Plutonium-239
Fissile isotope Pu-239 (half-life 24,110 y), primary fuel for breeders and MOX in LWRs.
Produced by U-238 neutron capture, two betas; thermal fission cross-section 750 b vs. 580 b for U-235. ~1% of spent LWR fuel; fast spectrum breeders recycle it. Weapons-grade if <7% Pu-240.
Why it matters now
Pu-239 recycling eyes 2030s via fast SMRs, easing HALEU dependence for data center nuclear PPAs.