The reactivity of a reactor – a measure for the deviation of the chain reaction rate from the stable state of equilibrium – depends upon a number of operating parameters, in a boiling water reactor, among others, on the steam bubble contents in the coolant in the core zone. In the case of an increase in the chain reaction rate and the resulting power and temperature increase, a negative steam bubble coefficient has the effect that the power is automatically limited by the growing steam bubble contents and thus declines. In the German licensing procedure it must be verified that the steam bubble coefficient is always negative. In the Russian RBMK reactor type the steam bubble coefficient is positive; therefore, a power and temperature increase causes an increasingly faster chain reaction rate, entailing further power and temperature increases, if they are not limited by other measures. This effect was one of the physical causes for the reactor accident in Chernobyl.

Evolution of reactor power under certain circum-stances at positive and negative steam bubble coefficients