with the exercise of normal precautions for an individual to venture into the vicinity of the second detector point (20 feet below Station 46) after only a few days (100 hr) following the final irradiation, whereas a waiting period of a week or so (200 hr) will be required before the dose rate at the first detector point (Station 46) dimenishes to a comparable level.
The effects of fewer irradiation periods has been accessed by comparing the dose rates at 50 and 500 hours following the 1st, 9th, and 10th irradiation periods to those following the 13th irradiation period. The ratios of these dose rates are as follows:
Times after Irradiation 50 hr 500 hr 1st/13th 0.223 0.049 9th/13th 0.225 0.273 10th/13th 0.995 0.550
At 50 hours following an irradiation: Cu-64 with a half life of 12.8 hours is the predominant radiation source; Na-24 with a half life of 15 hours is also significant; all other source terms combined account for less than 5 percent of the dose rate. With 30 days between irradiation periods there is no buildup of these significant sources. This is examplified by comparing the dose rate after the 1st irradiation period with that following the 9th irradiation period. The jump in the dose rate following the 10th irradiation period shows that the dose rate is nearly proportional to the length of the irradiation period which increased from 400 to 1800 seconds.
At 500 hours following an irradiation the dose rate is dominated by isotopes with half lives greater than a few hundred hours. There is a considerable buildup of these sources from previous irradiations. This is shown by comparing the dose rates following the 9th Irradiation period to that following the 1st irradiation period or by a comparison of the 13th to the 10th. The effect of buildup is of greater significance with the shorter irradiation periods.
From an operational standpoint fewer irradiation periods will significantly reduce the waiting time for entry into the area following an irradiation. This is especially true when the longer lined isotopes are withholding entry.
Neutron activation is used on purpose in nasty salted bombs. The bomb is intentionally cased in cobalt or other element that is particularly good at being neutron activated into a hideously radioactive isotope. This provides enhanced quantities of radioactive fallout. "Salted", as in "sowing the earth with salt so nothing ever grows there again." The salted bomb is conveniently almost non-radioactive while in storage, the radioactive isotopes are instantly manufactured at ground zero.
And let's not forget the "enhanced radiation bomb" aka "Neutron bomb". This is a nuclear warhead specially constructed so that less of the bomb energy goes into x-rays and more goes into neutrons. It was designed to do less blast damage to buildings and vehicles but do more radiation damage to people. But of course the extra neutron flux will naturally do more neutron activation to any material object near ground zero. I found mention that a main battle tank close to the detonation point would suffer enough neutron activation to render it lethally radioactive for about 48 hours.
Neutron activation analysis can be used to determine how much neutron radiation a hapless victim was exposed to. The doctor can examine the victim to determine how much of the body's sodium was neutron activated into sodium-24 and how much phosphorus was activated into phosphorus-31. This will provide an estimate of the acute radiation dosage.
Neutron radiation can make some materials become brittle by neutron-induced swelling and buildup of Wigner energy. High-energy neutrons striking metal gradually damage the metal's crystal lattice. This makes the metal more brittle and can eventually lead to failure. Steel has a so-called "ductile-to-brittle transition temperature", a temperature below which it becomes brittle. Neutron bombardment raises this temperature.
The brittleness can be healed by heating the material, this is called annealing. It might be possible to construct a reactor capable of annealing its structural members in place instead of removing it first. But of course you have to be very careful. You will be in for some real excitement if you accidentally catch the nuclear engine on fire.
Very-high energy neutrons not only cause neutron embrittlement, they also impart thermal energy to the nucleus they hit. Meaning it heats things up. Gamma rays heat things up as well. For the most part, fast neutrons tend to heat up things like liquid hydrogen, while gamma rays heat up engine components.
So here's the score:
- Very-high-energy neutrons: thermal heating of engine
- High-energy neutrons: embrittlement of engine
- Medium-energy neutrons: SAFE ZONE
- Low-energy neutrons: activation of engine (becomes radioactive)
Neutrons with energies in the medium-energy zone have no harmful effect on the engine, the metals composing the engine are transparent to the neutrons. Those neutrons have too much energy to be absorbed by nuclei and cause neutron activation, but not enough energy to interact with nuclei to cause neutron embrittlement or thermal heating.
It would be wonderful if researchers could invent a shield that would somehow cause the neutrons to enter the safe zone (the technical term is "neutron moderation"). But that would be tricky. And I am unaware of any way to speed up low-energy neutrons, moderators in nuclear reactors can only slow neutrons down.
Semiconductor electronics are also vulnerable to radiation (including particle beam weapons) due to mechanical disruption, as you can see from Anthony Jackson and Christopher Thrash (below).
However while the vacuum tubes (thermionic valves) in the Russian MiG fighters would not provide much protection from nuclear EMP, vacuum tubes are inherently more radiation resistant compared to semiconductors.
Subject: Re: radiation and computers From: email@example.com (Christopher Thrash) Date: 19 Nov 2000 14:06:00 GMT Message-ID: 6507 Newsgroups: sjgames.gurps.traveller On 16 Nov 2000 08:15:36 GMT, firstname.lastname@example.org (Anthony Jackson) wrote: > Realistically, what level of ionizing radiation will cause significant > software errors (and possible a soft reboot) in hardened electronics? > For that matter, what level of radiation will cause permanent damage, > I know that some of the jupiter probes were fried by multiple passes > through the jovian radiation belts. From _The Effects of Nuclear Weapons_, Glasstone and Dolan, 1977, Sec. 8.73-8.88: "The name commonly applied to the class of effects under consideration is "transient-radiation effects on electronics," commonly appreviated to the acronym TREE. In general, TREE means those effects occurring in an electronics system as a result of exposure to the initial radiation from an nuclear weapon explosion. The adjective "transient" applies to the radiation since it persists for a short time, i.e. less than 1 minute. The response, however, is not necessarily transient... "Radiation effects on electronics may be temporary or more-or-less permanent... The component responses of short duration are usually the result of ionization caused by gamma radiation and are dependent on the dose rate, e.g., in rads per second, rather than the dose. The more permanent effects are generally -- but not always -- due to the displacement of atoms in a crystal lattice by high-energy (fast) neutrons. In such cases the extent of the damage is determined by the neutron fluence, expressed in neutrons/cm2. When a permanent effect is produced in an electronic component by gamma radiation, the important quantity is usually the dose in rads." Neutron fluence (Fn) at a distance R from a nuclear detonation is approximately given by:
Fn = 1.4 × 1012 Y/R2where Y is in kilotons, R is in km, and Fn is in neutron/cm^2. Dose (Dg) from prompt radiation of an explosion is approximately:
Dg = 4 x 105 Y(2/3)/R2where Y is in kilotons, R is in km, and Dg is in rads (Si). Damage Thresholds (gleaned from the text):
Transistors 1011—1015 neutron/cm2 MOS Transisitors 104 rads (silicon) Capacitors 1015 neutrons/cm2 Precision Resistors 107 rads (carbon)/s 1014 neutron/cm2 NiCd Batteries 107 rads (air)/s 1013 neutron/cm2 Hg Batteries 10^16 neutron/cm2 Wiring Insulation: Silicon Rubber 2x1015 neutron/cm2 Polyethylene 107 rads (carbon) Teflon TFE 104 rads (carbon) Teflon FEB 2×106 rads (carbon) Polyolefins 5×109 rads (carbon)From _Space Mission Analysis and Design_, 3d Ed. (SMAD III), Wetz and Larson, 1999, pp. 214-240: Commercial Off the Shelf (COTS) and Rad Hard Parts Comparison:
Characteristics COTS Rad Hard Total Dose 103—104 rads 105—106 Dose-Rate Upset 106—108 rads(Si)/s >109 rads(Si)/s Dose-Rate Induced Latchup 107—109 rads(Si)/s >1012 rads(Si)/s Neutrons 1011—1013 n/cm2 1014—1015 n/cm2 Single-Event Upset 10-3—10-7 error/bit-day 10-8—10-10 error/bit-day Single-Event Latchup Single-Event Burnout <20 MeV-cm2/mg (LET) 37-80 MeV-cm2/mg (LET)LET is "linear energy transfer".
