Reactivity
Measure of reactor deviation from criticality, expressed as ρ = (k-1)/k where k is effective multiplication factor.
Positive reactivity accelerates fission rate; negative inserts shutdown margin. Units include percent delta-k/k, dollars (where $1=β_eff, ~0.0065), or pcm (10^-5 delta-k/k). Temperature, xenon, and boron effects alter reactivity over cycle.
Why it matters now
Reactivity control is core to SMR load-following for 2025 data center PPAs, with advanced designs targeting <1% swings via inherent feedbacks.