I didn't realize molten salt fuel is 10X less dense. But assuming either thorium or fast uranium reactors, they'll get 100X as much energy from the fuel, making

I didn't realize molten salt fuel is 10X less dense. But assuming either thorium or fast uranium reactors, they'll get 100X as much energy from the fuel, making 10X less fuel volume than conventional reactors. That doesn't necessarily imply anything about the waste volume. With high burnup, the waste is just fission products, which are entirely different elements than the fuel. One reason high burnup is easier in MSRs is that xenon, a neutron poison, is a gas that just bubbles out of the liquid fuel, instead of being trapped in a solid. With fast uranium or thermal thorium, you also don't have a large amount fuel that can't be fissioned. (Some early MSRs will be thermal uranium, and produce more waste.) For disposal of fission products, the general idea is to mix them into glass and bury them as solid blocks.