ML24312A053
Jump to navigation
Jump to search
| ML24312A053 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 11/07/2024 |
| From: | Shane Sandal NRC/RGN-II/DORS |
| To: | |
| References | |
| Download: ML24312A053 (12) | |
Text
Significance Determination Process Risk Evaluation for Watts Bar Unit 1 and 2 HP FLEX Makeup Fittings November 7, 2024 Shane Sandal Senior Reactor Analyst Region II, Division of Operating Reactor Safety Technical Support Assessment Branch
2 Condition Summary
- Watts Bar HP FLEX RCS makeup pump functionality was adversely impacted by the unavailability of fittings required to connect and align the pumps for operation
- Required detailed evaluation of the capability of mitigating strategies to ensure RCS makeup flow during ELAP events (for both flood and non-flood event initiators)
- Exposure period of 0.627 years (T/2)
3 Detailed Risk Evaluation
- Required evaluation of the likelihood (frequency) of precipitation events that could result in external flood/ELAP plant conditions
- Required adjustments to default SPAR model ELAP event sequences to match site-specific FLEX design and implementation
4 Flood Frequency Estimation
- Frequency determination had similarities to previous external flood DRE performed for Watts Bar in 2012 (ML13071A298).
- Two basic storm situations were found to have the potential to produce maximum flood levels at Watts Bar. These were:
- 1) a sequence of storms on the watershed above Chattanooga, and
- 2) a sequence of storms in the basin below major tributary dams
5 Flood Frequency Estimation
- Determined a location of rainfall in the watershed area of concern from external flood design basis information.
- NOAA Precipitation Frequency Data Server (PFDS) was used to obtain the frequencies of rainfall that would be indicative of a storm that could produce subject external flood conditions at Watt Bar.
6 Flood Frequency Estimation
- The data for 9-day durations were interpolated from the data, and then the frequency of a 22.17 9-day rainfall event (total of 6 antecedent storm and 16.17 PMP event) was extrapolated.
- Because there were two separate storm events within the watershed that could result in the external flooding event of concern, the resulting estimated storm frequency was doubled to account for the independent likelihood of either storm.
7 Flood Frequency Estimation
8 Flood Frequency Sensitivity
- To evaluate the sensitivity of the analysis results with respect to external flood event frequency, the application of NOOA derived precipitation event likelihoods were calculated at the 95% and 5% confidence interval values (e.g., orders of magnitude change in frequencies).
- As expected, the analysis results are highly sensitive (directly proportional) to the external flood event frequency that is used in the evaluation.
9 FLEX Modeling
- INL was consulted to update the SPAR model for external flood and non-flood ELAP sequences to reflect site-specific FLEX equipment and implementation procedures
- External flood events were dominant in the risk estimates
10 Risk Results
- Early/preliminary risk results indicated the potential for greater-than-green risk
- Refinement of the SPAR model to account for site-specific deployment of FLEX strategies reduced the risk estimates
- Availability of non-FLEX alternate charging flow path (above flood elevation) mitigated the estimated risk for flood scenarios
- The capability to power emergency (i.e., normal safety-related) high pressure injection pumps from a large, permanently installed FLEX DG mitigated the estimated risk for non-flood ELAP scenarios
- The DRE concluded the increase in risk was GREEN
11 Important Take-Aways (for all DREs)
- Early and periodic communication between SRA and site PRA risk analysts was important in refinement of the SPAR model
- It was sometimes challenging to address deterministic discussions and explanations when communications involved staff that do not have a PRA background
Questions Shane Sandal Senior Reactor Analyst U.S. NRC (404) 997-4513 shane.sandal@nrc.gov