Positive void coefficients reduction

The positive void coefficient, while it must be compensated for in an accident by the SDSs, has the advantage of resulting in fast and responsive neutronic trips for a number of accidents. It also ensures an inherent power reduction for rapid cool-down accidents such as steam main failure. 

The large positive void coefficient is valuable as a means for providing reactivity. For this purpose the reduction of primary circulation flow at low power is being considered so that approximately constant voldage conditions are maintained. At start-up, either steam or pressurized hot water would be injected at channel entry to create the voldage from which start-up reactivity is achieved. 

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. 

The stabilizing reactivity feedback caused by negative reactivity temperature coefficients for the fuel and coolant as well as the void reactivity coefficient mean that heating up the core structural components, including fuel, or water boiling in the core would eventually result in a spontaneous reduction or self-limitation of the reactor power irrespective of the positions of control rods, including scram rods. 

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