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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.
Types of Radiation
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. 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.
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, and particulate radiation like protons, neutrons, electron, or alpha particles.
In the context of this website, you will be dealing with radiation in several areas.
- Astronauts traveling from planet to planet are exposed to the natural radiation of space. This is generally always particle radiation, and the exposure time is "chronic" (see below). This radiation comes from galactic cosmic rays, solar storms, and spending to much time in the Van Allen radiation belts. 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.
- If the spacecraft uses a nuclear propulsion system, or has a nuclear power reactor, these are also sources of both electromagnetic and particle radiation. Again the exposure time is "chronic." 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.
- 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, while obviously particle beam weapons only emit particles.
From NASA-STD-3000 Man-Systems Integration Standards. (ed note: for the most part, almost all naturaly occuring ionizing radiation is particle radiation. One generally only encounters gamma rays and x-rays from artificial sources, such as nuclear reactors and nuclear weapons. Naturally any NASA document will be silent on weapons.)
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 220.127.116.11.1-1 summarizes the main types of ionizing radiation including their charge, mass, and source.
Figure 18.104.22.168.1-1 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.
Here is a good overivew of naturally occuring sources of space radiation.
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.
Effects of Radiation
The effect of radiation upon the crew is rather complicated (translation: I don't understand it) so take the following explanation under advisement.
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 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.
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.
Sieverts are determined from Grays. 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 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.
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)
Ignoring all the warning broadcasts, Floyd the dufus meteor-miner is poaching on an asteroid in the Trojan asteroid cluster claimed by the Jupiter-Equilateral corporation. 30 kilometers away (30,000 meters), the unaware local JE mining supervisor detonates a one kiloton nuclear mining charge on the surface of 4805 Asteropaios. Unfortunately for Floyd, there is a clear line of sight between him and the detonation.
Floyd will live, assuming that he can quickly get into a pressurized environment before his space helmet fills up with vomit.
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 rule of thumb, 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 factor, Q|
|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|
There are two kinds of radiation exposure: acute and chronic. 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.
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.
The point is, if an unfortunate person received a radiation dose of 1 Sievert over the course of 20 years, they will NOT suffer from radiation poisoning. But an unfortunate person who received a radiation dose of 1 Sievert over the course of five minutes certainly will.
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".
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.
Acute Radiation Syndrome Chart
|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 Sv, but occur consistently only above 0.50 Sv. 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 Sv 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 Sv, an exposure of 0.80 Sv has a 50% chance of causing temporary sterility in males. At 0.75 Sv 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 Sv, and become common at 2 Sv. Typical effects are mild to moderate nausea (50% probability at 2 Sv) , 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 Sv), malaise, and fatigue (50% probability at 2 Sv), 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 Sv and 100% at 3 Sv. 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 Sv), malaise, fatigue, diarrhea (50% prob. at 3.5 Sv), and hemorrhage (uncontrolled bleeding) of the mouth, subcutaneous tissue and kidney (50% prob. at 4 Sv). Suppression of white blood cells is severe, susceptibility to infection becomes serious. At 3 Sv 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 Sv (LD50) to 90% at 6 Sv (unless heroic medical intervention takes place). Hematopoietic tissues remain the major affected organ system. The symptoms listed for 2.0-3.5 Sv increase in prevalence and severity, reaching 100% occurrence at 6 Sv. 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 Sv 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 Sv 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|
Chronic Radiation Syndrome Chart
|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, minor: 0.005 Sieverts||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|
For a pregnant woman it is 0.005 Sievert total for the duration of the pregnancy.
Banana Equivalent Dose
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.
Electronics are also vulnerable to radiation (including particle beam weapons) due to mechanical disruption. You can find more details in this PDF document. Radiation damage can be combated by developing self-healing computers.
Modern rad-hardened electronics can survive a few hundred to a few thousand grays, and will generally continue functioning until destroyed; non-hardened electronics won't handle even one gray very well, and will crash more or less instantly. In general, more advanced chips, because they have smaller circuits, are more vulnerable to radiation than more primitive designs.
(ed note: here are some samples of rad-hard systems. The S950 is good for over 350 grays and uses a processor released in 2002.)
