Isotope
Atoms of same element (Z) with different neutron numbers (A), e.g., U-235 vs. U-238.
Stable or radioactive; fissile isotopes like Pu-239 sustain chains with thermal neutrons. Enrichment separates U-235 (0.7% natural) to 3-5% LEU. Transmutation creates new isotopes in core.
Why it matters now
Isotope separation is bottleneck for HALEU (19.75% U-235) supply in 2025-2026 SMR rollouts.
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