Since the 40s, the two possible weapons-grade fissionable materials were Uranium 235 And Plutonium 239. Unfortunately, U235 is a very miniscule component of natural Uranium which is primarily the inert U238. Since the two isotopes are chemically identical, separating them is extremely difficult and requires very slow and expensive "enrichment" technologies to achieve the moderate concentrations of U235 needed by reactors (about 5%), or the almost pure U235 used in weapons. This process takes a lot of energy (which is why Oak Ridge was located in TVA country). Even today, Uranium enrichment using centrifuges, mass spectrometer, or membrane technology can almost break a small country financially.

The alternative is to use another fissionable material, one not found in nature, Plutonium.
Plutonium can be manufactured, it is made when the inert U238 in a reactor absorbs stray neutrons to create the not-so-inert Pu239, which being chemically distinct from Uranium, can easily be removed from the spent fuel using chemical separation techniques. All Uranium reactors, even civilian LWRs, produce some Plutonium, which is removed from the spent fuel, during waste recycling (along with a host of other noxious substances) but specialized reactors designed for that specific purpose are much more efficient at it.

In the 70s, it was feared that the easily made and recovered Plutonium would become a global commodity, and that every other country in the world would start making its own nukes, so the Carter administration essentiually destroyed the nuclear industry to prevent this from happening. As it turned out, this was a tragic mistake, because it crippled an essential technology and did little to stop proliferation.