Assessment of the platinum deposition behaviour on stainless steel surfaces in a boiling water reactor plant

Abstract

With the aim of mitigating stress corrosion cracking (SCC) in reactor internals and recirculation pipes, the boiling water reactor plant Leibstadt (KKL) changed in 2008 from normal water chemistry to hydrogen water chemistry in combination with online noble metal chemical addition (OLNC). For this purpose, a Na₂Pt(OH)₆ solution is injected into the reactor feed water, where platinum (Pt) particles are formed and deposited on water-wetted surfaces. If a stoichiometric excess of hydrogen and a uniform surface coverage with extremely fine Pt particles exist simultaneously at critical locations, a low electrochemical corrosion potential, and therefore a low SCC susceptibility can be expected. To study the Pt deposition behaviour and to assess the effectiveness of the OLNC technology under real plant conditions, stainless steel coupon specimens were exposed to the reactor water for different durations at two locations in KKL. The specimens were then analysed at PSI for their Pt surface loading. In addition, a replica technique and transmission electron microscopy were used to determine the Pt particle size and distribution on the active specimens. The investigations indicated that the pre- and post-OLNC exposure times play an important role for the Pt deposition and retention on specimens and that a remobilisation of Pt in the plant takes place. Unfortunately, most of the released Pt does not redeposit on other steel surfaces in the reactor system. The flow velocity of the Pt containing reactor water across the specimens also has an impact on the Pt surface loading. The average Pt particle size is in the expected range, but the distribution of the Pt particles on plant specimens seems to be rather erratic.