The Benefits of Cycle Stretchout in Pressurized Water Reactor Extended-Burnup Fuel Cycles

Nuclear reactors are inherently capable of operating for a substantial period beyond their nominal end of cycle (EOC) as a result of negative moderator and fuel temperature coefficients and the decrease in xenon poisoning with lower core power levels. This inherent capability can be used to advantage to reduce annual uranium makeup requirements and cycle energy costs by the use of planned EOC stretchout. The benefits of planned stretchout are assessed in the context of extended-burnup fuel cycles for two methods of operation: normal power coastdown and feedwater-pressure augmentation (FWPA). In the latter method, feedwater temperature is reduced allowing extended operation at full rated core power but at a lower thermal efficiency. The extent to which FWPA can be practiced is limited, primarily, by turbine operating conditions, resulting in a differential benefit in uranium utilization of only ~0.5% above that of normal power coastdown.