The three anti-radiation protection methods are Time, Distance, and Shielding. Time means minimize the duration of exposure, Distance means get as far away from the radiation source as you can, and Shielding means get some radiation armor between you and the radiation source.
Remember, outside the engine room hatch will be a decontamination booth. And I'm sure over the hatch will be mounted an alarm with a red rotating light, so you don't have to put your ear on the bulkhead to hear Astro say "Oh SH*****T!!!". Past the hatch will be a radiation-shielded corridor, with a dog-leg bend in it, so you can get in but radiation cannot get out (radiation has to travel in straight lines, but crewmen can zig-zag). Be sure you are wearing your dosimeter.
Shortening the duration of exposure is difficult to do. A nuclear engine burn for a specific amount of delta V takes as long as it does, it cannot be shortened. You cannot stop a solar proton storm in progress to shorten exposure. About the only thing you can realistically do is make sure you perform any vital repairs in a radioactive zone as quickly as possible, and if you unexpected discover something radioactive nearby you should run away.
Difficult as it is, NASA scientists were looking into duration shortening strategies for a proposed Mars mission. Using a minimum delta V / maximum duration Hohmann trajectory the trip from Terra to Mars will take the better part of eight months. The Mars Science Laboratory actually traveled the route and measured the cosmic ray radiation exposure with the RAD. The scientists were aghast when they found that the round trip dose was much higher than they estimated, about 0.66 sieverts round trip (about 1.8 milliSieverts per day). Which is bad news if the career radiation limit for astronauts is 1.0 sieverts.
They tried to design radiation shielding that would reduce the cosmic ray exposure to something reasonable. Unfortunately this cut so deeply into the payload mass that there basically wasn't a mission any more.
But now the are focusing on reducing the time of radiation exposure. How? By developing more powerful propulsion systems like VASIMR. If such propulsion can increase the spacecraft's delta V capacity enough, it can afford a trajectory with trip duration reduced. Which would shorten the duration of cosmic ray radiation exposure.
There was some 1950's era spacecraft designs that attempted to substitute distance for lead shielding (since distance weighs nothing) thus utilizing the inverse-square law. They were practical designs for exploration craft but pretty silly for warships. Crew cabins on the end of hundred meter booms, or dangling at the end of kilometer long cables, that sort of thing. While you wouldn't want to use the designs, you understand the motivation. When every gram is limited, you don't want to waste it on a shield made out of one of the heaviest elements in existence. The break-even point is where the mass of the boom or cable is equal to the mass of the shadow shield. One source suggested that occurred at a cable length of one kilometer with a one megawatt reactor.
Remember the basic strategy. Use dense elements like lead, tungsten, and beryllium for x-ray and gamma-ray shielding. Use low-density elements like liquid hydrogen, dehydrated astronaut poo, lithium hydride, paraffin, hydrogenated polyethylene composite, or other hydrogen-rich compounds for particle radiation shielding.
Why? X-rays and gamma-rays are stopped by electrons, and high density elements have more electrons per cubic centimeter. Particle radiation is stopped by atomic nuclei and low density elements have more atomic nuclei per cubic centimeter than metals.
When shielding against neutron particle radiation, instead of using hydrogen compounds it is better to use neutron reflectors such as graphite, beryllium, steel, and tungsten carbide.
Beware of bremsstrahlung. If you place your shielding improperly you'll convert the inside of the spacecraft into a giant x-ray machine and fry your astronauts to death. To avoid this make sure the outermost shield layer (next to deep space or other radiation source) is particle shielding and the innermost shield layer (next to the astronauts) is the x-ray/gamma-ray shield layer. As a side note, only charged particles like protons and electrons cause bremsstrahlung, neutrons do not.
Why? Gamma-ray shielding is worse than useless against particle radiation. Charged particles hitting dense elements is the same operating mechanism used inside a dentist's x-ray machine. So if you have the gamma-ray shielding outermost, this is what happens:
- A charged particle from deep space slams into the x-ray/gamma-ray shielding
- This generates deadly x-rays which are emitted by the x-ray shielding
- The new x-rays find there is no x-ray shielding in front of them, only pathetic particle shielding
- The x-rays sail unharmed through the particle shielding, and kill the astronauts
When you place your shielding the right way, the particle shielding soaks up the particle radiation before it can hit the gamma-ray shielding.
Philip Eklund points out that in spacecraft combat an Orion drive rocket has built-in radiation armor. But it only works if you can keep the pusher plate aimed at the nuclear warhead. If you can manage that, you can laugh at most nuclear detonations.
On the other hand, there are certain propulsion systems that undergo catastrophic failure (i.e., they blow up) if minor damage happens to the fuel tanks. These include antimatter rockets, Zubrin's NSWR, and any form of metastable fuel.
Radiation shielding is rated in "Tenth Value Thickness" or TVT. One TVT is the depth of shielding required to reduce the radiation to one tenth of its initial value (i.e., it stops 90% of the radiation). Twice the TVT will reduce the radiation to one one-hundredth of its initial value (stops 99%), and so on.
Sometimes one will encounter "Half Value Thickness" and "1/e". HVT is the depth required to reduce the radiation by one-half, and 1/e is the depth required to reduce the radiation to approximately 37% (specifically to 1/e where e is approximately 2.718).
