Risk-Informing In-Vessel Effects Paul Stevenson - Westinghouse for PWROG

Inputs to In-Vessel Effects LOCA frequency Debris generation Debris transport Strainer bypass Chemical effects Boric acid precipitation

Approaches Screening approach STPNOC approach Deterministically screen as many scenarios as possible Evaluate the risk of the remaining scenarios conservatively assuming they lead to CDF STPNOC approach Use the applicable portions of the STPNOC RI process

Screening Approach Initial simplification Assume chemical effects are completely addressed in the fuel debris loading and results are acceptable Assume boric acid precipitation is deterministically screened Review impact on the approach if the above assumptions are not correct

Screening Approach Screen piping 2” or less NEI 04-07 Screen based on debris generated Consideration If prior sump strainer bypass testing was conservative consider repeating test for less conservative results Identify fuel debris loading limits PA-SEE-1090

Screening Approach Screen based on debris generated (cont) Identify break size that exceeds your limit Use current plant method for Debris generation Debris transport Sump strainer bypass Remember to also apply that break size on larger diameter piping

Screening Approach Screen based on debris generated (cont) Identify LOCA frequency for the break size that exceeds the limit Based on NUREG-1829 Represents the frequency for the plant Need to be near or below 1E-06 / yr 8.5” break size frequency is ~8.9E-07 /yr 6” break size frequency is ~2.4E-06 / yr 3” break size frequency is ~1.6E-05 / yr

Screening Approach Screen based on debris generated (cont) Considerations The larger the break size (that does not exceed the limit) the greater the margin Difficulty of the calculation for debris generation NRC questioned the use of geometric vs arithmetic mean values

Screening Approach If possible consider screening hot leg piping LOCA analysis PA-SEE-1090 will look at large break

Screening Approach Evaluate risk If the only screen used is debris generated LOCA frequency equals CDF Estimate the risk Modify PRA model Use PRA to calculate CDF and LERF with and without fiber insulation Compare to RG 1.174 criteria

Screening Approach Evaluate risk (cont) If hot leg screening is used Start with LOCA frequency identified from the debris generated Reduce frequency based on screening hot leg breaks Partition based on welds Apply weighting factor for degradation mechanisms, if necessary Estimate the risk

Screening Approach Consider chemical effects Chemical effects lead to unacceptable fuel debris loading limits Identify mitigation features that lead to acceptable fuel debris loading limits Switchover to hot leg recirculation before precipitation occurs Controlling temperature (slower cooldown)

Screening Approach Consider chemical effects (cont) Mitigation provides success but need to consider failure of mitigation Estimate the probability of failure of the mitigation feature(s) and apply to the applicable frequencies Estimate the risk

Screening Approach Consider boric acid precipitation Identify those scenarios where boric acid precipitation cannot be screened Conservatively add the frequency of these scenarios to the previously calculated frequency - don’t double count scenarios Estimate the risk

Screening Approach Consider boric acid precipitation (cont) Identify features that mitigate effects Provides success path Mitigation provides success but need to consider failure of mitigation Estimate the probability of failure of the mitigation feature(s) and apply to the applicable frequencies Estimate the risk

Screening Approach Consider time dependency Currently assume all debris bypassing the sump strainer ends up in the core Some of the water/debris goes out the break or through the containment sprays This debris then passes through the sump strainer again where more gets filtered Therefore less debris reaches the core A time dependency factor could be applied

STPNOC Approach Unable to get acceptable results using the screening approach Can use the applicable portions of the STPNOC RI process May be more beneficial to use a total risk-informed approach