Effects of Dissolved Hydrogen Content in PWR Primary Water on PWSCC Initiation Property

Environmental mitigation is being looked forward to as one of the promising preventive measures for better plant maintenance. Especially, it would be much more effective, when environmental measures are introduced in combination with mechanical measures, such as stress improvement, material replacement, and so on.

Dissolved hydrogen (DH) concentration in the primary cooling system has been controlled at the level of 25~35 ml STP/kg H₂O in all PWR plants in Japan, maintaining a sufficient margin to the utilities’ self-regulated band 15~50 ml STP/kg H₂O. The lower limit of this self-regulated band was determined to suppress reactor coolant radiolysis based on experiments using a test reactor over 50 years ago, though the test was performed at much lower than typical operational temperature and is considered over-conservative at present.

On the other hand, primary water stress corrosion cracking (PWSCC) events have occurred on Dissimilar Metal Welds (DMW) at vessel nozzles or at vessel head penetration in some PWR plants in Japan in recent years.  PWSCC growth test data have been showing that the current management band of DH content is around the peak of crack growth rate (CGR) at about 340ºC, corresponding to the operating temperature of the pressurizer. To avoid the higher CGR levels, the DH content band should be shifted to the higher side, or to the lower side. The authors suggest much lower DH content than that at present as an alternative control, since some experimental data on PWSCC initiation show monotonically decreasing dependency of initiation time with increasing DH content.

In this paper, we show the effects of DH content on PWSCC initiation property. Reverse-u-bend (RUB) tests and three-point-bend (3PB) tests were conducted for Alloy 600 base metal and Alloy 182, respectively, to examine the dependency of the lower DH content than currently controlled band on PWSCC initiation time.  Test results showed the advantage of lowering DH content down to around 5 ml STP/kg H₂O compared to 15~25 ml STP/kg H₂O. Analyses results of oxide and deposits on the surface of the test specimens were also shown to try to clarify how hydrogen interacts with nickel-base alloys and influences the propensity to PWSCC.  Based on the analyses results of surface oxide and deposits, an outline of the interface region was drawn at low and intermediate to high DH content.

Current status of the total studies on DH optimization and challenges left were summarized.