Such a condition is called a "positive void coefficient", and the RBMK reactor series has the highest positive void coefficient of any commercial reactor ever designed. Page 35, Annex I of INSAG-7. When a reactor has a positive void coefficient, the fission reaction speeds up as the coolant water turns to steam, rather than slowing down. Reactors that have a positive void coefficient can be unstable at low power and may experience a rapid, uncontrollable power increase. This makes it seem like an absurdly high positive void coefficient was intended to be safe. A positive void coefficient of reactivity means that the nuclear chain reaction speeds up whenever there is a loss of coolant accident. However, at the time of the accident at Chernobyl 4, the reactor's fuel burn-up, control rod configuration, and power level led to a positive void coefficient large enough to overwhelm all other influences on the power coefficient. The nuclear core in the RBMK-1000 actually became more reactive as it produced steam, creating a positive-feedback loop known as a "positive-void coefficient." Void Coefficient. The reasons for the sodium void coefficient becoming positive in large FBRs are well-known (Hummel and Okrent, 1970). Design efforts to make the overall sodium void coefficient negative are, there- fore, directed towards obtaining the reactor physics characteristics of … A positive void coefficient means that as power increases or water decreases, there is an increase in steam production and temperature of the fuel. The Chernobyl design had a positive void coefficient (VC), meaning that an increase in core temperature (more boiling) further increased power generation. Positive Void Coefficient of Reactivity and CANDUs. The void coefficient of reactivity is the dominant component of the power coefficient of reactivity of RBMK type reactors, reflecting a high degree of dependence of reactivity on the steam content of the core. The void coefficient depends on the composition of the core – a new RBMK core will have a negative void coefficient. It is expressed in units of pcm/%void.The value of void coefficient in PWRs may be of the order of -100 pcm/%void. In the unlikely event of a loss of coolant accident, or a big pipe break if you like, and that there is gas introduced. At a lower power level, however, the reactor could become overheated, unstable and prone to power surges because of the positive void coefficient … or water removed from the pipes, in a CANDU design, Chernobyl - Positive Void Coefficient Positive void coefficient is a term often associated with the RBMK reactors, the type involved in the Chernobyl disaster. This is undesirable because under adverse circumstances, a loss of coolant accident may be compounded by a loss of regulation as well (a power surge). The void coefficient is defined as the change in reactivity per percent change in void volume.. α V = dρ ⁄ d%void.
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