Rod Adams has an interesting post on thorium power over at Atomic Insights. I tend to think that nuclear power is more environmentally benign than the alternatives, such as paving thousands of square kilometers of our environmentally fragile desert southwest with solar collectors. If we do restart the nuclear industry, it will also make a lot more sense to build breeders of the type mentioned in Rod’s post, which produce more fuel than they consume during operation, than to just burn up all the uranium 235 we can find in natural uranium.
There are two basic breeder reactor fuel cycles. In the first, uranium 238, which makes up 99.3% of natural uranium, is converted to plutonium 239. In the second thorium 232, which is more abundant than natural uranium, is converted to uranium 233. Both are fissile reactor fuels. Both can also be used to make nuclear weapons. If we breed either of these isotopes, it is essential that we be sure of one thing; that they never fall into the wrong hands, either now or in 10,000 years from now. For that reason, it seems to me that thorium breeders are the better of the two options.
As noted above, both types of breeders would produce fissile material that could be used to make a bomb. In both cases, the material could be separated from spent fuel using relatively straightforward chemical methods. However, spent reactor fuel remains highly radioactive for many years after it is removed from a reactor core. It would be lethal to work with without highly specialized equipment unlikely to be available to other than technically advanced states. In the case of thorium breeders, however, the fissile uranium 233 would be contaminated with uranium 232, a short-lived, highly radioactive isotope that could not be separated from the U233, making it even more difficult to work with than plutonium.
In both cases, the levels of radioactivity of the spent fuel would decay exponentially over time, gradually making it easier to handle the material. Eventually, it would become possible for non-state actors to separate the bomb-grade material. It is immaterial whether this happens in a thousand years, or ten thousand years. We cannot simply put such material in a nuclear storage facility and leave it for future generations to deal with. In the case of plutonium, the only way to reliably eliminate it, other than, perhaps, rocketing it into the sun, would be to burn all of it up. However, in the case of U233, it could be “denatured” by mixing it with large amounts of non-fissile U238, rendering it, for all practical purposes, as difficult to convert to a weapon as natural uranium.