ML21316A233
| ML21316A233 | |
| Person / Time | |
|---|---|
| Issue date: | 11/15/2021 |
| From: | Fard M, Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| Hunter, Chris - 301 415 1394 | |
| References | |
| Download: ML21316A233 (12) | |
Text
Status of the U.S.
NRCs ASP Program
Mehdi Reisi Fard and Chris Hunter U.S. Nuclear Regulatory Commission Office of Nuclear Regulatory Research Division of Risk Analysis Performance and Reliability Branch Began evaluating hazards that are not currently included in all NRC SPAR models (internal fires, floods, etc.).
- No events were determined to be precursors based on this additional evaluation in 2020.
Program - Most of the events evaluated were associated with plants that did not have recent risk information provided to the NRC from risk-Status and informed license amendment requests.
Challenges Continue to see decrease in LERs reported.
- Recent experience shows that events that have historically been reported may not be in the future.
- It is a known limitation that some risk significant events may not always be reported in LERs (e.g., single train failures).
- Given these considerations, we are exploring the use of additional information sources for degraded conditions.
November 15-16, 2021 24th PSAEA Technical Meeting 2 Calendar # of LERs # of Potential # of LERs % LERs Year Reviewed Precursors Screened-Out Screened-Out 2016 368 62 306 83%
2017 307 48 259 84%
2018 252 38 214 85%
2019 199 35 164 82%
2020 137 29 108 79%
LER Data
November 15-16, 2021 24th PSAEA Technical Meeting 3 ASP Results and Trends The occurrence rate for total precursors and most precursor subgroups is decreasing for the past decade (2011-2020 ).
- No trends detected for precursors associated with emergency diesel Precursor generator (EDG) failures or those occurring at BWRs.
Trends There is no trend for the risk from all precursors as shown by ASP index.
- The majority (53%) of total risk associated due to precursors is dominated by 6 precursors with CCDP/CDP 10-4.
- The other 114 precursors contribute the remaining 49%.
November 15-16, 2021 24th PSAEA Technical Meeting 5 ASP Program Dashboard
November 15-16, 2021 24th PSAEA Technical Meeting 6 Recent Events High winds (100+ mph) from a derecho resulted in weather-related LOOP in August 2020.
- Offsite power was restored to safety-related buses in approximately 25 hours2.893519e-4 days <br />0.00694 hours <br />4.133598e-5 weeks <br />9.5125e-6 months <br />.
Duane Arnold Some damage occurred to SSCs (e.g., reactor building, FLEX building, cooling towers).
LOOP - FLEX equipment remained available.
Increased debris loading resulted in clogging of the train B essential service water (ESW) strainers.
- ESW flow decreased by 300 gpm; however, EDG remained running with no performance issues observed.
- Clogged strainers were manually bypassed by operators.
November 15-16, 2021 24th PSAEA Technical Meeting 8 The final analysis resulted in mean CCDP of 8x10 -4.
- Dominant accident sequence was the postulated failure of both EDGs resulting in a SBO with the postulated failures of both high-pressure injection sources.
This accident sequence accounts for approximately 35% the total CCDP for this event.
Final ASP 5% Median Pt. Estimate Mean 95%
1x10-4 6x 10-4 6x 10-4 8x 10-4 2x 10-3 Results Key Uncertainty Mean CCDP % Change No FLEX Credit 7.8x10-3 ~+900%
No FLEX Credit for Successful HPCI Scenarios 1.7x10-3 +120%
x2 increase 7.7x10-4 -3%
FLEX Hardware Reliability Multiplier x5 increase 9.3x10-4 +18%
x10 increase 1.2x10-3 +57%
FLEX HEPs x5 increase 1.9x 10-3 +135%
x5 decrease 5.8x 10-4 -27 %
72-Hour AC Power Recovery Requirement 1.1x10-3 +34%
Potential Effects of Bypassing ESW Strainer 1.5x10-3 +89%
November 15-16, 2021 24th PSAEA Technical Meeting 9 High winds, heavy rain, and localized flooding as part of Hurricane Ida resulted in a LOOP on August 29, 2021.
- Reactor was already shutdown (Mode 4) as part of storm preparations.
Waterford - Plant responded as designed.
LOOP During - Offsite power was restored to the safety buses in ~54 hours.
Hurricane Ida SPAR model FLEX modeling was changed based on review of the licensees final integrated plan.
- In addition, similar changes to the ones made for the Duane Arnold ASP analysis were made to the Waterford SPAR model.
November 15-16, 2021 24th PSAEA Technical Meeting 10 Analysis is ongoing; however, there are some key open questions.
- Availability of some FLEX equipment.
Preliminary - Temporary emergency diesel.
Results and Potential Changes
- Determine if the turbine -driven emergency feedwater pump Uncertainties can be credited reach safe/stable end state.
- Is there a need to perform detailed HRA on FLEX operator actions?
It is not believed that the HEPs would change significantly enough to affect the results because the FLEX equipment reliability is the dominant failure contributor.
However, the potential impact cannot be ruled out until an HRA is performed.
November 15-16, 2021 24th PSAEA Technical Meeting 11 The generic SPAR model FLEX structure requires a lot of changes to get a realistic Lessons results.
- Examples Safe/stable end state Learned and Mission time Extended turbine-driven pump operation Remaining - Consensus needs to be gained on these issues within the NRC Questions risk analyst community before making changes to all models.
If/when to perform an HRA for FLEX operator actions?
- If so, what method should be used?
November 15-16, 2021 24th PSAEA Technical Meeting 12