The economist has an Thorium story. I’ve been curious about this for years. Lot’s of hopeful rumors and iffy speculations about why it hasn’t happened. In this comment, there is an answer (right or wrong) which I can’t quite understand:
“Thorium itself is not fissile. If bombarded by neutrons, though, it turns into an isotope of uranium, U-233.”
The above is too simplistic. Herein lies the problem with thorium reactors. When bombarded by neutrons thorium-232, absorbs a neutron, becoming thorium-233, which quickly decays to protactinium-233, which very slowly decays to uranium-233. Protactinium-233 has a half-life of 27 days; its decay is slow indeed. Pa-233 also has a comparatively high affinity to absorb another neutron. 75% of neutrons hitting Pa-233 are absorbed. It that happens, it’s no longer possible for Pa-233 to decay to U-233 (because the Pa-233 has changed to non-fissionable U-234). To prevent Pa-233 absorbing that neutron it must be removed from the neutron source (the reactor) ASAP by ‘reprocessing’. The entire secret of thorium reactors boils down to how easy is to remove the Pa-233, to ‘reprocess’ it. Once reprocessed it is safe from absorbing that neutron and may be quietly left to decay into U-233. U-233 is the goal. Only the Chinese are moving in the right direction here because the molten salt reactor was designed to aid reprocessing. In theory reprocessing is easiest when thorium/protactinium-233 are in a blanket (surrounding the core neutron source). No one has figured out how to do that because no one has a material suitable for separating core from blanket.
If I had money, I’d put my pennies into a uranium-238 -> plutonium-239 fast reactor (ideally molten salt too). Such a reactor will not need thorium’s aggressive reprocessing, and there’s 1.5 million tonnes of depleted uranium in world stockpiles. In theory that’s enough to power all the world’s electricity needs for hundreds of years to come. Roll on thorium, but in the meantime, let’s be pragmatic.
PS: If Pa-233 captures a neutron it becomes Pa-234, which quickly decays to U-234. U-234 is not fissionable. It’s a dead end.