Axial Offset Anomaly: Coupling PWR Primary Chemistry with Core Design

摘要

Axial Offset Anomaly (AOA) in PWRs is a larger than expected negative flux depression beginning approximately 5000 MWD/MTU feed fuel burnup. AOA can threaten full power shutdown margin and thereby force a costly derate. More than twenty US cores have exhibited AOA, one seriously enough to have derated to seventy percent power for several months of the cycle. Deregulation of the US power market demands improvements in fuel economics. This means extended cycles with high boron at the beginning of core life, core designs with increased subcooled steaming, and shortened outages with less effective corrosion product cleanup. A growing fraction of the US PWR fleet is implementing these strategies, thus putting the cores at greater risk for developing AOA. The rootcause of AOA involves both primary chemistry and thermal hydraulics of core design. The most recent revision of the EPRI PWR Primary Water Chemistry Guidelines' addresses the primary-chemistry factors in new recommendations for the cycle pH program and options to control corrosion product transport during startup and shut-down. The EPRI Robust Fuel Program Fuel/Water Chemistry Working Group has addressed the thermal hydraulic issues through guidelines for reducing AOA core design risk, together with extensive programs to investigate means to prevent AOA via primary chemistry influence on formation and growth of fuel deposits. This paper will address the rootcause of AOA, updating recent changes to the primary water chemistry Guidelines as well as progress on fuel deposit characterization and AOA modeling by the Robust Fuel Program. It will demonstrate how primary chemistry and core design considerations must be coupled in a successful strategy to manage AOA risk.