Void Coefficient
Change in reactivity per unit void fraction in coolant, typically negative in LWRs for safety.
Voids reduce moderation and density, hardening spectrum and increasing leakage; PWRs show -1 to -3 $/m^3 void. Positive in early RBMK designs contributed to Chernobyl. Measured in beta/m^3 or pcm/%.
Why it matters now
Negative void coefficients are mandatory for 2026 SMR certifications, bolstering investor confidence in boil-off accident resilience for hyperscale PPAs.