Corrosion control for the NRU reactor vessel

The National Research Universal (NRU) reactor at the Chalk River Laboratories is a 135 MW (thermal) reactor that is used for research and medical isotope production. In May 2009, the NRU reactor entered a forced outage to repair a minor heavy-water leak from the vessel. Examination of the area of the vessel wall close to the leak site revealed that the leak was due to through-wall corrosion of the vessel from the gas filled annulus that surrounds the vessel. A review of the historical data showed that chemistry control of the water inside the vessel has been good and there has been very little degradation of the inside surface of the reactor vessel. This paper will describe the principal cause of the corrosion (the radiolytic production of nitric acid in the annulus around the reactor vessel) and explore the factors affecting the distribution of the corrosion identified by non-destructive examinations. The paper will discuss strategies being studied to improve chemistry control in the annulus and to minimize the rate of future corrosion. The importance of understanding the rationale underlying chemistry specifications and the lessons learned from the review of the chemistry in the annulus will also be presented. (author)