Power versus volume flow

A reduced velocity (Boure, 1966 Ishii and Zuber, 1970) involving the flow rate-versus-heating power ratio or its reciprocal [phase change number in Ishii and Zuber (1970)]. This group includes the specific volume ratio in Boure (1966) and in Ishii and Zuber (1970). 

It was at this point that the positive reactivity coefficient of the RBMK at low power quickly created the one of the worst nuclear accidents in history. The coolant was already very close to its flash point. A small increase in core power, or a small decrease in coolant flow, would generate a significant amount of steam within the core. As the coolant flow decreased due to the lower amount of power being suppUed to half of the coolant pumps, the temperature spiked, and the amount of steam inside the core increased. Because fewer gaseous steam molecules occupy the same volume as liquid water molecules, the now larger steam volume could not absorb as many neutrons as the previously liquid water had been able to absorb. This increased the abundance of neutrons within the core, produced a positive feedback, and led to a rapid and dramatic increase in core power. Post-accident analyses have suggested core power levels reached over lOOx greater than the normal full-power level, about 300,000 MWt versus 3140 MW (US NRC, 1987). 

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