Water radiolysis analysis and in-core ECP measurement in PWR primary cooling system

Hydrogen addition into PWR primary water has conventionally being performed to prevent structural material, e.g. austenitic stainless steel, from Stress Corrosion Cracking (SCC). The concentration of dissolved hydrogen (DH) has been controlled high enough to suppress the reactor radiolysis within 25-30 cm{sup 3}STP/kg-H{sub 2}O. Problems come real recently, however, such as Primary Water SCC (PWSCC) of nickel base alloys, radiation dose increase of piping due to the deposit of nickel corrosion products on fuel surface, Axial Offset Anomaly (AOA), etc., under strong reductive environment. It's common knowledge that such problems are strongly influenced by DH. In addition, since thermal power up-rate and adoption of high burn-up fuel are being discussed for field application in Japan, it is estimated that risk of the problems mentioned above would be increasing. Therefore, it is necessary to optimize DH in PWR primary coolant to reduce the risk of such problems as well as suppression of radiolysis. Therefore, we have estimated the Critical Hydrogen Concentration (CHC), which is the minimum required DH to suppress radiolysis, using a model for PWR verified by an in-pile loop test. A calculation result of reactor radiolysis with PWR conditions, showed that the CHC was about 2 cm{sup 3}STP/kg-H{sub 2}O. Besides, we measured Electrochemical Corrosion Potential (ECP) value at the top of core in Tsuruga-2 (PWR, 1160 MWe, Commercial Operation started in 1987) as a first trial in actual PWRs, to verify the predictability of the model within the safety regulated band, 15-50 cm{sup 3}STP/kg-H{sub 2}O. From this result, it was confirmed that the reactor radiolysis could be well suppressed when DH was reduced to about 15 cm{sup 3}STP/kg-H{sub 2}O, because measured ECP value was very low and stable, and the inclination of measured ECP behavior was consistent with prediction by the model. Also, the first step has been cleared by this experience toward reviewing CHC in PWRs. (author)