Plutonium is always associated with critical mass and nuclear fission. The 1930's name red uranium is for alloying uranium or plutonium with mercury to get a distinctive red color that warns employees that they are handling somthing fissionable. Red gold of Atlantis prized by sorcerers in the Hero Game System Altantean Age has today's breeder nuclear reactor the equivalent of the more prized form of red gold. It also is very difficult to purify uranium-235 or safely obtain plutonium-239. You could run a huge carbon dioxide laser off a nuclear submarine for the equivalent of an Atlantean Age sorcerer. Beryllium metal as an additive has 1/9 of it's mass available to endothermically fission, to create two slower neutrons from one fast neutron. High melting point fuel cladding could fission electroplated hafnium which is a nuclear waste fission product, that is fissioned since the 1960's in the hafnium wall nuclear reactor.
Muon catalysed nuclear fusion, exists is why it is worth studying if the approach of critical mass also catalyses nuclear fusion. Uranium deutride close to critical mass physically separated by hafnium from the rest of the nuclear reactor to keep gaseous hydrogen away from any carbon in use, would produce helium-3 that would not be found elsewhere in the nuclear reactor. Hot nuclear fusion would use tritium obtained by cold nuclear fusion as the supply, and a lithium deutride limiter would keep deuterium ready to use one the plasma is hot enough by tritium-tritium nuclear fusion.