The other even more exotic possibility is to use something like this to enrich ordinary reactor grade plutonium to weapons grade plutonium-239. The amount of mass to process with plutonium is orders of magnitude less because spent fuel plutonium is already more than 50% Pu-239, versus 0.7% U-235 in natural uranium, and a bare sphere critical mass of Pu-239 is only 10 kg vs 52 kg for U-235:
https://en.wikipedia.org/wiki/Critical_mass#Critical_mass_of...
The United States considered enriching waste fuel plutonium to weapons grade in the 1980s, when it contemplated another big nuclear weapons buildup against the USSR, but the laser based separation technology to be used was much more complicated than centrifuge separation. The project ended shortly after the USSR dissolved. It was called the Special Isotope Separation Project.
The 1988 environmental impact statement for the project gives some background information:
https://www.energy.gov/sites/prod/files/2015/06/f24/EIS-0136...