IMPACT OF CORROSION OF NICKEL BASED COATINGS FOR FLOW ACCELERATED CORROSION CONTROL ON BWR RADIATION FIELDS

摘要

Boiling water reactor (BWR) primary coolant systemradiation fields are mainly caused by the deposition ofactivated corrosion products. Co-60, formed by theneutron absorption of non-radioactive Co-59, is theradioisotope that is the main contributor to BWRradiation fields and personnel collective radiationexposure.While there are Co-59 sources within the BWRreactor vessel and reactor recirculation system, the maininput of Co-59 into the BWR reactor vessel is from thefeedwater system due to the erosion and corrosion ofvarious materials used in piping and components in thecondensate, feedwater, heater drains, and steam systems,including process valves with hard facing cobalt alloytrim. Further, materials that contain nickel, such asstainless steels and high nickel alloys (Alloys 600, 690,625, etc.), may contain a high content of Co-59, unlessspecified for low cobalt content. Raw nickel ores maycontain up to 13% cobalt (by weight).At one dual unit BWR 5, high nickel content coatingsapplied inside the high pressure turbine-moistureseparator/reheater cross-under pipes in preparation foran extended power uprate successively minimized carbonsteel flow accelerated corrosion (FAC). In the followingoperating cycle, however, an order of magnitude stepchange increase in feedwater nickel concentrationoccurred from less than 0.1 μg/L to near 1 μg/L at bothunits. Coincidentally, long term increasing trends inreactor coolant Co-60 and Co-58 concentrations wereobserved, including a nearly two orders of magnitudeincrease in Co-58 (a neutron activation product of Ni-58)and an order of magnitude increase in Co-60, leading tohigh primary system radiation fields. The forwardpumped heater drains design at these plants results in allheater drains bypassing the condensate treatment system.This paper examines why apparenterosion/corrosion of the nickel based coatings hasoccurred, how long will erosion/corrosion of the nickelcoatings continue, and what options are available formitigating nickel and cobalt ingress into the reactor fromthe feedwater system.