Subject: Re: radiation and computers From: firstname.lastname@example.org (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, email@example.com (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 x 10^12 Y/R^2 where Y is in kilotons, R is in km, and Fn is in neutron/cm2. Dose (Dg) from prompt radiation of an explosion is approximately: Dg = 4 x 10^5 Y^(2/3)/R^2 where Y is in kilotons, R is in km, and Dg is in rads (Si). Damage Thresholds (gleaned from the text):
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 10^3-10^4 rads 10^5-10^6 Dose-Rate Upset 10^6-10^8 rads(Si)/s >10^9 rads(Si)/s Dose-Rate Induced Latchup 10^7-10^9 rads(Si)/s >10^12 rads(Si)/s Neutrons 10^11-10^13 n/cm2 10^14-10^15 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".
Transistors 10^11-10^15 neutron/cm2 MOS Transisitors 10^4 rads (silicon) Capacitors 10^15 neutrons/cm2 Precision Resistors 10^7 rads (carbon)/s 10^14 neutron/cm2 NiCd Batteries 10^7 rads (air)/s 10^13 neutron/cm2 Hg Batteries 10^16 neutron/cm2 Wiring Insulation: Silicon Rubber 2x10^15 neutron/cm2 Polyethylene 10^7 rads (carbon) Teflon TFE 10^4 rads (carbon) Teflon FEB 2x10^6 rads (carbon) Polyolefins 5x10^9 rads (carbon)
"Wrecked. Null-gee and high radiation. I'll have to put you in the safe for a while." Deston shoved the oldster into a small room, gave him a line, and turned to Barbara. "My tell-tale reads twenty-pink - so we've got a few minutes...
...He glanced at his telltale. Thirty two. High red. Time to go...
...In the lifecraft he closed the port, cut in the launcher, and slammed on a one-gravity drive away from the ship. Then he shucked Barbara out of her suit and shed his own. He unclamped a fire-extinguisher-like affair; opened the door of a tiny room. "In here!" He shut the door behind them. "Strip, quick!" He cradled the device and opened four valves.
Fast as he was, she was naked and ready for the gush of thick, creamy foam from the multiplex nozzle. "Oh, Dekon?" she asked. "I've read about it. I rub it in good, all over me?"
"That's right. Short for 'Decontaminant, Complete; Compound, Absorbent, and Chelating; Type DCQ.' It takes care of radiation, but speed is of the essence. All over you is right." He placed the foam-gun on the floor and went vigorously to work. "Eyes, too, yes. Everywhere. Just that. And swallow six gulps of it . . . that's it. I slap a gob of it over your nose and mouth and you inhale once-hard and deep. One good one's enough, but if it isn't a good one you die of lung cancer, so I'll have to knock you out and give it to you while you're unconscious, and that isn't good - complications. So make it good and deep?"
"Will do. Good and deep." She emptied her lungs.
He put a headlock on her and slapped the Dekon on.
She inhaled, hard and deep, and went into paroxysms of coughing. He held her in his arms until the worst of it was over; but she was still coughing hard when she pulled herself away from him.
Note that Doc Smith to the contrary, chelating decontamination doesn't quite work this way.
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.
The crew is protected by a "shadow shield" between the atomic engine and Polaris' crew quarters (lead for gamma shielding, paraffin for neutron shielding). As a matter of fact, you probably want to design the ship so that everything is between the engine and the crew.
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")
The ship's structure as well should be inside the shadow. Neutrons can cause neutron embrittlement, which is not healthy for struture in general and load-bearing members in particular. In the above-right diagram, 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").
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.
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.
You also have to worry about "backscatter." Outside objects can bounce radiation around the shield. This is why nuclear powered aircraft never caught on, the very atmosphere itself would cause backscatter.
Spacecraft will have Storm Cellars where the crew can cower and wait out solar storms. However, storm cellars only have particle radiation shielding. Any warship that expects to encounter hostile nuclear warheads had better add some x-ray shielding.
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 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.
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:
|Reduction Factor||Additional cm of Lithium Hydroxide for Neutron Attenuation, (+kg/m2)||Additional cm of Tungsten for Gamma Attenuation (+kg/m2)|
|x0.5||+0.205 cm (+2.99 kg/m2)||+0.564 cm (+109 kg/m2)|
|x0.2||+0.477 cm (+6.96 kg/m2)||+1.308 cm (+252 kg/m2)|
|x0.1||+0.683 cm (+9.97 kg/m2)||+1.872 cm (+361 kg/m2)|
|x0.01||+1.365 cm (+19.92 kg/m2)||+3.744 cm (+722 kg/m2)|
|x0.001||+2.048 cm (+29.90 kg/m2)||+5.616 cm (+1084 kg/m2)|
Say a gamma attenuation factor of 0.00105 is not enough, you need 0.000525. This is a reduction factor of 0.000525 / 0.00105 = x0.5. Looking this up in the table reveals that the shield will need an additional 0.564 centimeters of tungsten, for a grand total of 2.0 + 0.564 = 2.564 centimeters. This will increase the mass of the shadow shield to 3,500 + 109 = 3609 kg/m2.
