SCC mitigation in BWRs by platinum addition: effect of environment and injection rate

On-line noble metal chemical addition is a technology developed by General Electric to mitigate stress corrosion cracking (SCC) in reactor internals and recirculation pipes of boiling water reactors (BWRs) avoiding the negative side-effects of the classical hydrogen water chemistry. For a more efficient reduction of the electrochemical corrosion potential (ECP) Pt is injected into the feed water during power operation. Pt is claimed to deposit as very fine metallic particles on all water-wetted surfaces and to stay electrocatalytic over long periods. To assess this SCC mitigation technique, the deposition and distribution behaviour of Pt on stainless steel coupon specimens, exposed to simulated BWR water conditions, using a sophisticated high-temperature water loop, has been investigated in detail at PSI. During the tests Pt solution was injected into the loop and Pt was deposited on the specimens. Scanning and Transmission Electron Microscopy techniques and Laser Ablation - Inductively Coupled Plasma - Mass Spectrometry were used to characterise the Pt distribution and concentration on the specimens. In this paper results from a set of experiments with varying concentrations of dissolved H₂ and O₂ in the high-temperature water, as well as from tests with different Pt injection rates are presented and discussed. These experiments revealed a clear difference in the Pt deposition behaviour in terms of size and homogeneity in distribution depending on the water chemistry and injection rate. A reducing environment and slow Pt injection rates resulted in the most effective Pt deposition, thus potentially in a better protection against SCC. (authors)