In case of large temperature increase, in an accidental configuration, vapor bubbles may appear in the coolant. For PWR reactors this has a negative effect on the reactivity, because of the dominant influence of the spectrum hardening. For RMBK reactors the effect is strongly positive because of the dominant effect of the decrease of the capture rate in the coolant. In sodium cooled reactors the effect on reactivity is positive, but it occurs at a much higher temperature than for PWR reactors. In lead cooled reactors the void coefficient is negative[38]