ICONE11-36553 BWR SCC Mitigation Strategies and their Effectiveness

Abstract

Intergranular stress corrosion cracking (IGSCC) of boiling water reactor (BWR) internals materials is a well understood phenomenon. The degree of stress corrosion cracking (SCC) varies from plant to plant depending on weld heat affected zone residual stresses and the operating water chemistry history of each plant. Since late 80's plants have paid a significant attention to reactor water chemistry control and have seen the beneficial effect of reduced incidents of SCC as a result of good water chemistry control. In addition to water chemistry control by lowering the reactor water impurity ionic chloride, sulfate, copper and maintaining feedwater oxygen and iron at the desired levels, plants have also attempted to lower the electrochemical corrosion potential (ECP) of internal surfaces by practicing hydrogen water chemistry (HWC) to reduce the propensity of materials to undergo SCC. Majority of the BWRs worldwide have employed HWC to mitigate IGSCC of reactor internals. Since recently, many BWRs have also employed a new technology called NobleChem^<TM> and low HWC which improves the effectiveness of hydrogen to lower the ECP to HWC specification value of -230 mV(SHE) or lower. This paper highlights some of the shortcomings of the SCC mitigation practices that need further attention to derive the full benefit of HWC and other mitigative actions against IGSCC. The topics that will be discussed in this paper will include, 1) operating BWRs at low feedwater hydrogen levels, 2) effect of hydrogen interruptions and the importance of high hydrogen availability, 3) dissolved oxygen monitoring as a means of controlling SCC, 4) ECP measurement at the end of long sample lines and its impact on SCC mitigation decisions, and 5) using main steam line dose rate as a criterion to determine SCC mitigation effectiveness.