Correlation of corrosion and activity build-up in structural components in relation to the swedish nuclear utilities requirements

Abstract

In the nuclear industry there are specific requirements which refers to materials that are subjected to radiation and/or primary water environment. One important requirement for materials in nuclear power plants is the amount of cobalt (Co) present in the materials that are in contact with primary water exposed to radiation. The Co present in the materials can be activated to the activation product 60Co by the combined effect corrosion and neutron capture. Since 60Co also causes an activity build-up in the structural components,it is important to see if the strict requirements on the Co level for systems,structure sand components that can be exposed to direct radiation are relevant and fulfilled to be within Ringhals framework of nuclear-specific requirements. This was one of the parts the project aimed to investigate. These requirements differs depending on where the material or component is located and how large the surface are a is. The subject is also important to investigate from an activity production and radiation safety point of view since a higher radioactive build-up could cause higher doses to personnel. The second aim of the project was therefore to investigate what Co content is reasonable from an "As Low As Reasonably Achievable" (ALARA) and operational perspective. Literature studies, collecting data and information from reports and documents at Ringhals and estimation calculations of Co levels in the primary circuit were performed. By comparing it to measured chemistry data at Ringhals it was seen that regarding materials and components used inside the reactor pressure vessel (RPV) and its internals, the requirement of 0.05 wt% cobalt is still adequate and should be kept strict. Outside the RPV and its internals, it was seen that the requirement could be less strict if the surface area is≤10 m2. This could potentially be applied to some larger surface are as aswell if the corrosion rate could be kept low. It was also seen that it is of great importance to keep the low cobalt content in the steam generator tubings since most of the Co is released from there.