Water Chemistry in the Primary Coolant Circuit of a Boiling Water Reactor During Startup Operations

Abstract

The coolant in a boiling water reactor (BWR) usually contains a relatively high level of dissolved oxygen from intrusion of atmospheric air during a cold shutdown. Accordingly, the structural materials in the primary coolant circuit (PCC) of a BWR could be exposed to a strongly oxidizing environment for a short period of time during a subsequent startup operation. Due to limitedmeasurable water chemistry data, a well-developedcomputer code DEMACE was used in the current studyto investigate the variations in redox species concentration and in electrochemical corrosion potential (ECP) of components in the PCC of a domestic BWR during startup operations. Our analyses indicated that the dissolved hydrogen level in the reactor coolant at a low power level without steam generation in the core was higher than that at a power level with a minor amount of steam generated in the core. The dissolved oxygen concentrations in the reactor coolant would be relatively high and more than 1000 ppb during startup operations at power levels greater than 2.5%. In the meantime, the concentrations of hydrogen peroxide could be more than 2000 ppb at the core outlet region during startup operations, which rendered a strongly oxidizing coolant environment. The electrochemical corrosion potentials of structural componentsin the PCC of the analyzed BWR generally followed the concentration trend of H2O2. It was predicted that the coolant environment in a BWR during a plant startup could be highly oxidizing, and the structural components would therefore suffer from a more serious corrosion problem than that under operations at the rated power level.