The influences of ECP and metal dopants on radiation field build-up on stainless steel surfaces

In LWRs, water chemistry control has been effectively applied to countermeasure radiation field build-up and materials integrity issues. Numerous plant observations and laboratory examinations indicate that the radiation field build-up is very sensitive not only to the concentrations of various impurities of reactor water but also to the electrochemical corrosion potential (ECP). The mechanisms behind these observations have not been fully understood due to possibly rather complex interactions among fuel CRUD, reactor water, and material piping surfaces In Studsvik, an experimental loop system has been constructed to simulate BWR water conditions and applied Co-60 tracer technique to online measure radioactivity build-up and release on stainless steel surfaces under various controlled water chemistry conditions. We have also examined the microstructures of the oxide films formed under those conditions. In this paper, we report on radioactivity uptake on and release from stainless steel material under simulated cyclic oxidising and reducing conditions. Metal dopants of iron, nickel and zinc species in different concentrations were used to study the influence of water chemistry. The activity build-up and release were studied in different exposure intervals during a 3-month long autoclave experiment. The radioactivity uptake rate was seen to increase when shifting water chemistry from oxidising to reducing conditions. The corresponding changes from reducing to oxidising chemistry resulted in a decreased rate. Different activity uptake rates were observed with and without zinc injection in the presence of iron and nickel dopants. With zinc injection, instantaneous release of Co-60 was seen in conjugation to any water chemistry shift from oxidising to reducing or from reducing to oxidising conditions. The results of this study are compared with that obtained previously to shed some light on the possible mechanisms behind the activity build-up and the influence of water chemistry conditions. (author)