– “I believe that the Fukushima reactors themselves were intact and resiliant to the tsunami but the cooling infrastructure was debilitated enough for the reactors to overheat.”
Yes. An inevitable design problem with pressurised water reactors; cooling must continue no matter what caused reactor shutdown or heat from the fission products will melt the core causing complete loss of control of the reactor.
I have read, but not confirmed, that the height of the land was actually lowered when the Fukushima site was built, so that less powerful seawater cooling pumps could be used. Had the original height been retained the diesel generators would not have been swamped and the reactors would have shut down in good order.
– “The USA looked into thorium molten salt reactors back in the 50’s but abandoned it. Perhaps materials science has caught up enough to allow them?”
They had already solved the materials science problem, as demonstrated by the Molten Salt Reactor Experiment MSRE at Oak Ridge, 1964 – 1970:
https://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment
Nixon had it shut down – (1) to save money, (2) because it wasn’t intended to produce plutonium for weapons and (3) it wasn’t in his home state. Weinberg was sacked for promoting it rather than Pressurised Water Reactors, another of his own inventions, which he described as “not safe enough for civilian use”.
MSRE never actually ran on thorium because thorium, like U238, is not quite a nuclear fuel; it is just short of being able to sustain a chain reaction. Thorium and U238 are ‘fertile’ not ‘fissile’; they become fissile when irradiated in a reactor – thorium becomes U233 which is fissile, and U238 becomes plutonium, which is fissile. This is how ‘breeder’ reactors can “make more fuel than they consume”; by nudging a fertile material just across the line into becoming fissile.
MSRE was the most versatile power-production reactor ever built. Without requiring redesign or major reconfiguration, they ran it on U233, U235 and, to an extent, plutonium. It could be possible to develop the design to “cook down” spent fuel; if so, there are a couple of centuries of free electricity as a by-product of disposing of existing “nuclear waste”.