ML24365A080

Clinch River CP Saf Docs - Transmittal of PRA Risk Insights Clarification Questions

ML24365A080
Person / Time
Site:	99902056
Issue date:	12/30/2024
From:	NRC
To:	NRC/NRR/DNRL
References
Download: ML24365A080 (6)

Text

Final Clarification Questions on PRA Insights for Discussion with TVA/GEH December 30, 2024 Risk Insights Questions

1. In developing risk insights to support the construction permit review, staff has identified several instances where information was not provided on the portal. While staff can develop a preliminary list of risk insights with the information provided, the information requested below or an understanding of when the information below will be available, will allow staff to contextualize the completeness of the list of risk insights that will be available to support the construction permit review. The following questions are related to the timeframe for these analyses:

Regarding the GEH PRA maturity, in the Probabilistic Safety Assessment Summary, Table 5-3 Internal Hazard Quantitative Screening Analysis the line item Heavy Loads Drop states that it is to be covered by the internal events PSA in the future. It appears that heavy loads drop has been analyzed in for the low power and shutdown PSA. Is Table 5-3 referring to heavy load drops for the at power internal events PSA? If so, when will it be available for review?

In the Scoping Seismic PSA for the BWRX-300 (Audit Document 006N7493), the "Executive Summary" section (Page 7) it states that The report presents the initial scoping seismic PSA for the BWRX-300 standard plant design. This analysis uses the seismic hazard curve for the Darlington site. When is the site-specific seismic PSA based on a seismic hazard curve expected to be complete?

In GEH 008N0504 Revision 1 January 2024 PSA Summary, Table 5-4 List of Assumptions, in the BWRX-300 External Hazard Screening, W03 High Water Level, it states that only a 100-year flood is assumed, which is the frequency of an Anticipated Occupational Occurrence. In addition, the flood is assumed to be gradual. Based on recent flooding experience in North Carolina from Hurricane Helene, external flooding may not be gradual. Does the external flooding analysis consider less frequent floods with greater consequences or external flooding that is not gradual? Is this information available or expected to be provided with the CP application?

In GEH 008N0504 Revision 1 January 2024 PSA Summary, it states that dam failures were qualitatively screened. Is there more discussion regarding the screening of dam failures or will this be provided with a CP application?

The staff could not find the focused PRA results for the RTNSS analysis to determine if SSC functions relied on for at-power and shutdown conditions meet the Commission goals of a core damage frequency (CDF) of less than 1x10-4 each reactor year and a large release frequency (LRF) of less than 1x10-6 each reactor year. Is a focused PRA going to be performed and when will these results be available?

BWRX-300 PSA Model Results and Insights, DBR-0054796, Rev. C, Section 7.5 Inadvertent Open Relief Valve, states RPV overpressure protection valves have been added to the PSA model. There are various design options being considered and these are currently not postulated to contribute to a LOCA initiating event in the PSA. The final option and configuration selected have potential to change this assumption and thus add this as a potential contributor to CDF. Since this element

of the design could have an impact on the PSA event sequences and CDF, will it be completed and factored into the PSA as part of the PSAR application?

BWRX-300 PSA Model Results and Insights, DBR-0054796, Rev. C, Section 5, Discussion of Result and Insights, reports that a top initiating event contributing to CDF is Loss of Plant Pneumatics. The report states Plant Pneumatics supplies motive air to many plant valves including Feedwater and Control Rod Drive Injection.

Loss of the plant pneumatics system fails many of these valves in an undesirable position for PSA success. Loss of the Isolation Condenser System or failure of the reactor to trip (in an ATWS scenario) prevent sufficient heat removal and lead to core damage. The Tennessee Valley Authority, Clinch River Nuclear Site Unit 1, Probabilistic Safety Assessment Summary, 008N0504, Rev. 1, Section 4.2.18, Plant Pneumatic System states There is no detailed PPS design information available at the time of analysis, as such, no detailed system model or documentation notebook is developed. Describe how the point estimate screening value was developed for use in the PSA and discuss the associated uncertainty. Identify if the PSA will provide detailed PPS design information and detailed system modeling at the time of the PSAR application.

2. In the Design Basis Record Design Notes, DBR-0071331, Revision A, BWRX FPIE Level 1 PSA Uncertainty and Sensitivity Analyses, Section 4.2.1, ICS Passive Reliability, it states, without a formal passive reliability assessment for the ICS, it is desired to understand the sensitivity of the BWRX-300 PSA results to the potential increase in ICS functional reliability. Will a passive reliability assessment will be performed that determines the thermal-hydraulic phenomena that are significant to ICS passive reliability?

3. The Isolation Condenser System (ICS): From DBR-005203, Rev. E, PRA System/Functional Reliability Scoping Modeling, it states There is only one function for the ICS in the PSA model. This function is to provide a controlled reactor pressure vessel depressurization and shutdown cooling until the shutdown cooling system is in service.

The steam and feedwater line breaks and excessive loss of coolant accident result in excessive cooldowns so the ICS is not effective and therefore not credited. Confirm that providing makeup water to mitigate a LOCA, even small LOCA, is not a capability of the ICS.

4. GEH 008N0504, Rev. 1 Probabilistic Safety Assessment Summary (page 98 of 175) refers to Modes 3-6 for the BWRX-300 Low Power and Shutdown (LPSD) model, but the document does not define these modes. The staff also reviewed GEH DRAFT NEDO-34024-Revision B-October 2023 on Technical Specifications, but the staff could not find a definition of operational modes in the draft Technical Specifications.

a. Please provide the definition of operational modes used in Technical Specifications.

b. Please define the LPSD modes of operation used for the LPSD PSA model, including any partitioning of the modes for LPSD modeling purposes (for example modes 6-1, 6-2, 6-3) to help the staff understand the difference between modes defined in Technical Specifications versus LPSD modes of operation used to partition an outage into different plant operating states to quantify the LPSD PSA model.

