Axial offset anomaly: coupling PWR primary chemistry with core design

Abstract

Axial offset anomaly (AOA) in pressurised water reactors (PWRs) is a larger than expected negative flux depression beginning approximately at 5000 MWd/MtU feed fuel burnup. AOA can threaten the full power shutdown margin and thereby force a costly derate. More than 20 US cores have exhibited AOA, one seriously enough to have derated to 70% 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 sub-cooled steaming, and shortened outages with less effective corrosion product clean-up. A growing fraction of the US PWR fleet is implementing these strategies, thus putting the cores at greater risk for developing AOA. The root cause of AOA involves both the primary chemistry and thermal hydraulics of core design. The most recent revision of the Electric Power Research Institute (EPRI) PWR Primary Water Chemistry Guidelines addresses the primary-chemistry factors in new recommendations for the cycle pH programme and options to control corrosion product transport during start-up and shutdown. 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 programmes to investigate means to prevent AOA via primary chemistry influence on the formation and growth of fuel deposits. This paper will address the root cause of AOA, updating recent changes to the primary water chemistry guidelines as well as progress on fuel deposit characterisation and AOA modelling 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.