ML25171A175
| ML25171A175 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 06/20/2025 |
| From: | Samson Lee Plant Licensing Branch IV |
| To: | Lee N Wolf Creek |
| Lee S, 301-415-3168 | |
| References | |
| EPID L-2024-LLA-0170 | |
| Download: ML25171A175 (1) | |
Text
From:
Samson Lee To:
Nathan Lee
Subject:
Wolf Creek TSTF-505 audit questions (APLC, SCPB) dated June 20, 2025 (EPID: L-2024-LLA-0170)
Date:
Friday, June 20, 2025 9:10:00 AM By letters dated December 17, 2024, (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24352A438), Wolf Creek Nuclear Operating Corporation (the licensee) submitted license amendment requests (LARs) to amend the license for Wolf Creek Generating Station (WCGS, Wolf Creek), Renewed Facility Operating License No. NPF-42. The LAR would adopt Technical Specifications Task Force (TSTF) Traveler TSTF-505, Revision 2, Provide Risk-informed Extended Completion Times - RITSTF [Risk-Informed TSTF] Initiative 4b.
The U.S. Nuclear Regulatory Commission (NRC) staff has identified the need for a regulatory audit to examine the licensees non-docketed information with the intent to gain understanding, to verify information, or to identify information that will require docketing to support the basis of the licensing or regulatory decision. The NRC staff issued an audit plan dated April 2, 2025 (ML25090A217). The NRC staff has formulated initial audit discussion questions below for certain technical areas. If time allows, please prepare responses to these questions in advance. It would facilitate the audit discussions, especially if responses can be posted in the online portal as they become available. The NRC staff plans to issue additional audit discussion questions for other technical areas as available to support breakout audit meetings for efficiency.
Wolf Creek TSTF-505 Audit Questions (APLC, SCPB 6-20-2025)
PRA [Probabilistic Risk Assessment] Licensing Branch C (APLC)
APLC-01 In the audit document, Seismic Hazard Analysis to Support Wolf Creek RICT [risk-informed completion time] LAR, the licensee indicates that the baseline seismic core damage frequency (CDF) and large early release frequency (LERF) values presented in table E5-1 of the LAR, used as seismic penalties for the RICT, are based on plant-level fragility parameters (high confidence low probability of failure (HCLPF) peak ground acceleration (PGA) = 0.3g and c = 0.3). These values were derived from fragility evaluations in the licensees unfinished seismic probabilistic risk assessment (SPRA),
which has not undergone independent peer review.
The licensee is requested to:
0.2g and c = 0.4) published in the NRCs Generic Issue 199 (GI-199) report for the seismic penalty factor (ML100270731), or
- b. Provide the completed SPRA fragility evaluation notebook, including the detailed steps used to derive the plant-level HCLPF and c values, along with documentation of independent peer review results for NRC staff review.
APLC-02
In the audit document Seismic Hazard Analysis to Support Wolf Creek RICT LAR, the licensee states, A new SEL [seismic equipment list] was created for a SPRA for WCGS.
While the SPRA SEL does not fully overlap the original individual plant examination of external events (IPEEE) SEL, it encompasses a large number of components that were originally in the IPEEE SEL.
If the licensee chooses option b in response to APLC-Q1, identify the criteria that differentiate the IPEEE SEL and the new SPRA SEL.
APLC-03 of the LAR does not address the incremental risk associated with seismic-induced loss of offsite power (LOOP) that may follow a design basis seismic event. While accident scenarios involving unrecovered LOOP may be partially accounted for in the internal events PRA, demonstrate that the impact of seismic-induced LOOP on the RICT calculations is negligible.
APLC-04 of the LAR states, For high winds events less than 125 MPH, a fragility analysis is performed to calculate a conditional LOOP probability. The fragility analysis uses the product of the transmission line fragility and conditional probability of losing all transmission lines at varying wind speeds to determine the conditional LOOP probabilities.
Further, in the audit document WCNOC-PSA-057 High Winds Fragility Assessment, the licensee indicates that fragility curves were developed for various systems, structures, and components (SSCs).
Clarify the development and use of the fragility curves to calculate conditional LOOP probability for all high wind events less than 125 MPH and for the high winds RICT penalty calculations.
APLC-05 In section 7 of the audit document WCNOC-PSA-057 High Winds Fragility Assessment, the licensee states, It is possible that a wind event in the range of 75-125 mph could affect the plant without causing an initiating event, as demonstrated by operating experience at Wolf Creek.
The licensee is requested to:
- a. Describe and illustrate how high wind events in this range affect the plant without causing an initiating event.
- b. Identify the operating experience that demonstrates the impacts to the plant.
APLC-06 Include the following in enclosure 4 of the LAR, with appropriate explanation, to support the staffs review of the licensees seismic and high wind penalty factors:
- a. Information included in Table 3-1 of the audit document Seismic Hazard Analysis to Support Wolf Creek RICT LAR, or its updated version.
