Positive steam voidage coefficient

However, steam voidage can also have another effect because, as well as slowing down neutrons, water also absorbs some neutrons. Hence, if the neutrons are already fuUy moderated (e.g., by the graphite in the RBMK reactor core) then increased steam voidage will just mean that fewer neutrons are absorbed in the water this means that increased steam voidage can lead to an increase in reactivity - exactly the effect that we do not want This is called a positive steam voidage coefficient. 

Difficulties in steam dmm water level control at this juncture led the operators to override the reactor trip signals generated by low drum level. They then topped up the drum water level under manual control, with relatively cold feedwater. This caused a fall in reactor temperature and therefore a reduction in the amount of steam voidage inside the reactor core hence (via the positive steam voidage coefficient) the control rods had to be withdrawn yet further to maintain reactor power level at 200 MW. 

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