A comprehensive investigation of the platinum application to BWRs to mitigate stress corrosion cracking

Stress corrosion cracking (SCC) is a well-known degradation mechanism for boiling water reactors (BWRs). Noble metal chemical application (NMCA) to mitigate SCC has been developed by General Electric to reduce the negative side-effects of hydrogen water chemistry and NMCA is now widely used as an online process (OLNC). However, the understanding of the parameters that control the formation and deposition of the noble metal (Pt) particles in a BWR was still incomplete. To fill this knowledge gap, a joint project (NORA) between the PSI, the Swiss Federal Nuclear Safety Inspectorate (ENSI) and the Swiss nuclear power plants of Leibstadt (KKL) and Mühleberg (KKM) was started. The present paper summarises the most important findings gathered during the 3.5 years this project lasted. Experiments in a sophisticated high-temperature water loop revealed that the flow conditions, water chemistry, the Pt injection rate, and the pre-conditioning of the stainless steel surfaces have an impact on the Pt deposition behaviour. Slower Pt injection rates and stoichiometric excess of H₂ over O₂ produce smaller particles. Surfaces with a well-developed oxide layer retain more Pt particles. Furthermore, the pre- and post-OLNC exposure times play an important role for the Pt deposition on specimens exposed at the KKL power plant. Redistribution of Pt in the plant takes place, but most of the Pt apparently does not redeposit on the steel surfaces in the reactor system. Comparison of lab and plant results also demonstrated that plant OLNC applications can be simulated reasonably well on the lab scale. (author)