Well I'm not a physicist, so take everything I say with a grain of (molten) salt, but it has to do with the kind of fuel used, which produces different waste pr

Well I'm not a physicist, so take everything I say with a grain of (molten) salt, but it has to do with the kind of fuel used, which produces different waste products. The half life of most of the waste product from a gen-3 uranium reactor is measured in millions of years (235U's half life is ~700 million years, for example). In some gen-4 molten salt reactor designs, most of the waste product would have a half life measured in hours, so they wouldn't need to be put into long-term containment at all. And the rest of the waste products would only require a few hundred years before they are emitting only background level radiation. Hopefully someone with actual education on this can give a more complete answer, but that's my Reader's Digest understanding.