Void coefficient RBMK reactors

All RBMK reactors have positive void coefficients which means that increasing the boiling rate increases the steam fraction in the core which increases reactivity causing more steam void which causes more reactivity and so on. Competing factors provide stability, but startup, shutdown and maneuvering below about 600 MWt are unstable, hence, there is a rule prohibiting extended operation below 700 Mwt. 

This defect is met in commercial BWR designs by additional circulation of the coolant around the reactor core, over and above the coolant removed as steam through the turbines. Controllers link the flow rate to the power demand as felt on the turbine, so that the increased coolant input sweeps out the steam bubbles and compensates for the void effect. The RBMK has a similar additional coolant circulation which is necessary, perhaps, for its satisfactory operation with a negative water void coefficient but would exacerbate the disadvantage of a positive coefficient. This again was a feature of the Chernobyl accident. 

Shortly after the beginning of the test, the reactor power began to rise sharply. The bulk of the coolant was very close to the saturation point at which it would flash to steam, because the operators had earlier run an excessive level of coolant flow with all eight pumps on during low power reactor operation. The RBMK reactor, with its positive void coefficient, responds to any such formation of steam with an increase in reactivity and power, and further increases in temperature and steam production-producing a runaway condition. 

Unlike Western reactors that have negative feedback coefficients in all phases of operation, the RBMK reactors had a positive feedback coefficient under some circumstances. While the graphite provided the vast majority of neutron moderation, the cooling water also absorbed some neutrons. Under certain operating conditions, especially at lower power levels, an increase in power could turn some of the water in the core into steam. This increase in steam would then cause a reduction in neutron absorption because of the reduction in liquid water density. Thus, if power increased while at a low power state, the positive void feedback coefficient would drive the power higher, and potentially exacerbate an accident situation. 

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