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 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

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 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