Uranium-235
Uranium-235 (U-235) is the fissile isotope of uranium that sustains nuclear fission in reactors. It constitutes 0.72% of natural uranium and requires enrichment for most power reactors.
U-235 undergoes fission when absorbing a thermal neutron, releasing energy, neutrons, and fission products. In light water reactors, fuel is enriched to 3-5% U-235. Higher enrichments up to 20% are used in some research reactors or HALEU for advanced designs. Natural uranium reactors like CANDU use unenriched fuel relying on U-238 breeding.
Why it matters now
U-235 demand rises with reactor restarts and SMR deployments in 2025-2026. HALEU (up to 19.75% U-235) shortages bottleneck advanced reactors for data center PPAs.