So the theory is you calculate radiation flux from the atomic engine, multiply it by the appropriate attentuation factors of the shadow shield, and see of the resulting dose is within acceptable limts.
Which brings us to the problem of calculating the radiation flux from the atomic engine. This is a bit complicated, but there is a first order approximation here.
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.
Ten minutes later he was back. "Captain," he stated darkly, "that number two jet ain't fit. The cadmium dampers are warped."
"Why tell me? Tell the Chief."
"I did, but he says they will do. He's wrong."
The captain gestured at the book. "Scratch out your name and scram. We raise ship in thirty minutes."
Rhysling looked at him, shrugged, and went below again.
It is a long climb to the Jovian planetoids; a Hawk-class clunker had to blast for three watches before going into free flight. Rhysling had the second watch. Damping was done by hand then, with a multiplying vernier and a danger gauge. When the gauge showed red, he tried to correct it -- no luck.
Jetmen don't wait; that's why they are jetmen. He slapped the emergency discover and fished at the hot stuff with the tongs. The lights went out, he went right ahead. A jetman has to know his power room the way your tongue knows the inside of your mouth.
He sneaked a quick look over the top of the lead baffle when the lights went out. The blue radioactive glow did not help him any; he jerked his head back and went on fishing by touch.
When he was done he called over the tube, "Number two jet out. And for crissake get me some light down here!"
There was light -- the emergency circuit -- but not for him. The blue radioactive glow was the last thing his optic nerve ever responded to.
* * *
Rhysling obliged, then said, "You youngsters have got it soft. Everything automatic. When I was twisting her tail you had to stay awake."
"You still have to stay awake." They fell to talking shop and Macdougal showed him the direct response damping rig which had replaced the manual vernier control which Rhysling had used. Rhysling felt out the controls and asked questions until he was familiar with the new installation. It was his conceit that he was still a jetman and that his present occupation as a troubadour was simply an expedient during one of the fusses with the company that any man could get into.
"I see you still have the old hand damping plates installed," he remarked, his agile fingers flitting over the equipment.
"All except the links. I unshipped them because they obscure the dials."
"You ought to have them shipped. You might need them."
"Oh, I don't know. I think--" Rhysling never did find out what Macdougal thought for it was at that moment the trouble tore loose. Macdougal caught it square, a blast of radioactivity that burned him down where he stood.
Rhysling sensed what had happened. Automatic reflexes of old habit came out. He slapped the discover and rang the alarm to the control room simultaneously. Then he remembered the unshipped links. He had to grope until he found them, while trying to keep as low as he could to get maximum benefit from the baffles. Nothing but the links bothered him as to location. The place was as light to him as any place could be; he knew every spot, every control, the way he knew the keys of his accordion.
"Power room! Power room! What's the alarm?"
"Stay out!" Rhysling shouted. "The place is 'hot.'" He could feel it on his face and in his bones, like desert sunshine.
The links he got into place, after cursing someone, anyone, for having failed to rack the wrench he needed. Then he commenced trying to reduce the trouble by hand. It was a long job and ticklish. Presently he decided that the jet would have to be spilled, pile and all.
First he reported. "Control!"
"Control aye aye!"
"Spilling jet three -- emergency."
"Is this Macdougal?"
"Macdougal is dead. This is Rhysling, on watch. Stand by to record."
The ship was safe now and ready to limp home shy one jet. As for himself, Rhysling was not so sure. That "sunburn" seemed sharp, he thought. He was unable to see the bright, rosy fog in which he worked but he knew it was there. He went on with the business of flushing the air out through the outer valve, repeating it several times to permit the level of radiation to drop to something a man might stand under suitable armor.
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.
Another nasty problem is "neutron embrittlement". Neutrons striking metal gradually compress 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. Embrittlement can be reversed by annealing. It might be possible to construct a reactor capable of annealing its structural members in place.
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.
"Does that give anyone a notion of why the Mayflower was assembled out in an orbit and will never ever land anywhere?"
"Too hot," somebody said.