Water has a TVT of 25.4 centimeters vs particle radiation (including neutrons), but only 61 centimeters vs gamma rays. Lead has a TVT of 5 cm vs gamma, and basically doesn't do diddly-squat vs particle radiation. Steel has a TVT of 11 cm vs gamma and also does poorly vs particle radiation. By my calculations carbon should have a TVT of 22.5 cm vs gamma rays, but I have no idea what its TVT is vs neutrons.
X-ray and gamma-ray shielding boils down to how much mass is between the radiation source and the crew. 45 g/cm2 is a TVT (meaning that behind each square centimeter of shield surface area is 45 grams worth of shield material of a thickness determined by the material's density). As a wild guess, the interior of the spacecraft has a density of about 0.25 grams per cubic centimeter. This means a crew member would get a "free" 1 TVT from X and gamma-rays if they were 1.8 meters from the hull, from the shielding provided by the bulkheads, machinery, pipes, and structural materials (45 g/cm2 / 0.25 g/cm3 = 180 cm). Keep in mind that 1 TVT isn't all that much, and the free shielding obviously goes up the further from the hull the crew is. This is an argument for putting the control room of a combat spacecraft near the center.
Cosmic rays will need a TVT of about 450 g/cm2. You will need 450 g/cm2 to get OSHA-legal exposure limits on a timescale of years, say for a space colony or long duration space mission.
Extremely high energy particle beam weapons act like cosmic rays, with a TVT peaking at a whopping 100 to 300 g/cm2.
If you have a thickness which stops a known amount of radiation of a known and constant type, then if you have a new thickness, you can calculate how much it stops by:
Stoppage = 1 - ((1 - AmountStoppedByKnownThickness)^(NewThickness / KnownThickness))
Example: if six centimeters of polka-dotted Kryptonite will stop 90% of x-rays, then eighteen centimeters of polka-dotted Kryptonite will stop:
Stoppage = 1 - ((1 - AmountStoppedByKnownThickness)^(NewThickness / KnownThickness))
Stoppage = 1 - ((1 - 0.9)^(18 / 6))
Stoppage = 1 - (0.1^3)
Stoppage = 1 - 0.001
Stoppage = 0.999 = 99.9%
Just to make our lives more difficult, mixed radiation such as is found in space has verying penetration. So if shield material X stops 90% of the quote "radiation" unqote, this will mean something like stopping 99% of the low-penetrating radiation and 50% of the high-penetrating radiation. And doubling the thickness of the shielding might only bring the radiation stoppage up to aroung 96%.
For comparison purposes, a typical NASA space suit has 0.25 g/cm3, the hull of an Apollo command module is rated at 7 to 8 g/cm3, the Space Shuttle is rated at 10 to 11 g/cm3, the storm cellar of the International Space Station is rated at 15 g/cm3, and future lunar bases are planned to exceed 20 g/cm3.
Now to calculate the radiation that penetrates a shield:
Rd = Rh * Vf(Ad / Vd)
- xy = raise x to the power of y
- Rd = Radiation dose that penetrates the armor (grays or whatever)
- Rh = Radiation strength hitting the armor (grays or whatever)
- Vf = Value Factor (0.1 for TVT, 0.5 for HVT, 0.37 for 1/e)
- Ad = Armor depth (centimeters or whatever)
- Vd = Value depth (e.g., 61 cm if armor is water and radiation is gamma rays)
The safe design would be to totally encase the engine and reactors in radiation shielding. But this sharply reduces the ship's payload since radiation shields literally weigh tons, and Every Gram Counts. Shadow shields are the bare minimum of shielding: it only stops the radiation heading for the habitat module and other vital parts of the spacecraft. The radiation freely sprays in all other directions, which makes it dangerous to approach an atomic rocket outside of the safe shadow cast by the shield.
Another implication is that the ship's docking port is probably best placed on the ship's nose. This will allow two ships to dock nose-to-nose, while keeping each other in the shadow of their shadow shields.
Note that the larger the distance between the crew and the atomic engine, the narrower the angle the shadow has to be, thus the smaller the shadow shield. Since the shadow shield is several tons of rocket mass that is not payload, the smaller it is, the better. Also note that radiation weakens with distance due to the inverse-square law, which is another argument in favor of plenty of distance. As previously mentioned, if the rocket has multiple atomic engines one wants them clustered closely or they will require a larger shadow shield, or even one shield for each engine. (for "cluster closely" read: "have the radioactive components as close to the axis of the spacecraft as possible")
I have seen a few Mars mission studies that try to use the shadow shield to protect against solar proton storms in lieu of a storm cellar. The idea is to point the engine at the Sun when a storm comes and hope it provides the same protection as the penalty-mass laden storm cellar which was omitted from the design. I do not have precise figures on how effective this strategy is, but the fact it is not a standard design feature in all Mars missions tells me the concept has problems.
When the reactor is idling, the shadow shield does not have to be as thick. In order to widen the area of shadow (for adding side tanks or whatever), the secondary shadow shield could extrude segments as extendable side shields.
This NASA technical report had a clever idea. It is a better way to widen the area of the shadow.
During a propulsive burn the neutron-stopping part of the shadow shield (Lithium Hydroxide LiH or something) and the gamma-ray-stopping part (Tungsten W or Mercury Hg) have to be on top of the engine, to shadow the rest of the spacecraft. After the burn, the reactor stops emitting neutrons and gamma rays. However, neutron activation has transmuted girders and other engine parts into radioactive isotopes. These will emit dangerous gamma-rays at a gradually decreasing rate. After a couple of days the gamma-ray flux will be reduced by about three orders of magnitude (1/1,000th). Which means a thinner shield can provide the same protection. A pity that the shield is welded in place.
In the report they say Not So Fast, there may be a way. What if you used liquid mercury instead of solid tungsten as the gamma shield? In the diagram above the mercury is in the upper tank during the burn. The thick layer can cope with burn-levels of gamma rays, at the price of only protecting a narrow shadow.
But after the burn, the neutron radiation stops and the gamma-rays are at a reduced level. The mercury is pumped into the lower tank. The mercury shield is a thinner layer which can cope with the reduced gamma-rays. The advantage is a thinner layer means there is more mercury to go around. The lower tank creates a mercury layer that covers more of the engine, expanding the size of the shadow. This could come in handy if the spacecraft is a tail-sitter.
RADIATION FLUX FROM ENGINE
So the theory is you calculate effective radiation flux from the atomic engine, multiply it by the appropriate attentuation factors of the shadow shield, and figure the effective dose given the total burn time for the mission. The idea is to keep the effective dose is within acceptable limts.
