Included for comparison, Figure D.18 shows the calculated flaw rates (non-through wall). Listed in Table D.10 are the RR and FW weld failure rates that represent the state-of-knowledge at T = 15 years of operation. Listed in Table D.11 are the RC and HPI/NMU prior weld failure rates. The failure rates represent the frequency per weld-year of a through-wall flaw resulting in a leakage of less than or equal to the Technical Specification limit for unidentified leakage. In summary, the derivation of prior weld failure rates includes the following steps:

• Determine the number of through-wall leaks from PIPExp database. Includes performing a trend analysis.

• From the PIPExp database, determine the appropriate exposure terms.

• Establish the susceptibility of different weld locations to degradation.

• Combine the output from the previous steps to determine prior failure rates applicable to welds in the RR and FW systems.

1.00E-02

1.00E-06

Figure D.18 Prior Frequency of Non-Through Wall IGSCC in RR Piping