A Numerical Model for Evaluating the Impact of Noble Metal Chemical Addition in Boiling Water Reactors

The technique of noble metal chemical addition (NMCA), accompanied by a low-level hydrogen water chemistry (HWC), is being employed by several U.S. nuclear power plants for mitigating intergranular stress corrosion cracking in the vessel internals of their boiling water reactors (BWRs). An improved computer model by the name of DEMACE was employed to evaluate the performance of NMCA throughout the primary coolant circuit (PCC) of a commercial BWR. The molar ratios of hydrogen to oxidizing species in the PCC under normal water chemistry and HWC are analyzed. The effectiveness of NMCA is justified by calculated electrochemical corrosion potential (ECP) around the PCC and in a local power range monitoring (LPRM) housing tube, in which practical in-vessel ECP measurements are normally taken.

Prior to the modeling work for the BWR, the Mixed Potential Model, which is embedded in DEMACE and responsible for ECP calculation, was calibrated against both laboratory and plant ECP data. After modeling for various HWC conditions, it is found that the effectiveness of NMCA in the PCC of the selected BWR varies from region to region. In particular, the predicted ECP in the LPRM housing tube is notably different from that in the nearby bulk environment under NMCA, indicating that cautions must be given to a possible, undesirable outcome due to a distinct ECP difference between a locally confined area and the actual bulk environment.