Recent experience in pressure vessel materials irradiation

The reactor pressure vessel has been repeatedly cited as a primary concern in assessment of pressure boundary structural integrity and in planning for plant life extension programs. The life of the reactor pressure vessel will be limited by radiation-induced embrittlement; this is monitored in Westinghouse designed nuclear steam supply systems by testing samples of base metal, heat-effect-zone and weld metal in the form of Charpy V-notch, tensile, and fracture mechanics specimens which have been irradiated in surveillance capsules adjacent to the wall. The earliest reactor vessel material radiation surveillance program was the Yankee Rowe program which started in 1961. As data became available from power reactor surveillance and test reactor programs, estimates of radiation-induced changes in mechanical properties were predicted in the form of radiation damage trend curves which provide methods for calculating numerical estimates of changes in mechanical properties as a function of chemistry and fluence. For example, the proposed Revision 2 to Regulatory Guide 1.99 provides estimates of shifts in transition temperature as a function of copper and nickel content and fluence. Slight variations in chemical analyses for copper and/or nickel can result in limitation on heat-up and cool-down rates or compliance with regulatory rules, such as the PTS screening criteria. Automatic submerged arc welding was employed in the fabrication of reactor vessels in Westinghouse designed nuclear steam supply systems. The type of flux material utilized in the welding process is important because mechanical properties can differ depending upon what flux is used. This paper correlates the results from over 50 surveillance capsules with the welding practice and concludes that radiation damage trend curves can be developed for welding practice. By using trend curves based on welding practices, discrepancies in chemical analyses are eliminated and credibility is restored to structural integrity assessments.