5. In general, plant procedures may require a plant shutdown when a hurricane is approaching and thus, the plant may be in hot or cold shutdown during the hurricane event. The staff did

not identify the following information in the High Wind Scoping PSA for the BWRX-300 (Audit Document 006N7287). What is the approach to address hurricane wind for a given site? Will hurricane only be considered as a shutdown event, or will it also be part of the consideration of the straight wind for the at-power PRA model?

6. The Scoping Seismic PSA for the BWRX-300 (Audit Document 006N7493), Section 9.1 provides quantification results with seismic hazard truncated for annual frequency of exceedance less than 1E-6/yr, in accordance with IAEA-TECDOC-1791, which results in seismic CDF of 5.1E-8/yr. The document also provides in Section 9.2 quantification results without hazard truncation, not following the guideline in IAEA-TECDOC-1791, which results in seismic CDF of 2.8E-7/yr, a significant increase from the case of 1E-6/yr hazard truncation. Clarification would be needed on the approach to be used in developing the SPSA for a CP application.

7. In GEH Level 2 PSA document, 006N7608 Revision B, June 2022, Section 2.1 Mission Time, it states that for active systems, the mission time is 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. For passive systems, the document states that the mission time is 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. In contrast, GEH 008N0504 Revision 1 January 2024 PSA Summary, Section 6.1.4.1.2 Functional Success Criteria states that the mission time of the PRA is 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and does not differentiate between active and passive systems. Please confirm the mission time for a CP application PRA regarding active and passive systems.

8. In the Design Basis Record Design Notes, DBR-0071331, Revision A, BWRX FPIE Level 1 PSA Uncertainty and Sensitivity Analyses, Table 4-8: Sensitivity Results - ICS Passive Reliability, are the results per system or per train?

Case CDF/year Difference Baseline 1.06E-8 n/a Sensitivity 1.3E-8 22.7%

9. The Internal Fire Scoping Evaluation states [t]he Turbine Building has two non-safety-related standby diesel generators, and the two diesel generators are also in separate areas, but the Internal Flood Scoping Evaluation states that [t]he TB also has two non-safety-related standby diesel generators, and the two diesel generators are in a common room. Are the non-safety-related standby diesel generators in separate fire areas (actually different rooms), but considered to be in the same room for a flooding scenario?

Submittal Focused Questions During its review of risk insights, staff identified a non-exhaustive list of potential questions that may come up during the review when a CP is submitted. Responses to these items do not support the TVA risk insights collection and are not requested.

1. In Section 3.1.3, of the High Wind Scoping PSA for the BWRX-300 (Audit Document 006N7387), it is not clear how the tornado hazard frequencies were estimated. How were the frequencies compared to the approach documented in NUREG/CR-4461 which uses a life-line model and a point model and adds the two frequencies for an overall tornado hazard frequency?

2. The results in Section 4 of the High Wind Scoping PSA for the BWRX-300 (Audit Document 006N7387) indicated that the low wind speed bins (bin 1-3) contributed more than 60% of High Wind CDF (about 88% of Straight Wind CDF and 66% of Tornado CDF). How will the PSA address potential overlapping of the wind event category frequencies with the weather-related LOOP category in the internal events model?

3. The Scoping Seismic PSA or the BWRX-300 (Audit Document (006N7493), Table 6-1 (Human Failure Events in the BWRX-300 Seismic Model) provides HEPs for internal events but not for seismic. GEH states, below Table 6-1, that "The methodology FRANX uses to assess HEPs for each ground motion bin is discussed in the FRANX software manual." As the FRANX software manual is not available to the staff, it is suggested that GEH include seismic HEPs for each ground motion bin for staff review when the construction permit application is submitted.

4. In BWRX-300 PSA Model Results and Insights, DBR-0054796, Rev. C, Section 5, Discussion of Result and Insights, the top two initiating events and the top four event sequences contributing to CDF are LOCAs. The only credited source of injection for a LOCA is the CRD purge pumps taking suction from the CST. Failure of either one of the CRD purge pumps leads to CDF. Justify why these systems are not considered safety class 1 or safety class 2 systems for demonstrating high reliability, considering the CRD purge system failure probability to mitigate a LOCA is greater than 1E-1. (The reference is from PRA System/Functional Reliability Scoping Modeling, DBR-0052031, Rev. E, Appendix I: G12-CRD Injection System Notebook,Section I.4.3, System Model Quantification, page 111 -

provided as PSA System Notebooks and System Reliability. The actual reliability specified is 1.01E-1.)

5. In BWRX-300 PSA Model Results and Insights, DBR-0054796, Rev. C, Section 5, Discussion of Result and Insights, it states Containment and RPV Isolation is highly sensitive to its reliability data. The PSA modeling of the system includes the RPV Integrated Isolation Valves which are the primary means of isolating various LOCAs. Therefore, maintaining its current reliability is highly important...High sensitivity will contribute to more operational burdens if the system displays any reliability glitches, or if there is uncertainty associated with the novel implementation of the integrated valves. Providing an alternate method of performing this function would reduce the sensitivity to its reliability. Describe what is meant by the operational burdens and the novel implementation of the integrated

valves. What reliability data is being used and how is it justified considering it is a novel implementation. Discuss the uncertainty associated with the reliability of the valves and provide any sensitivity studies that have been performed.