Why plutonium fuel?
Plutonium fuel fabrication

The use of uranium in thermal neutron reactors is limited: 0.5 to 0.8% of the uranium nuclei initially extracted from the deposit are burned, directly by fission of ²³⁵U or rapid fission of ²³⁸U, or indirectly by conversion of ²³⁸U into plutonium and fission of the latter.

Unused uranium is found, on the one hand, in the form of depleted uranium from enrichment plants and, on the other, in irradiated fuel, associated with a small fraction of plutonium.

Uranium mining resources at a cost not exceeding $35/lb...