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Risk-Informing In-Vessel Effects

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

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Risk-Informing In-Vessel Effects

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

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

  3. Approaches • Screening 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

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

  10. 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

  11. 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

  12. 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)

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

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