A consideration of scatter in radiation damage trend curves for reactor pressure vessel steel plate materials

The ability to accurately predict irradiation damage in reactor vessel materials is important for assuring structural integrity of vessels. The present irradiation damage trend curves in Regulatory Guide 1.99, Revision 2 were based on statistical regression analyses of an EPRI irradiated steel database containing approximately 151 shift data points for plates and 65 shift data points for welds and the NRC 177 data points database. Due to data scatter, the accuracy of the correlations is characterized by the observed (i.e., actual-predicted) mean residual error which was determined to be 17 F (9 C) for plates and 28 F (16 C) for welds. New data have been gathered since the fundamental database used in developing the original Regulatory Guide 1.99, Rev. 2 correlations. These new data have been included in the EPRI PREP4 database, and hyperbolic tangent curve-fits were performed on all Charpy V-notch data. A calculation of the 30 ft-lb (41 J) shifts was also performed to establish an expanded database for monitoring embrittlement trends. These new shift data values were compared to the Reg. Guide predictions, and mean residual errors calculated for weld and base materials. The results are presented here for comparison to the 2-sigma margin terms used in Regulatory Guide 1.99, Revision 2 which are intended to bound the actual measured data