positive void coefficient
양의 보이드 계수
A nuclear reactor physics coefficient describing a condition where reactivity increases as steam bubbles (voids) form in the coolant. In the RBMK design, water served as coolant and graphite as moderator, so when coolant boiled away, neutron absorption decreased while moderation continued, creating a dangerous positive feedback loop. This was a direct cause of the 1986 Chernobyl disaster; at the time of the accident the RBMK-1000's void coefficient reached +4.7 β, and Western reactors are designed to exclude a positive void coefficient entirely.
In depth
Principle
When coolant water boils inside a reactor core and forms steam bubbles (voids), two competing effects on neutron economy arise. First, reduced coolant density diminishes neutron moderation. Second, neutron absorption by the coolant decreases. In most Western reactors (PWR, BWR), water serves as both coolant and moderator, so the first effect dominates: reactivity decreases as voids increase, yielding a negative void coefficient that contributes to inherent reactor safety.
The RBMK anomaly
The RBMK reactor employed a design unique among commercial power reactors: the coolant (light water) and moderator (graphite) were separate. When water boiled into steam, the graphite moderator remained in place to continue slowing neutrons, while the water's neutron-absorbing effect was lost. This created a positive feedback loop: void formation, increased reactivity, higher power, more steam, and more voids. The effect was especially dangerous at low power levels, where the reactor was least stable.
Chernobyl and post-accident modifications
On April 26, 1986, during a safety test at Chernobyl Unit 4, reactor power dropped sharply, leading operators to withdraw most control rods. When power surged moments later, the positive void coefficient (measured at +4.7 β, far beyond what control systems could handle) drove a runaway chain reaction culminating in a steam explosion. After the disaster, all operating RBMK reactors were modified to bring the void coefficient below +0.7 β through higher-enrichment fuel and additional fixed absorbers, substantially suppressing the positive feedback potential.
Western comparison
US reactors are not permitted to operate with a positive void coefficient. Pressurized water reactors (PWRs), where coolant and moderator are the same water, have an inherently negative void coefficient. Boiling water reactors (BWRs) also feature negative void coefficients and use them for power regulation. Canada's CANDU reactors have a modest positive void coefficient, but it is small enough that control systems can respond adequately, and independent shutdown systems provide additional protection.
Sources
- Wikipedia (RU) Russian Wikipedia: definition of steam/void coefficient of reactivity and its physical basis; explains competition between moderation and absorption effects
- Wikipedia (EN) English Wikipedia: comprehensive explanation of positive vs negative void coefficients; RBMK had +4.7 β before Chernobyl, reduced to +0.7 β after; US reactors prohibited from having positive void coefficient
- ans.org American Nuclear Society webinar (2022): reactor physicist Christopher Perfetti confirms positive void coefficient as an unsafe design feature prohibited in US reactors; explains the over-moderated graphite/water split in RBMK
- world-nuclear.org World Nuclear Association: detailed explanation of positive void coefficient in RBMK reactors, its dominance in the power coefficient, and the positive feedback mechanism that drove the Chernobyl accident