Much ado about thorium-fueled nuclear reactors around the blogosphere. Much of it is hype, as per usual, but there is a valuable core(!) in the debate — and, as usual, it’s political, not technical. The thorium cycle is more complex than a reactor using uranium needs, and it operates at higher temperatures, so the engineering aspects are somewhat more difficult. Balancing that, thorium is both more abundant and “hotter” — it produces more energy per pound of fuel than the uranium cycle does — and is not radioactive in its fuel-ready form, making transportation easier.

One of the political arguments made in favor of using thorium instead of uranium + plutonium is that the thorium cycle does not produce materials that can be used for a bomb (advocacy site), and thus that it reduces the likelihood of “proliferation” (PDF), the spread of nuclear weapons to smaller states and even terrorists. As several people including blogosphere treasure Steven den Beste have pointed out, this isn’t strictly true — the Wikipedia article makes it clear that, in any nuclear cycle employing fission to produce energy, there will be fissionable uranium in there somewhere, and probably plutonium. I hadn’t seen this person before, but he’s even more emphatic about that, with details, and what he says about some other subjects matches what I remember. Whether it’s so or not, anybody who suggests that uranium and plutonium will go away is dreaming of unicorns. Proliferation is a fact that future generations will have to live with — but perhaps it can be slowed a bit.

The best argument that the thorium cycle is resistant to proliferation comes indirectly from the original developers of nuclear power. They wanted bombs, they wanted them as soon as possible, and they selected the fuel cycle that would yield explosive materials as soon as possible — and it wasn’t thorium. This is a strong hint that the thorium cycle is less apt to fueling proliferation than uranium plus plutonium.

If the new, accelerator-based technology works (a bigger “if” than may be first apparent) there will be one attractive feature to the thorium cycle that no uranium- or plutonium-based reactor can match: a core that never contains a critical mass. A uranium reactor requires control rods (yes, oversimplified) that poison the reaction and shut it down if it gets out of control, and those are mechanical components that can fail. If the neutrons causing fission come from outside rather than other parts of the core, shutting down the accelerator shuts the reactor down (or materially reduces its output). This strikes me as a Good Thing.

But the advantages are, as stated, political. Environmentalists have painted themselves into a corner in which they are obliged to demand that we become poorer, because they have so demonized nuclear power that demonizing fossil fuels as well leaves no way to keep warm or run industry. If the thorium cycle is, or can be, a fig leaf that allows the Greens to back off — “ah, well, we’re against uranium and plutonium power ’cause it makes bombs; thorium doesn’t so it’s OK” — it will be to everyone’s advantage, whether or not the statement is literally true in an absolute sense.

CO2 is pretty nearly irrelevant, whether or not it causes global warming (or “climate change”). It simply wouldn’t make sense to burn all the good lubricants and synthetics feedstocks in order to stay warm, even if the result of that was pure oxygen and clean water, and coal makes a mess before, during, and after use as fuel. Transportation requires liquid fuel (no, electrics won’t make a dent; not enough energy density, and overcoming that would create an horrific safety problem). The ideal fuel would be hydrogen, because burning hydrogen produces only water, but hydrogen is too small a molecule and therefore too elusive — its energy density is almost as low as that of batteries, and it has the nasty habit of escaping through any material at all, and ruining the tank material on the way out. Practical fuels use carbon as a ball and chain to tie the hydrogen down, keep it confined, and get the molecules close enough together for decent energy density.

This suggests a vision: Carry the baseload on nuclear reactors, including electrifying the railroads where practical; other transportation fuels would be produced by reforming methane (natural gas, available in vast quantities) or electrolyzed hydrogen into liquids with high hydrogen content using the abundant energy from reactors. Result: minimum pollution of all types, including a dramatic reduction in CO2 output, and near-zero dependence on foreign sources. This has been proposed before, but environmentalists have always balked at it; the present-day Green movement started out as anti-nuclear, and it is now so much their signature that they are incapable of backing away from that stance. If the Greens can be talked (or suckered) into a face-saving withdrawal from absolutism using the thorium cycle as a wedge, the future could be bright all round.