I was looking at CANDU reactors, Canadian Deuterium Uranium reactors. They use a heavy water design which can run un-enriched uranium which is less than 0.7% U235, the good stuff. Because the heavy water doesn’t capture as many neutrons, they can get a bigger reactor fissioning just a little U235 and still ends up with a lot of spent fuel when the fraction of U235 gets even lower over time, which is why these types of reactors can breed U233 from fertile Thorium. U238 is fertile; it can be transmuted into Plutonium 239, which is admittedly scary stuff, but useful. We desperately need it for RTGs for outer solar system space probes, and we can burn spent fuel in MOX, mixed oxide fuel which is a mix of fissionable oxides, plutonium with natural uranium, repossessed spent uranium fuel or depleted uranium. We want to burn this crap up anyway, and a molten chloride reactor using MOX would be one way.