Why is it so expensive to purify plutonium or enrich uranium?

The reason why uranium enrichment is expensive is because the separation of U-235 from U-238 is based on a tiny difference in mass between compounds of the two isotopess. Enrichment using centrifuges requires a compound in the gas phase. read more

Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238 U isotope, with 235 U only constituting about 0.711% of its mass. 235 U is the only nuclide existing in nature (in any appreciable amount) that is fissile with thermal neutrons. read more

The other thing is that the uranium is enriched as UF6, but once it’s enriched it has to be converted to uranium oxide (U3O8 and then UO2), and that’s made into a powder, which is sintered into fuel pellets, which are then clad and loaded into fuel rods. read more