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This is not something I've looked into extensively, but my understanding is that D-T fusion is the easiet type of fusion to achieve by a retarded margin.  If I'm understanding this equation correctly, the energy that the nuclei have to overcome in order to fuse is a function of the product of their atomic numbers.  Add to this that D-T fusion is four times punchier than D-D fusion, and I've read that it has far lower bremsstrahlung losses than other fusion plasmas.


Come to think of it, I'm not sure if that was "far lower bremsstrahlung losses than other plasmas" or "far lower bremsstrahlung losses relative to energy density than other plasmas."


D-D fusion seems like less of a pain in the ass than D-T fusion for commercial power production (you know, assuming you can ever get that to work) because less of the energy is released as insanely fast moving neutrons.  About 80% of the energy from D-T fusion is released as neutron kinetic energy, and the neutrons from D-T fusion are about seven times more energetic than those released by nuclear fission.  This strikes me as a large practical barrier to practical commercial fusion power.  In D-D fusion neutrons are only carrying about 33% of the energy, and they're about as energetic as the neutrons released by fission.  D-D fusion also produces half as many neutrons per mol compared to D-T fusion.


If you want to go the other direction, and make lots of neutrons, T-T fusion sounds absolutely hilarious for bombs, since it will produce a 65% increase in neutrons per gram of fusion fuel over D-T (possibly more in practice, due to the elimination of D-D reactions in the D-T fuel).  That'll learn ya!

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...Right, I realized that was written largely in jargon.


Since I am still working on my nuclear energy mega-effort-post, I shall quickly summarize the issues here:


-With nuclear fission you have basically three fuels of any interest; U-235 (which is naturally occurring, but a bit of a PITA to get), U-233 (which is made from Th-232), and Pu-239 (which is made from U-238).  Once you've got the fuels though, their performance is broadly similar and not the topic of much discussion unless you're making high-performance bombs.  Commercial reactors will happily gobble up plutonium from recycled warheads (e.g. MOX fuel), for example, with only modest changes to their operating parameters compared to using their normal fuel.


-With nuclear fusion, there are a lot of potential fuels to consider, and they make a big difference in performance.  At this point in our species' engineering acumen, "performance" with regards to fusion reactors means "whether or not the goddamn thing will work."


-The biggest problem in fusion is actually getting the damn atoms to fuse with each other.  For reasons explained in the hyperlink above, it's easiest to get elements with a low atomic number to fuse.  For this reason, fusion bombs work by fusing deuterium and tritium (abbreviated to D-T), which are two isotopes of hydrogen (they should be called H-2 and H-3 respectively, but everyone likes using the fancy names instead).  This is why fusion bombs are sometimes called "hydrogen bombs."  D-T fusion is also the easiest fusion reaction to use in power-producing fusion reactors.  However, there are problems.


-Unlike fission reactions, D-T fusion does not leave any radioactive byproducts from the reaction itself.  However, D-T fusion produces an insanely high neutron flux (the same neutron flux from D-T fusion is what makes neutron bombs work), and this insanely high neutron flux could potentially turn the fusion reactor itself radioactive.  The extent of this problem would depend on what the neutron reactor is made out of, and how that material responds to neutron bombardment.  Additionally, since a high percentage of the energy from D-T fusion is in the neutrons, it's harder to use the energy from D-T fusion for anything useful, since it is difficult to harness the energy of neutrons.


-There are various fusion reactions which don't have the neutron problem, or at least have it to a much smaller extent, but they are harder to achieve.

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My suspicion is that if anyone ever gets power-producing fusion reactors to work, D-D will be the reaction of choice, at least initially.


D-D is only a little bit harder than D-T; ignition temperature is 15% higher or so IIRC.  Energy yield is lower, but the energy is in a form that's far easier to recover so it may be about the same.


Neutron flux is halved, which is very nice indeed.


Deuterium is .0156% of all naturally occurring hydrogen by mole, or .0312% by mass.


One liter of water contains about 3.33 x 1025 molecules, or 6.66 x 1025 atoms of hydrogen.  So that's 1.04 x 1022 atoms of deuterium per liter of water.


Each nuclear fusion requires two atoms of deuterium and produces 3.7 MeV, or 2.86 x 10-13 joules per atom.  So, that's 3 x 109 joules of energy from fusing all the deuterium in a liter of water.  Coal has an energy density of 24 MJ/kg, so the energy provided by fusing all the deuterium in one liter of water is equivalent to the energy of burning 125,000 tonnes of coal.  US energy consumption for 2011 was 9.17 x 1019 joules, so 3 x 1010 liters of water would suffice.  The US uses that much water in about three weeks for agriculture, showers, mud wrestling and suchlike.



Someone had better double-check that math before they go quoting it; I have been known to err.

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