The effective radiation flux is the atomic engine's total radiation flux reduced by the free shielding of the remoteness of the habitat module
Divide the radiation flux by the square of the distance between the engine and the habitat module to get the effective radiation flux. That is:
Fluxeff = Flux / d2where:
Flux = engine radiation flux (Severts/sec)
Fluxeff = effective radiation flux (Severts/sec)
d = distance between engine and habitat module (meters)
x2 = square of x (x × x)
Which brings us to the problem of calculating the radiation flux from the atomic engine.
Anthony Jacks says as a quick-and-dirty first order approximation, for a fission reactor, the radiation flux is one-half the power production (in kilowatts) of the reactor core (which is greater than the power output due to inefficiency). This assumes 1 kilowatt of reactor core power produces 1.26 kilowatts of radiaion, mostly neutrons.
Flux = 0.5 * Pcorewhere:
Flux = engine radiation flux (Severts/sec)
Pcore = power production of reactor core (kW)
As a rough guess, for atomic engines with a thermal power level of one megawatt to one gigawatt, the shadow shield will be from 1.0 to 0.1 kilograms per kilowatt. This assumes that the spacecraft is long and skinny, which reduces the angular size of the shield. The shield will be a composite of gamma ray shielding materials and neutron shielding materials.
In Space Propulsion Analysis and Design they give the specs on a typical shadow shield. Starting at the atomic engine, the gamma rays and neutrons first encounter 18 centimeters of beryllium (which acts as a neutron reflector), followed by 2 centimeters of tungsten (mainly a gamma-ray shield but also does a good job on neutrons), and finally 5 centimeters of lithium hydroxide (To stop the remaining neutrons. Hydrogen slows down the neutrons and lithium absorbs them.). This attenuates the gamma flux to a value of 0.00105, and neutron flux to 4.0e-9. This has a mass of 3,500 kilograms per square meter of shadow shield (ouch!).
For a rough estimate of the surface area of the shadow shield it should be a disk with a radius equal to the radius of the reactor core.
To estimate the size of the core is over my head but it is covered in SPAD. In the example from the book a 1000 megawatt reactor had an radius of 0.45 meters so the shield surface area is 0.64 m2. The mass would be 3,500 * 0.64 = 2,240 kg.
A 2000 MW had a radius of 0.75 meters, surface area 1.77 m2, mass 6,195 kg.
If an attenuation factor of 0.00105 for gamma and 4.0e-9 for neutrons is not enough, the factors can of course be increased by adding more thickness to the layers in the shadow shield. The SPAD has a handy table which I adapted:
|Additional cm of|
for Neutron Attenuation
|Additional cm of|
- Size and nature of the power source
- Type of radiation
- Configuration of the spacecraft or platform and its payload (radiation flux level decreases by a factor of 1/distance2 from the radiation source)
- Generic operational procedures and requirements for the mission
- Length of the mission
- Total permitted levels of radiation dosage
To design the shield correctly, we need to compare the radiation flux from the engine with the allowable dose. For example, if the engine is releasing 107 rem per year and the payload is allowed a dose of only 10 rem per year, we need to attenuate the radiation by a factor of 106. For shield design in a nuclear rocket, we usually attenuate the radiation flux to avoid excess propellant heating while the propellant is still in the tank. The structure of the tank, the propellant, and the additional distance of the payload from the radiation source then reduce the radiation further (see Fig. 8.22).
By combining different shielding materials, we can tailor the shield to a particular form of radiation. For example, usually a shadow shield (shields the reactor from the tank) is directly under the tank. Figure 8.23 shows a typical cross section of a shadow shield. In this example, the radiation first sees a neutron reﬂector material such as beryllium (Be). Next, it encounters a thin layer of heavy material used to shield gamma rays. Tungsten (W) is a good candidate for this part of the eld because it has a high neutron-absorption cross section plus high gamma attenuation. Finally, a lighter-weight material finishes attenuating the neutron flux. We often suggest using lithium hydride (LiH2) because it has good neutron-slowing properties from the hydrogen component and a high neutron-absorption cross section from the lithium component.
Table 8.11. Physical Properties of Shielding Material and Effectiveness for Sample Shield Layout Parameter/Shield Material Be W Li-H2 Density (ρ — kg/m3) 1850 19,300 500 Molecular weight (gm/mol) 9.01 183.86 Li—6.94;
σa - absorption cross section (barn) 0.009 19.2 Li—71;
Attenuation factor for 1MeV gamma rays
(μ — cm-1)
0.104 1.235 0.0444 Shield thickness (cm) 18 2 5 Gamma attenuation of incident beam 0.1538 0.0854 0.8011 Number density (atom/(barn·cm)) 0.124 0.063 0.047 Σa - absorption cross section (cm-1) 0.001 1.214 3.373 Attenuation of incident neutron beam 0.9802 0.0882 4.7×10-8 Integrated gamma-flux reduction 0.1538 0.00131 0.00105 Integrated neutron-flux reduction 0.9802 0.0864 4.0×10-9
(ed note: Fig. 8.23 says tungsten (W) is 5 cm thick, table 8.11 says 2 cm thick. I cannot quite calculate which one is the correct figure.)
Table 8.11 summarizes the properties and resultant radiation attenuation across each thickness and for the overall shield. Because the size and mass of shields are significant, we keep the layers as thin as possible for an effective shield. But the shield shown in Fig. 8.23 can reduce the gamma ray flux by a multiplication factor of 0.001 05 and the neutron flux by 4.0 (10)-9, which is adequate for most applications. We can use the data in Table 8.12 to increase or decrease the attenuation. For example, adding 1.872 cm of tungsten further decreases the gamma-ray flux by a factor of 10.
The information in the last two lines is most important. As gamma rays pass through the complete shield (Fig. 8.23), they are attenuated by a factor of 1000 (0.00105). Similarly, neutrons are attenuated by 9 orders of magnitude (4×10-9).
For preliminary design, we use the shield cross section shown in Fig. 8.23 as our baseline. This shield configuration has a mass of 3,500 kg /m2.
(ed note: I tried calculating the mass but came up with 969 kg/m2. I am unsure what I am doing wrong.)
As a first cut in sizing our shield, we assume it has a radius equal to that of the reactor core (Rcore).
(ed note: I'm still trying to figure out how to calculate Rcore. The math in the book is rather dense. A supplied graph showed a 1000 megawatt reactor had an Rcore of 0.45 meters for a surface area 0.64 m2 and a mass of 3,500 * 0.64 = 2,240 kg. A 2000 MW had a radius of 0.75 meters, surface area 1.77 m2, mass 6,195 kg.)
We can change this baseline shield configuration a bit for lower or higher reactor power, burn duration, and reactor type. However, scaling this shield is highly non-linear and requires complicated computer analysis. The simplified approach we discuss here gives us an adequate estimate.
