> A reactor burning higher transuranics will be right on the knife edge of prompt supercriticality. Meh, that might be a bit of an overstatement. Yes, plutoniu

> A reactor burning higher transuranics will be right on the knife edge of prompt supercriticality. Meh, that might be a bit of an overstatement. Yes, plutonium-driven fast reactors have a delayed neutron fraction around 0.0035 down from 0.0065 in a U-235-fueled fast reactor. The relatively large delayed neutron fraction from fast fissions of U-238 helps out along the way. You can reduce the radiotoxicity of TRU even if you continue to load some U-238. The inert matrix fuels with no U-238 at all get more interesting from this perspective (also from the Doppler effect), but most people don't envision using these. In fluid systems, more than half of the fuel inventory is off in a heat exchanger out of the core, which means the delayed neutron fraction is cut roughly in half in those systems too (the delayed neutron precursor decays happen where neutron importance is near zero). Plutonium-fueled fast reactors can be operated with good margins of stability.