A limited scope benchmarking exercise was performed to compare predicted weld failure rates with the reported service experience. The benchmarking was limited to NPS12 BWR reactor recirculation welds susceptible to IGSCC. Probabilistic fracture mechanics (PFM) calculations using the WinPRAISE computer code generated predictions about the weld failure rate for different assumptions about the normal operating stresses (cNO).18 Bayesian reliability analysis was used to derive weld failure rates from service experience data. Figure D.50 shows the results of the benchmarking exercise. Table D.28 includes a description of the different cases of the benchmarking exercise.

D.7.4 Comparison to Historical LOCA Frequency Estimates

Figures D.51 and D.52 compare the Base Case results to historical LOCA frequency estimates. Direct (one — to-one) comparisons are not feasible due to different LOCA definitions and estimation approaches. Listed below are the selected BWR and PWR LOCA frequency references.

BWR Large (> Cat3) LOCA Frequency Estimates (Figure D. 51)

• SKI 98:30 (FW/RR); the displayed value range is taken from Reference [D.18]. It excludes contribution from thermal fatigue in Code Class 1 feedwater system piping. The feedwater system design is unique to the pilot plant in SKI Report 98:30 and it is therefore not applicable to BWR-2.

• NUREG/CR-5750 (Appendix J) provides recommended pipe LOCA frequencies. The given value range accounts for all Code Class 1 pipe failure contributions.

• GRS-98 is a probabilistic safety assessment of the German plant Gundremmingen; a BWR plant designed and built by Kraftwerk Union. This reactor design has no external recirculation loops; the given LOCA frequency value range accounts only for contributions from Code Class 1 feedwater pipe failure.

• BFN-1 (NUREG/CR-2802) is the 1982 probabilistic safety assessment of Browns Ferry Unit 1 performed as part of the NRC-sponsored Interim Reliability Evaluation Program. The given LOCA frequency value range accounts for Reactor Recirculation pump suction piping failure.

PWR Large (> Cat3) LOCA Frequency Estimates (Figure D. 52)

• NUREG/CR-5750 (Appendix J) provides recommended pipe LOCA frequencies. The given value range accounts for all Code Class 1 pipe failure contributions.

• Surry-1 (1990 Expert Elicitation). Surry-1 is a 3-loop Westinghouse reactor, similar to the PWR- 1/PWR-2 reference design. The given LOCA frequency value range applies to RCS pipe failure and resulted from a NRC-sponsored expert elicitation.[13]

• EPRI TR-100380 (Piping Failures in U. S. Commercial Nuclear Power Plants, 1992) includes recommended BWR and PWR LOCA frequencies that are based on statistical analysis of service data. The given LOCA frequency value range accounts for all Code Class 1 pipe failure contributions.