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Another thing that should be spelled out is proliferation, aka keeping the fissionables away from the bad guys. There are 114 different elements in the table so far, but more than 2600 known isotopes. An isotope is an atom with extra neutrons. For example hydrogen, deuterium, and tritium are chemically nearly identical but the weight is different. Chemical separation is difficult but doable with commercially available technology. Isotope separation is so hard that it takes entire governments years of effort to get it done in useful amounts.
Why we care is because making bomb grade material from natural ore requires isotope separation. But spent nuclear fuel has a bunch of nifty fissionables in it which can be extracted using chemical separation (aka partitioning). Unforch, that knowledge is available around the world. Only the difficulty of isotope separation prevents well financed garage tinkerers from building a DIY bomb out of uranium ore off ebay. Spent nuclear fuel tends to be self-guarding since it radiates a fatal dose in less than 5 minutes. But its also well guarded by folks who totally are authorized to shoot you with a 30mm autocannon.
Current reactor designs only use about 5% of the fissionables due to fission and decay products that catch too many neutrons and stop the show. To put that in perspective, 99% of the material out of a typical uranium mine is overburden. Then of that 1% uranium metal, only about 1/2% is the right isotope. So for every ton of fissionable metal delivered, we have a 20,000 ton hole in the ground. Throwing away over 95% of that is just stupid expensive. But even using light water reactors, there is more energy contained in known uranium ore deposits than all the coal, oil, and gas that has ever been burned. And its nearly carbon-free, not just carbon neutral.
The alternative, and the tiger we must ride, is chemical partitoning of spent fuel. Or in the case of a salt reactor continuous processing of a small sidestream of molten salt. The concern is if some clever bastard steals a gram here and a gram there, it only takes a few years to build a bomb. And if a rogue government takes over a power plant, there's all the equipment lined up to separate fissionables. A primary goal of all modern nuke plant designs is to make it verifiably impossible for any of that to happen in secret. We watch like a hawk and if there's any funny business then we send in the rude boys.
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