Table 8.12. Radiation Attenuation for Shielding Example Reduction
cm Thickness Required for
cm Thickness Required for
Φ(x)/Φ(0) Be W LiH2 Be W LiH2 0.5 622.7
3.794 1.365 44.281 3.744 103.721 0.001 6,206 5.691 2.048 66.421 5.616 155.581
To reduce gamma rays by 50%, we would need a tungsten shield 0.564 cm thick.
As an example, NASA's Reusable Nuclear Shuttle concept used a NERVA NTR engine with 334 kiloNewtons of thrust with a shadow shield massing 1360 kilograms which protected a 10 degree half-angle area. The distance between the habitat module and engine (a bit less than 49 meters) provided extra protection, as did the mass of the propellant.
The crew protected by the shadow shield, distance, and propellant would still suffer a radiation dose of 0.1 sieverts every time the shuttle did a standard burn. Anybody outside of the shadow cast by the shield and closer to the engine than 16 kilometers would suffer a whopping 0.25 to 0.3 sieverts per hour. The safe distance outside of the shadow is no close than 160 kilometers.
A standard burn was a delta V between 1 and 2 kilometers per second.
NASA has a career limit of 4 sieverts for astronauts, so an astronaut exposed to 40 standard burns would be permanently grounded.
I have found minimal references to low mass shields for space nuclear reactors that were layered tungsten-lithium hydride, layered boron carbide-beryllium, and layered lithium hydride-beryllium. The lowest mass one is the tungsten-lithium hydride shield.
In Heinlein's "The Green Hills of Earth", atomic spacecraft designers are guilty of scrimping on shadow shields in order to save mass. The designers were under pressure to maximize payload mass without worrying about trivial incidentals like the health of the engine crew. This is why the jetmen working next to the atomic engines find it so hard to get insurance, and seldom have children. At least ones that are not mutants.
Keep in mind that these are called "shadow" shields because it is too expensive to put radiation shielding all around the hot stuff ("expensive" in terms of reduction of payload mass). This means that if one ventures outside of the spacecraft, you run the danger of moving out of the shadow and into the deadly glow of the unshielded engine. When the spacecraft is designed, it is also important to ensure that no part of the ship scatters the lethal radiation around the shadow shield and into the crew. The heat radiators, for instance. If lifting off from a planet with an atmosphere, said atmosphere can also create pesky neutron backscatter.
This does make exiting a landed ship somewhat challenging, and makes an argument for a ground crew wearing lead suits. In the Tom Corbett books, any ship that was to be on the ground for more than three days would have its liquid fissionable fuel removed by the "hot soup" wagon. Keep in mind that the neutron flux from the engine would transmute the elements composing the rocket's structure, making the aft end of the spacecraft radioactive even if all the fissionables are removed. Spaceship designers should also construct the aft end of the spacecraft out of materials that are not only strong, but that will transmute into materials of still acceptable strength.
You will find more discussion on embarking/debarking from a radioactive rocket here.
In the interest of radiation safety, the corridor to the atomic engine room is going to have dogleg bends in it. Radiation travels in straight lines but people don't have to. This allows the crew to quickly move out of direct line of sight with the reactor. The corridor exit will have an adjacent decontamination booth.
Nope, nice try, but you'd do best to keep every single part of the ship inside the radiation shadow. Especially the heat radiators, which are huge extended structures that want lots of room. There are three reasons why:
- Neutron radiation can cause Neutron Embrittlement. Becoming brittle is not healthy for struture in general and load-bearing members in particular. Unless you have a perverse reason to want your spacecraft to snap like a twig when you goose the rockets.
- Neutron radiation can cause Neutron Activation. Having components of the spacecraft transmuted into radioactive isotopes is a health hazard. Especially since the isotopes will not be behind a radiation shield. They will be free to spray the habitat module with deadly radiation.
- Spacecraft strutures protruding outside of the radiation shadow can cause "backscatter", bouncing deadly radiation around the shadow shield and irradiating the habitat module. This is one reason why nuclear powered aircraft never caught on, the very atmosphere itself would cause backscatter.
In the first diagram below, note how the lower part of the external propellant tanks are cut at an angle so they do not stick outside of the shadow (the "half-cone sections"). In the other diagrams, note how the square heat radiators are trimmed to a triangular profile when they are near the shadow shield. This also gives the viewer an indication of the outline of the (otherwise invisible) radiation shadow.
Hull armor is specifically to protect the ship and crew from the natural radiation from space, and from hostile weapons fire.
Different kinds of armor are required for different kinds of ionizing radiation: particle radiation or electromagnetic radiation. Neutron, cosmic rays, solar protons and the like are particle radiation (because they are subatomic particles). X-rays and gamma-rays are electromagnetic radiation. Particle shielding is generally something with lots of hydrogen in it, like water, liquid hydrogen propellant tanks, lithium hydride, paraffin or a hydrogenated polyethylene composite. X-ray/gamma-ray shielding is generally something very very dense, like lead, tungsten, or an alloy with a lot of heavy elements in it.
The hull armor will be arranged differently than than shadow shield.
First off, the armor is probably only going to be on the habitat module, and any radiation-sensitive equipment. It is not going to be over the entire spacecraft.
Secondly, unlike a reactor, cosmic rays and solar storms contain charged particles, mostly protons. Charged particles can create "Bremsstrahlung" or braking radiation. (Keep in mind that the hull of the spacecraft will probably never encounter natural gamma rays in the space environment. Gamma rays will probably only come from artificial sources, such as nuclear weapons.)
You see, gamma shielding is worse than useless against charged particle radiation. Such particles striking lead actually creates deadly x-rays, making the radiation problem much worse (the same principle is used in a doctor's x-ray machine). Please note that this only applies to charged particles, neutrons from the reactor do not generate Bremsstrahlung.
And please do not confuse "neutral particle beams" with "neutron particle beams." The former will produced Bremsstrahlung, the latter will not. Neutral particle beams are beams of protons and electrons (which are charged) in a neutral electrical balance. Neutron particle beams are beams of neutrons (which are uncharged).
So for the hull shielding it is best to arrange things so that the incoming radiation first encounters the paraffin to soak up all the particle radiation, then have a layer of tungsten to stop the gamma rays.
Anthony Jackson on the topic of Carbon as radiation shielding says:
This graph is from Proceeding of the Symposium on Manned Planetary Missions 1963/1964 page 92.
For solar proton storms occuring during missions lasting from 300 to 700 days, the graph shows the radiation dosage the crew will receive to their skin given aluminum shield weight. The dose to the crew's blood forming organs will be roughly half the skin dose.
