Fusion Fuel

Fission weapons (aka "atomic bombs") did bring an end to World War II, but nuclear scientists did not rest there. The second way to use nuclear physics to release vast quantities of energy is by nuclear fusion. By 1951 the first fusion weapon had been designed, the Teller-Ulam thermonuclear weapon (aka "h-bomb"). Fission was now old-hat, fusion was tapping the same source of power as the freaking sun. It was the energy of the future.

And as year overtakes year, fusion power remains the "energy of the future", it never becomes the energy of today. As with most things it is far more difficult to do something constructive than to do something destructive. Scientists all over the world have been trying to develop fusion power since the 1950's, and they are still far away from the "break-even" point (where they actually get more energy out of the fusion reactor than they put in to kick start it). They keep working on it, though, because the benefits are huge. You get more energy from a gram of fuel, there is no chance of a runaway reaction (it is hard enough just to keep the reaction running), no chance of large-scale releases of radioactivity, little or no atmospheric pollution, the fuel is mostly harmless light elements in small quantities, waste has only short-lived radioactivity, and it does not produce weapons grade plutonium as a by-product.

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  Discovery II fusion reactor and engine.

There are two basic operations possible in the universe, analysis and synthesis. That is, breaking one large object into smaller parts, or assembly smaller parts into one larger object. The ancients called this "solve et coagula." With fission, you take one large unstable atom and break it into fission fragments (aka "split the atom"). With fusion, you take two or more small atoms and fuse them into one larger atom.

In both cases, when you weigh the things you start with and weigh the result, you will find the result weighs less. This is know as the binding energy mass defect. It represents the amount of matter that is turned into energy. Everybody knows that e = mc², but unless you've had a physics class you may not know that c (the speed of light in a vacuum) is a mind-boggling huge number, and squaring a mind-boggling huge number makes it astronomically huger. Bottom line is that microscopic amounts of matter create titanic amounts of energy.

The conversion is 1 atomic mass unit = 931.494028(±0.000023) MeV.

As a side note, fission and radioactive decay makes atoms become smaller atoms, until the atoms become atoms of lead, where they are stable (i.e., they do not decay or otherwise undergo fission). Fusion, on the other hand, releases energy as you fuse larger and larger atoms, until the atoms grow such that they are atoms of iron. After than, fusing heavier atoms actually consumes energy instead of releasing it.

Golden-aged science fiction authors E.E."Doc" Smith and John W. Campbell jr. noted this and postulated space-opera science that required elements in the middle of the periodic table for direct conversion of all the mass into energy. In Doc Smith's "Skylark" series the element was copper and in John Campbell's The Space Beyond the element was iron. But I digress.

The Particles table gives the symbols of the various fusion fuels. The particle mass is given if you want to amuse yourself by calculating the binding energy mass defect of various reactions.

Tritium is annoying since it has a fast half-life of only 12.32 years; e.g., after about twelve years half of your tritium has decayed into Helium-3. Use it or lose it. This is why there are no tritium mines. Most reactor designs that use tritium incorporate a tritium breeder. This usually takes the form of a tank of liquid lithium surrounding the reactor, sucking up the neutrons and transmuting the lithium into fresh tritium and helium-4.

The infamous Helium-3 is often touted as an economic motive for space industrialization, unfortunately it is not a very good one. There are no Helium-3 mines on Terra, so it is hard to obtain. Space enthusiasts trumpet the fact that there are helium-3 deposits on the moon that can be mined, but they don't mention that it is in a very low concentration. You have to process over 100 million tons of Lunar regolith to obtain one lousy ton of helium 3.

It is possible to manufacture the stuff, but it takes lots of neutrons. Basically you breed tritium and wait for it to decay. There is lots of helium-3 available in the atmosphere of Saturn and Uranus, if your space infrastructure is up to the task of traveling that far from Terra. Helium-3 concentration is estimated at about 10 parts per million, which beats the heck out of Luna. Jupiter has helium-3 as well, but its steep gravity well makes it uneconomical to harvest.

There are many fusion reactions, but only a few are suitable for use as power sources or rocket fuels. There are lots of limitations that you can read about here. Of the candidates, you want to use those with low Lawson criterion, which measures how hard it is to start and maintain the reaction. It is a plus if the reaction only produces charged particles, since these can be turned into electricity directly, instead of having to be converted into heat first.

Finally it is a plus if the reaction does not release neutrons, because they are not only dangerous radiation, but they have the nasty habit of weakening engine parts ("Neutron embrittlement"), and transmuting engine parts into radioactive elements ("Neutron activation"). Unless you are using the neutrons to breed tritium.

The D + ³He reaction is of particular interest for rocket propulsion, since all the products are charged particles. This means the they can be directed by a magnetic field exhaust nozzle.

The Reaction table displays the various fusion reactions that look promising for power plants and spacecraft. Note that the Deuterium + Deuterium reaction has two possible outcomes and thus two rows in the table. Each outcome has about a 50% chance of occurring. The two lithium reactions are not power reactions, they are the tritium breeding methods mentioned above.

The Reaction column shows the starting fuels and the end products of each reaction (for example the first row shows that fusing one nuclei of deuterium with one nuclei of tritium results in one helium-4 nuclei, one neutron, and 17.6 MeV of energy). The MeV/fusion column shows the energy generated by one reaction in mega-electron-volts (as per the equation above). The TJ/kg column shows how many terajoules are produced by burning one kilogram of the fuel mix (I calculated this, so use at your own risk). The 1000 MW burn column is how many grams of nuclear fuel must be totally burnt (fusioned) each second to produces 1000 megawatts of thermal energy (I calculated this as well, treat this also as suspect). The L-C column shows the Lawson criterion value for that reaction. The A-N column shows if the reaction produces neutrons or not (Aneutronic), please note that even if there is no n symbol in the results column neutrons can still be produced by side-reactions. And the Exhaust column gives the theoretical maximum rocket exhaust velocity as a percentage of the speed of light. Equations for calculating exhaust velocity can be found here.

The Deuterium-Tritium reaction is easy to ignite (low Lawson criterion), but it uses that pesky decaying tritium. Hydrogen-Boron (a proton is an ionized hydrogen atom) has the advantage of being aneutronic, but is very difficult to ignite, with a whopping Lawson criterion of 500! Helium-3+Helium-3 is also aneutronic, but helium-3 is hard to come by. Which is probably why I could not find any source quoting its Lawson criterion.

Proton-proton fusion is what the Sun uses, and what Bussard Ramjets would like to use. Four protons fuse to create an atom of helium-4 and 26.73 MeV of energy. Trouble is that the Lawson criterion is off the top of the chart. Trying to get four protons to simultaneously fuse is almost impossible, short of using an actual star.

The two entries for neutron-Lithium are not for power generation, those are the reactions for breeding Tritium.