"'Too hot' is an understatement. If the Mayftower had blasted off from Mojave space port the whole Los Angeles Borough of the City of Southern California would have been reduced to a puddle of lava and people would have been killed by radiation and heat from Bay City to Baja California. And that will give you an idea of why the shielding runs right through the ship between here and the power plant, with no way at all to get at the torch."
We had the misfortune to have Noisy Edwards along, simply because he was from the same bunk room. Now he spoke up and said, "Suppose you have to make a repair?"
"There is nothing to go wrong," explained Mr. Ortega. "The power plant has no moving parts of any sort."
Noisy wasn't satisfied. "But suppose something did go wrong, how would you fix it if you can't get at it?"
Noisy has an irritating manner at best; Mr. Ortega sounded a little impatient when he answered. "Believe me, son, even if you could get at it, you wouldn't want to. No indeed!"
"Humph!" said Noisy. "All I've got to say is, if there isn't any way to make a repair when a repair is needed, what's the use in sending engineer officers along?"
You could have heard a pin drop. Mr. Ortega turned red, but all he said was, "Why, to answer foolish questions from youngsters like yourself, I suppose." He turned to the rest of us. "Any more questions?"
Naturally nobody wanted to ask any then. He added, "I think that's enough for one session. School's out."
I told Dad about it later. He looked grim and said, "I'm afraid Chief Engineer Ortega didn't tell you the whole truth."
"In the first place there is plenty for him to do in taking care of the auxiliary machinery on this side of the shield. But it is possible to get at the torch, if necessary."
"There are certain adjustments which could conceivably have to be made in extreme emergency. In which case it would be Mr. Ortega's proud privilege to climb into a space suit, go outside and back aft, and make them."
"I mean that the assistant chief engineer would succeed to the position of chief a few minutes later. Chief engineers are very carefully chosen, Bill, and not just for their technical knowledge."
It made me feel chilly inside; I didn't like to think about it.
Different kinds of armor are required for different kinds of ionizing radiation: neutron and other particle radiation on one hand, and x-rays/gamma-rays on the other. 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 shielding is generally something very very dense, like lead or tungsten.
The hull armor will be arranged differently than than shadow shield. 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:
Carbon's decent (better than aluminum or steel, worse than hydrogen or hydrocarbon plastics) against neutrons and cosmic rays (including particle beams), and has the useful secondary property of not becoming radioactive when bombarded with such particles. It's inferior against gamma rays and electrons (electrons are not hard to shield against even with a bad material, however). Within the context of a space radiation environment, it's probably overall a good material.
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.
Extremely high energy particle beam weapons act like cosmic rays, with a TVT peaking at a whopping 100 to 300 g/cm2.
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)
PFC Floyd, on a nameless jungle planet, is surprised by a Blortch storm-trooper. Floyd jumps into a pool, dives to the bottom, and holds his nose. The Blortch unlimbers his deadly gamma-ray zap gun and caroms a bolt into the pool.
Poor Floyd has only three feet of water (91 centimeters) between him and the 100 Gray gamma-ray bolt. Water you will recall has a TVT of 61 centimeters vs gamma rays.
- Rd = 100 * 0.1^(91 / 61)
- Rd = 100 * 0.1^1.49
- Rd = 100 * 0.032
- Rd = 3.2 Grays
As Blortch storm-trooper oozes away, Floyd floats to the surface. Floyd is very ill with radiation sickness, but alive.
An absurdly powerful one-GEV particle beam weapon would have a 1/e of about 100 g/cm2. In Attack Vector: Tactical one armor layer is 5 g/cm2. If a warship had 14 layers of armor, a particle beam strike would be reduced by:
- Rd = 1 * 0.37^((5 * 14) / 100)
- Rd = 1 * 0.37^(70 / 100)
- Rd = 1 * 0.37^0.7
- Rd = 1 * 0.5
- Rd = 0.5
in other words, by one-half.
A one kiloton nuclear warhead from the Asteroid Revolutionary Navy goes off fifteen kilometer from the scoutship Peek-a-Boo. At fifteen klicks, the radiation flux will be about 1500 joules per square meter. 90% is x-rays and the rest is neutrons, or 1350 j/m2 x-rays and 150 j/m2 neutrons. The Peek-a-Boo has 5 centimeters of lead around the crew compartment, which is one TVT vs x-rays and zero TVT vs neutrons. Only 135 j/m2 of the x-rays penetrate, but the entire 150 j/m2 neutrons comes sailing on through.