The curves are for the probability of exceeding the listed radiation dosage, probabilities of 0.001, 0.01, and 0.1 (i.e., one in a thousand, one in a hundred, and one in ten).
For example, say you were concerned with the crew having a skin dose over 103 rads (10 grays) and the mission was 700 days. Find 103 on the vertical scale on the left. Look at the three curves: 700 days at P=0.001, 700 days at P=0.01, and 700 days at P=0.1
You draw a horizonal line starting at 103, and draw a vertical line where it hits each of the three 700 day lines. Here it makes vertical red, gold, and green lines. See where the vertical lines hit the bottom scale.
The red line says that if the shielding is 3 gm/cm2 of aluminum, there will be a one in ten chance that the crew will receive a skin dose of over 103 rads on a 700 day mission. The gold says 10 gm/cm2 will only have a one in a hundred chance, and the green says 17 gm/cm2 will only have a one in a thousand chance.
Anti-weapon armor (lasers and kinetic energy weapons) is discussed here.
Storm cellars are specifically to protect the ship and crew from the natural radiation from space, specifically when the radiation suddenly increases. Much like people take shelter in a conventional storm cellar when a tornado suddenly appears. In NASA-speak a storm cellar is called a "biowell". "Sudden increases" in radiation usually means a solar storm, though occasionally it means an unavoidable pass through a planetary radiation belt.
There are also storm cellars in Orion nuclear pulse driven spacecraft, since detonating hundreds of nuclear devices for propulsion will also cause a sudden increase in enviromental radioactivity.
A storm cellar is a radiation-shielded room near the ship's center, barely large enough for the entire crew. If it can be located in the middle of dense things, like fuel tanks or cargo, so much the better. The shielding is generally a material that contains lots of hydrogen since storm cellars typically protect against particle radiation, not x or gamma rays. NASA is currently working on a new shielding material, a hydrogenated polyethylene composite. Not only is it a better shield than aluminum, it has less mass as well.
Cellars might use an as yet un-invented magnetic anti-radiation field. Such fields are currently science fiction, and in any event will only provide protection against charged particle radiation, not x or gamma rays (for that you'll need an honest-to-Doc-Smith force field or ray screen). Keep in mind that almost all natural radiation hazards are charged particle, x and gamma rays generally come from human sources (such as poorly shielded fission reactors and nuclear weapons).
To protect against a significant solar storm, the shielding on the biowell should be at least 500 grams per square centimeter (5,000 kg/m2). This will give good protection against neutrons as well.
I have seen a couple of designs for Mars missions wit solid core nuclear thermal rockets try to make a poor-man's storm cellar by aiming the rocket's shadow shield at Sol and hoping it stops enough solar storm radiation so the crew doesn't die. I am trying to find some figure on how effective this would be, but it sure looks like a dangerous mass-cutting short cut to me.
A storm cellar surrounded by water tanks can be found in John Campbell's THE ULTIMATE WEAPON, Robert Heinlein's PODKAYNE OF MARS and Lee Correy's (AKA G. Harry Stine) SPACE DOCTOR. Both Heinlein and Stine call the cellar a "caisson" or "cofferdam". A caisson is actually a pressurized working area surrounded by water that is used when building the submerged pylons of a bridge, but I suppose the description is whimsically close enough to a spacecraft storm cellar.
The crew will occupy the cellar when the sun kicks up a solar storm of radiation. As these can last for days, one had better include a few crew-days worth of food, water, and tranquilizers. And a porta-potty. If you are relying upon algae for your oxygen, it deserves space in the storm cellar as well. This probably also applies to stored food too. I have heard that particle radiation can destroy a lot of the vitamins in food, especially pyridoxine and thiamine. Alas, computers and other digital electronics are also vulnerable to radiation. Don't forget repeaters for the gauges on the major ship systems, and one monitoring radiation levels outside the cellar. The latter tells you when it is safe to come out. The former tells you if there is a critical failure outside, meaning it is time to start drawing straws to decide who gets to heroically commit suicide by saving the ship. After the storm the crew can emerge and go check the dosimeters they thoughtfully left in the modules of the spacecraft vulnerable to radiation.
As a matter of fact, in many early NASA designs for Mars missions, the storm cellar is also the control room. Don't just have repeaters in the cellar, have all the spacecraft controls. It is a really bad idea to leave the control room unmanned, and the crew will be reluctant to expose themselves to an agonizing radioactive death because the blasted control room ain't shielded. Yes the control room will be a bit crowded when the sun raises up a ruckus, but you can't have everything.
To protect against galactic cosmic radiation and solar proton storms, lots of mass is required. A spacecraft has to carry its own shielding. But a planetary base can use regolith as shielding, i.e., bury the base by shoveling tons of the readily available local dirt over it. Alternatively the base can be located in artificial or naturally occurring caves and tunnels deep underground. This explains NASA's interest in Lunar and Martian lava tubes.
It is estimated that Lunar lava tubes can have a diameter of up to 300 meters and lying under 40 meters or more of basalt. In addition to protecting from galactic cosmic radiation, lava tubes will also protect against meteorites, micrometeorites, and ejecta from impacts. They will also provide a stable temperature of about -20 °C (instead of varying from -173 °C to +100 °C) and access to underground resources.
Martian lava tubes are estimated to have a roof thickness of around 30 meters. In 2010 a "skylight" (lava tube with hole in the roof) was observed in the Pavonis Mons region of Mars. The skylight was estimated to be about 190×160 meters wide and at least 115 meters deep.
Using regolith is more work, but you cannot always count on a convenient lava tube near the proposed base site.
The table below assumes that regolith has a bulk density of 1.3 grams per cubic centimeter.
to Terra Sea Level
|1,000 g/cm2||7.7 m|
|700 g/cm2||5.4 m|
In the Lunar base design below, regolith is stuffed into long bags and coiled around the dome.
Radiation shields composed of matter are quite massive, and Every Gram Counts. Researchers have been looking into using magnetic and electrostatic fields to protect against particle radiation, since such fields have no mass. Unfortunately the generators of such fields do have mass. And the field strength will have to be strong enough that the word "superconductor" will soon be mentioned. In addition, such powerful fields might be health hazards to the astronauts. It is worthless if the field simultaneously protects the astronaut from particle radiation, but also instantly kills them by being strong enough to straighten out all their DNA molecules.
There are some medications that can offset the harmful effects of acute radiation exposure, but there is a limit to the protection they can offer.
If there is nuclear fallout or a release of radioisotopes/fission fragments into the air, people in the area should immediately take a potassium iodide tablet. While none of the fission fragment elements are particularly healthy, Iodine-131 is particularly nasty. This is because ones thyroid gland does its level best to soak up iodine, radioactive or not. Your thyroid will quickly become saturated with deadly iodine-131 and thyroid cancer will ensue. Potassium iodide pills load one's thyroid with safe iodine, so it be sated and thus ignore any deadly iodine-131 that passes by.