Condensed tables from the audit documents Att1_WCGS HW RICT Pnlty CDF.pdf and Att1_WCGS HW RICT Pnlty LERF.pdf, presented so that the following are clearly included:
Baseline sequence conditional CDF (CCDP) and conditional LERF (CLERP)
Baseline bounding CDF and LERF Relevant values from the high winds model Containment & Plant Systems Branch (SCPB)
SCPB-01 Section 2.3.2, Technical Variations, of the LAR states:
The following variations from the TSTF-505-A template for NUREG-1431 are considered to be technical in nature:
- 4. WCGS TS 3.7.2, Condition A, One MSIV actuator train inoperable. The main steam isolation valve (MSIV) actuators (skid-mounted at the valve) consist of two separate system-medium actuation trains. In the event of a single failure that prevents one entire MSIV actuation train from performing its required function, safety function is preserved by the other operable MSIV actuation train. Therefore, two independent MSIV actuation trains are required to be operable in Modes 1, 2, and 3 to satisfy TS LCO 3.7.4. Since Condition A does not result in a loss of function, it is acceptable to add an RICT to RA
[Required Actions] A.1, Restore MSIV actuator train to OPERABLE status.
- 5. WCGS TS 3.7.2, Condition B, Two MSIV actuator trains inoperable for different MSIVs when the inoperable actuator trains are not in the same separation group. For each MSIV, one actuator train is associated with separation group 4 (yellow), and one actuator train is associated with separation group 1 (red). A single active failure in one power train would not prevent the other power train from functioning. Additionally, the dual-redundant actuator train design ensures that with only one actuator train on each of two affected MSIVs inoperable, each MSIV is still capable of closing on demand. Since Condition B does not result in a loss of function, it is acceptable to add an RICT to RA B.1, Restore one MSIV actuator train to OPERABLE status.
- 6. WCGS TS 3.7.2, Condition C, Two MSIV actuator trains inoperable when the inoperable actuator trains are in the same separation group. For each MSIV, one actuator train is associated with separation group 4 (yellow), and one actuator train is associated with separation group 1 (red). With an actuator train on one MSIV inoperable and an actuator train on an additional MSIV inoperable, the dual-redundant actuator train design for each MSIV ensures that a single inoperable actuator train cannot prevent the affected MSIV(s) from closing on demand. With two actuator trains inoperable in the same separation group, an additional failure (such as the failure of an actuation logic train in the other separation group) could cause both affected MSIVs to fail to close on demand.
The primary design basis events that require MSIV closure are the spectrum of breaks on the secondary side of the steam generator. Of these, the limiting analysis is the Main Steam Line Break (MSLB). In the limiting case, the MSIV is assumed to fail (open). Closure of the faulted loop MSIV does not terminate the break flow from the faulted steam generator, since the limiting break is postulated to be located between the steam generator and the MSIV. However, the faulted loop MSIV would isolate the break from the remainder of the steamline and the other steam generators. If the faulted
loop MSIV fails to close, blowdown from multiple steam generators is prevented by the closure of the corresponding MSIV for each intact steam generator. The MSIV failure on the faulted loop increases the unisolable steamline volume, containing steam which will be released to the containment.
Since Condition C does not result in a loss of function, it is acceptable to add an RICT to RAC.1, Restore one MSIV actuator train to OPERABLE status.
TSTF-505 does not include ConditionsA, B &C of TS3.7.2 for Westinghouse plants.
These Conditions are unique to the U.S. Standardized Nuclear Unit Power Plant System (SNUPPS) plants (i.e., Callaway Plant and Wolf Creek). The NRC staff notes that the front stop Completion Times authorized for the Callaway Plant TSTF-505 implementation are significantly more restrictive than those proposed for the same Conditions at Wolf Creek.
The front stop completion times for ConditionsA, B &C of TS3.7.2 at the two SNUPPS plants are summarized in the following table. The differences are shown in red:
CONDITION REQUIRED ACTION COMPLETION TIME - Wolf Creek Proposed COMPLETION TIME - Callaway Approved TSTF-505 Amendment 229 (ML22301A007)
A. One MSIV actuator train inoperable.
A.1 Restore MSIV actuator train to OPERABLE status.
7 days OR In accordance with the Risk Informed Completion Time Program
72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> OR In accordance with the Risk Informed Completion Time Program
B. Two MSIV actuator trains inoperable for different MSIVs when the inoperable actuator trains are not in the same separation group.
B.1 Restore one MSIV actuator train to OPERABLE status.
72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> OR In accordance with the Risk Informed Completion Time Program 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> OR In accordance with the Risk Informed Completion Time Program C. Two MSIV actuator trains inoperable when the inoperable actuator trains are in the same separation group.
C.1 Restore one MSIV actuator train to OPERABLE status.
24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> OR In accordance with the Risk Informed Completion Time Program 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> OR RICT Not an Option F. One MSIV inoperable in MODE 1.
F.1 Restore MSIV to OPERABLE status.
8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> OR In accordance with the Risk Informed Completion Time 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> OR In accordance with the Risk Informed Completion Time
Program Program
The NRC staff has reviewed the relevant final safety analysis report (FSAR) passages and figures pertaining to the MSIVs as well as the technical specification Bases (TSB3.7.2).
Based on that review, the NRC staff cannot justify these significant deltas. In particular, the proposed change to the Completion Time for CONDITIONC for Wolf Creek per Table E1-2 of the LAR, Example RICT Calculations for WCGS, allows up to a RICT Estimate of 30days. In contrast, the Completion Time for Callaway is 4hours.
Please discuss the bases for Wolf Creeks Completion Times and provide additional justification for these significant deltas between the two U.S. SNUPPS plants.
Docket No. 50-482