Floyd is the only crewmember. He's about 68 kilos, and 168 centimeters tall, which gives him a surface area of about 1.78 square meters. Figure that Floyd's cross section is one quarter of his surface area, or 0.445 m2. He will intercept 60 joules of x-rays and 67 joules of neutrons. Dividing by his mass we discover that Floyd has been exposed to 0.88 Grays of x-rays and 0.98 Grays of neutrons. This makes a grand total of 1.86 Grays. He will start upchucking his lunch in a few hours but he'll live.
Philip Eklund points out that 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.
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.
The ship will need a "storm cellar" (a "biowell" in NASA speak). This 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, or it can be 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). 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.
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.
The SFO satellites - Solar Flare Observatories - running unmanned in a different part of geosynchronous orbit have detected activity that normally precedes solar flares. So we've been operating in the Flare Watch mode. If we go to Flare Alert, we'll have no more than ten minutes to get into P-suits and make it to the caisson, our storm cellar.
Seven hex modules in a circular honeycomb make up the caisson. It's surrounded by the water tanks of GEO Base: twelve hex modules nested in honeycomb around the caisson. There are seven half-hex modules on each end of the caisson, and they're water tanks, too. We've never had to worry about water in GEO Base for several reasons, not the least of which is the fact that every human in the station puts about seven quarts of water per day into the system through metabolism and urine. However, that's only a small part of GEO Base water supply, which amounts to over 3700 tons of water. We'll never use that much in GEO Base. It's here for the primary purpose of radiation shielding.
Dan hills had once asked me to check his figures on the mass of water required to knock down the radiation of a solar flare to something that would give a person in GEO Base less than a twenty-five rem (0.25 Sievert) exposure from the largest solar flare recorded to date - the one of 23 February 1956. Well, if we get another one like that, we'll all have to be shipped back to Earth ... and that will be the end of our space traveling. However, the average solar flare will give us less than a single rem inside that caisson.
Dan's a bright one. All the pre-Eden studies of space habitats assumed extraterrestrial materials for shielding. We aren't that far along. But we always need water, and water is handy to have around to break into hydrogen and oxygen for propellants. It's nontoxic and easy to transport. Only problem is that it weighs eight pounds per gallon. But it's easily moved around by piping and pumps.
My med team has its own duties inside the caissons if the alarm goes off. We all wear dosimeters during the Watch. In the course of the emergency, we'll be prepared to handle radiation sickness, although there isn't much we can do if a guy shows up with more than four hundred rems (4 Sieverts) in him...
..."Flare Alert! Flare Alert! All personnel to the caisson! Nine minutes and counting!...
...Tons of water were being pumped by computer control into the tankage surrounding the caisson.
They were on station with two minutes to spare.
It was a human sardine can.
P-suited figures were stacked in honeycomb cubicles that were just big enough to hold a single person in a P-suit - a volume of thirty-six cubic feet. The med team wasn't much better off, except that they were in the middle of the honeycomb and able to move anywhere in the caisson for medical purposes. And they had the luxury of a tiny emergency surgical volume not much larger than Tom's sleeping quarters.
Caisson stewards, chosen not only for their ability to keep cool but also because they were big and brawny, moved quickly among the hundreds of people stacking up like cord wood. They shifted P-suit supply hoses from backpacks to the caisson supply system and plugged the comm system into each person...
..."These usually don't last much more than twenty-four hours," Fred volunteered.
"All personnel, this is Base Engineer Pratt.", the big man's voice boomed through their individual helmet loudspeakers. "Relax. We'll know in a few minutes if everybody made it. Sorry we don't have channels enough to allow you to talk to one another. There's music on Channel B. If you're short on sack time, I'd suggest you use this period to catch up. Under all circumstances, stay quiet and keep your activity to a complete minimum; we have limited life-support oxygen and regeneration in flare emergency mode. If you don't cool it, I guarantee one of the stewards will be around with something to send you beddy-by fast. If you're in trouble, press your call button. If you didn't get a chance to hook up your urine and fecal bags, do it now while we've still got pressure in here. If we happen to lose pressure for some reason, and if you're not hooked up, you'll just have to stew in your own juices."
There was a pause, then he went on. "This is a Class One flare. Solar protons should peak in five hours. With luck, we'll be out of here in fifteen hours. Sorry about the lack of room, but better you're alive in thirty-six cubic feet than dead with all of space to roam in. Hang in there!"