Obviously potassium iodide tablets provide zero protection against any of the many other radioisotopes.
The body's hematopoietic (blood forming) tissues are seriously damaged with exposures of 1 gray or more. This is mainly the bone marrow, causing damage to blood-cell production and the immune system.
The standard treatment is Granulocyte colony-stimulating factor (kicks the surviving bone marrow into overdrive), but G-CSF must be administered as soon as possible or it does not help. A new treatment combines thrombomodulin with activated protein C, this will help if administered within 24 hours of exposure.
The gastrointestinal tract is increasingly damage with exposures above 5.5 grays. The regenerative peptide TP508 (rousalatide acetate) will significantly increase survival and delay mortality by activating stem cells (though I am curious as to the exact definition of "significantly"). TP508 may also activate stem cells in other organs besides the GI tract, helping them recover from radiation exposure as well.
The drug dimethyloxalylglycine helps protect the GI tract from radiation damage by blocking PHD proteins. It should be administered within 24 hours of exposure.
Radiation damages a cell's DNA. If a cell discovers too much DNA damage, it commits suicide (to avoid the risk of becoming cancerous). If too many cells suicide, the person dies. Unfortunately the mechanism is set too conservatively, it will kill the cell even if the damage is slight enough to be repairable.
The drug 2-[4-(1,3-dioxo-1H,3H-benzoisoquinolin-2-yl)butylsulfamoyl]benzoic acid (mercifully abbreviated to DBIBB) delays cell suicide and speeds up DNA repair, giving the cells a fighting chance to heal themselves.
Tardigrade are microscopic animals that do not grow larger than 1.5 millimeters or so. Ordinarily they would be very forgettable creatures, were it not for the disconcerting fact that the blasted things are almost indestrutable, or at very least invincible.
They can withstand pressure of 6,000 atmospheres (about six times the water pressure at the bottom of the Mariana trench). They can withstand the zero pressure of outer space. They can survive a temperature of −20° C for about thirty years. They can survive a temperature of 151° C for a few minutes. They can officially survive being dehydrated for ten years, though there was one report of a 120-year-old dehydrated specimen waving one of its arms.
One wonders if laminated tardigrades would make good combat armor.
But more to the point, those little adamantine monsters can withstand 1,000 times more radiation than other animals. 10 grays is certain death for a human being. The median lethal dose (LD50) for a tardigrade is 5,000 freaking grays of gamma rays or 6,200 freaking grays of heavy ions. This means you could irradiate a bunch of tardigrades with sixty times the radiation that would instantly put a human into a coma and kill them in 24 hours and half of the blasted tardigrades would survive!
Naturally scientists were interested in [a] how the heck do they do this? and [b] can we teach humans to do this as well?
Scientist Takuma Hashimoto et al figured it out, and published their results in a paper Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein
Using tandem mass spectrometry they discovered a previously unknown protein that they gave the boring name of Damage suppressor (Dsup). The stuff stays inside the nuclei of tardigrade cells and apparently wraps itself around the nuclear DNA.
Using standard HEK 293 cells as experimental vectors, the researchers did genetic engineering to give the experimental cells the gene for Dsup. Then they subjected the cells to 10 grays of radiation. The Dsup cells had only 48% of the single-strand break radiation damage, 40% of the double strand break radiation damage, and only 25% of the reactive oxygen species radiation damage; as compared to ordinary HEK 293 cells. Which is an amazing increase in radiation resistance, expecially just from a single new stupid protein.
The bad news is that apparently the only way to obtain this radiation protection is to do genetic engineering on the astronaut's cells. Which has quite a few ethical problems. But study of the Dsup protein will eventually reveal the mechanism of its protection, and may lead to radiation protection that is a bit less invasive than mutating your cells.
Dsup is probably not the only anti-radiation measure in the tardigrade's genome. For example, its genome contains more copies of an anti-oxidant enzyme and a DNA-repair gene than any other animal.
This section is sort of a case study on the many ways atomic radiation impacts on atomic engine design. This is taken from Radiation Effects on a Nuclear Rocket Engine Systems (1961)
The report is about the Kiwi-B2 experient of the Los Alamos Scientific Laboratory. It goes into great detail on the design challenges created by the radiation enivronment. The report details what it would take to alter the Kiwi-B2 to create a practical engine.
Obviously the radiation affects everything. Neutron heating and Gamma heating puts thermal stress on the engine components, structure, and propellant tanks. Neutron embrittlement damages sensitive materials and electronic equipment. Ionizing radiation disrupts electrical systems. The grim spectre of neutron activation will transmute once-safe engine components into radioactive isotopes spitting death in all directions. This will present a severe radiation hazard to the crew tasked with maintenance, disassembly, and inspection.
If that wasn't enough, allowances must be made to ensure the blasted engine will actually operate in the space environment. Passing tests on the ground with flying colors is no guarantee it will work in space. On the ground there is an atmosphere and the ground. Both can backscatter neutrons back into the reactor core, acting like additional neutron moderators that will not be present in space. Real embarrassing if the reactor won't react once you get it into orbit. Plus the atmosphere will give additional convective cooling to the reactor, which will also be absent in space. Melting in orbit is even more embarrassing.
The priority focus of the report is trying to optimize the balance between "reactor — tank" separation and radiation shield weight. Because the rest of the design hinges on it. Remember that radiation shields have outrageous high weights but are compact. Distance is anything but compact. In theory distance has no mass, but in reality you at least need a structural spine whose mass increases with distance. By using graphs one can determine the sweet spot: that perfect balance of shield mass and distance mass which adds the lowest additional mass for a given radiation protection level.
Secondary focus was on specifying components such that they were light-weight, resistant to radiation damage, and resistant to neutron activation.
As per the design parameters the engine assumes the use of a LASL Kiwi-B3 reactor, a Rocketdyne Mark 9 pump + turbine, and a modified Kiwi B nozzle. The diagrams above show the nozzle with lines for expansion ratios of 50:1, 75:1, and 100:1.
The turbines can be energized in any of three different ways
- Generator: turbine driven by hot gases created in a separate gas generator
- Bleed Cycle: turbine driven by hot gasses tapped from the reactor exhaust
- Topping Cycle: turbine driven by hot gasses created by pumping stolen cold propellant through special heat exhanger pipes passing through nozzle and reactor
Kiwi-B2 design chose to use Bleed Cycle because it has a performance advantage over the gas generator method, and because the topping cycle lacks over-all engine flexibility.
