Half-Life
Time for half of radioactive nuclei in a sample to decay.
Defined by λ = ln(2)/T_{1/2} where λ is decay constant. Varies from ms (e.g., I-135) to 10^6 years (U-235: 704 Myr). Governs isotope activity A = λN and decay heat evolution.
Why it matters now
Half-lives dictate spent fuel cooling times (10 years for Cs-137 30y) and HALEU handling in 2025 recycling pilots.