Precise evaluation of water radiolysis and ECP under complex water flow condition

In recent years BWR primary loop water chemistry has been extensively investigated by actual plant measurement as well as computer code simulation. At first the precise evaluation of water chemistry was focused on recirculation line regarding piping IGSCC problems. The pressure boundary components and in-core internals have attached wide attention nowadays from the viewpoint of plant life extension. Hydrogen water chemistry (HWC) has been applied to commercial BWRs as one of IGSCC countermeasures to reduce the oxidizing species in reactor water. Precise evaluation of water chemistry, therefore, is of very importance. Under the complex water flow condition, the estimation of water radiolysis products concentration is very difficult because of variety of residence time of water in radiation field. The corrosion potential of stainless steel is one of the most important indicators of HWC effectiveness and the IGSCC susceptibility of component materials. The corrosion potential is affected not only by water chemistry, but also by the water flow velocity. The effect of flow velocity on corrosion potential increases where there is a low oxidant concentration in the reactor water (Refs. 1 and 2). In this document, two types conditions are considered. One is the complex flow in low radiation field such as BWR bottom region, the other is, in high radiation field such as down comer region of a BWR. In the former case, water velocity is very important to evaluate ECP of the structural materials, the latter case, radiolysis products concentration would be dominant. (authors)