The reactor power is 1500 megawatts, propellant mass flow through reactor is 83 lb/sec, chamber pressure is 816 psia, and propellant temperature at reactor outlest is 4500° R.
In the design process it became glaringly obvious that the propellant tank was going to need protection from reactor radiation. Or the propellant would start violently boiling and the tank would explode, this is a bad thing. This is the reason for the focus on balancing the distance separation with the radiation shield weight: if the boiling is a show-stopper you must prevent it, but every gram counts.
Radiation Shadow Shield
True, the primary job of the radiation shield is to prevent lethal radiation from destroying the ship and killing the crew. But there are other concerns besides how much radiation they stop.
The shield should be as light as possible because every gram counts. It should be strong enough to withstand rocket acceleration, requiring additional supporting structure is adding more grams. Since it is stopping radiation it is going to grow hot, so it will need adequate cooling. This also means it has to be constructed of materials that have good heat transfer properties and high-temperature strength.
Obviously it will have to stop both neutrons and gamma rays. Just stopping one type will be a deadly mistake.
An important point is it should not reflect neutrons. Otherwise it will increasing the amount of free neutrons in the core, and could cause a runaway reaction.
In the report, they figured that the best shield material that fulfilled all the requirements was boronated graphite.
Balancing "Reactor — Tank" Separation with Radiation Shield Weight
In the three graphs below, Reactor — Tank Separation is displayed on the bottom scale ( abscissa ) and Radiation Shield Weight is indirectly displayed at the "Attenuation Factor" curves (the attenuation implies the thickness of the radiation shield). The side scale (ordinate) is the system weight change. The idea is to find the lowest value for system weight change.
There are three graphs because there are three mathematical propellant flow models: Potential Flow Model, Completely Mixed Flow Model, and Recirulation Flow Model.
The minimum system weights for each of the three models is:
which you can see is the lowest curve point of all the curves, in the three graphs below.
The report is vague, but I gather they went with figure 5: separation distance of 17 and attenuation factor of 6. These were used in the radiation flux plot below. Figure 3 was thrown out because an attenuation factor of 2 is not enough to prevent severe radiation damage to the engine and crew.
Radiation at the "top" has been attenuated by a factor of 6 by using a 14-inch 20% void boronated graphite shadow shield. This protects the crew, but also the feed system and propellant tank (and the propellant, if it starts boiling that will be very bad).
The Kiwi test engine included some explosive devices for emergency break-up and scattering of the reactor core. Alas the reliability of these drastically degrades as they are exposed to gamma radiation (neutron radiation has little effect because explosives have relatively small numbers of hydrogen atoms). There are two radiation effects. First the explosive force is diminished as the radiation breaks down the molecular structure. This can be measure by the amount of gas evolved by a unit dose of radiation, to allow selection of the most resistant explosive type. Second, the radiation can make the explosive unstable and sensitive, so it might "predetonate" (explode whenever it damn well pleases instead of waiting for you to push the button).
A glance at the graph shows that the obvious winner in the explosives contest is good ol' TNT. Its gas evolved line is practically flat, meaning that the radiation has little effect. The maximum dose at the horizontal centerline of the engine is about 1011 erg gm-1 (C) (i.e., way off the right edge of the graph) but the TNT line is flat enough that it seems safe to extrapolate.
The various controls, tongs, and remote control "waldoes" will reach around or penetrate the anti-radiation shadow shield, and there may be auxiliary lead baffles. Peeking around the baffles is how Rhysling lost his sight in Heinlein's "The Green Hills of Earth".
Remember that the shadow shield will be in the floor, with the engine below that. Closed-circuit TV monitor will help Astro see what he is doing, but if they are damaged, he'll have to make do with mirrors and/or doing it by touch. What he really needs is one of Tom Swift Jr.'s Giant Robots, which were designed to do maintenance inside nuclear power plants. There is more about robots here.
For external repairs, the chief engineer might use something similar to the amazing Canadarm 2, which is currently on active duty on the International Space Station. Unlike the first Canadarm, this one is not attached at either end. Instead, either end can plug into special sockets ("power data grapple fixtures") built at strategic spots on the surface of the station. Canadarm 2 can literally walk on the surface of the station to where it is needed, moving end-over-end like a giant metal inch worm. The main limitation is that each "step" must end at a socket, but this is due to power and control signal issues. A more advanced version might be self contained enough to not require sockets, just hand-holds or other protrusions that it could grab.
Canadarm 2 is quite large, 17.6 meters (57.7 feet) long when fully extended. It can move payloads with a mass up to 116 metric tons.
On your atomic rocket, one would use arm(s) long enough to reach any spot on the radioactive engine.
Refueling and maintenance on radioactive spacecraft out on a landing pad will need something with lots of waldoes and probably treads. In olden days (the 1950's) they figured these things would be controlled by men inside lead-lined control cabs, using TV cameras. Nowadays it would make more sense for the vehicles to be remotely controlled drones.
In the old Tom Corbett Space Caded novels, such vehicles were called "hot soup wagons", because the spacecraft in the novels used liquid core nuclear thermal rocket propulsion. Though in reality I doubt that a landed rocket would keep the plutonium fuel molten just for ease of pumping it into the wagon.
One of the more interesting examples of a hot soup wagon is the Beetle. It was built in 1961 by Jered Industries on contract for General Electric's Nuclear Materials and Propulsion Operation division. It was going to be used in the US Air Force Special Weapons Center to service and maintain a planned fleet of nuclear powered Air Force bombers. The bombers were never constructed and Beetle was scrapped.
You can find all the details in the report AD0402748 USAF Shielded Cab Vehicles Test And Evaluation. This includes the layout of all 120-odd buttons on the control panels, which I didn't bother to include.
On the Beetle, the engine and transmission are located in the front of the chassis, while the operator cab and manipulators are mounted on the rear. The cab walls are solid lead 12 inches thick, clad with a one inch steel shell on the outside and a 0.5 inch steel shell on the inside. The five operator windows are 23.25 inches thick made out of seven panes of leaded glass (same radiation shielding level as 12 inches of solid lead). The cab can lift up to a height of 26 feet off the ground since nuclear bombers are quite tall. Each arm can lift 2,000 pounds yet are delicate enough to pick up an egg without breaking it.
|Height - cab down||11' 7"|
|Height - cab up||26' 7"|
|Ground Pressure||35 lbs/in2|
|Rotates||360° at 0.8 rpm|
|Lead wall thickness||12"|
|Hatch opens||1 minute|
|0% grade forward||8 mph|
|0% grade reverse||5 mph|
|10% grade both||5 mph|
|Max weight lift|
|Max weight lift|
|Flood light||250 foot-candles at 15'|
|-30°F to 130°F|
|Drawbar pull||85,000 lbs|