NOBLE METAL APPLICATIONS FOR SCC MITIGATION IN BWRS: PLATINUM NANOPARTICLE PENETRATION INTO CREVICES AND CRACKS UNDER CONTROLLED FLOW CONDITIONS

摘要

Most of the US boiling water reactor (BWR) fleet and three European BWRs use noble metal injection to mitigate stress corrosion cracking (SCC) of reactor components. Despite the wide use of this technology there are still open questions regarding the chemical and physical parameters affecting the application process and the possible improvements to it. Since 2010 the Paul Scherrer Institut (PSI), in the frame of a joint effort with the Swiss Nuclear Safety Inspectorate (ENSI) and the Swiss nuclear power plants of Leibstadt (KKL) and Muehleberg (KKM), is addressing these questions. A recurrent question is how deep the Pt nanoparticles can enter into cracks, crevices and other confined spaces and thus provide -in conjunction with H_2- the desired lowering effect on the electrochemical corrosion potential. With this in mind, tests were performed using a high-temperature water loop at PSI where coupon specimens with crevices or cracks were exposed to water carrying Pt particles. In parallel computational fluid dynamics calculations were performed to better understand the flow conditions in the crevices. It was found that under a set of given flow conditions, the penetration of the Pt into the cracks and crevices was strongly dependent on the opening width and orientation with respect to the flow direction. Active flow in the crevice is essential to achieve significant Pt surface loadings. Local variations in Pt surface loading seem to be correlated with changes in flow velocity and direction.