IR 05000395/2026092
| ML26082A236 | |
| Person / Time | |
|---|---|
| Site: | Summer  (NPF-012) |
| Issue date: | 03/31/2026 |
| From: | Mark Franke Division of Operating Reactors |
| To: | Carr E Dominion Energy |
| Shared Package | |
| IR 2026092 | List: |
| References | |
| IR 2026092 | |
| Download: ML26082A236 (0) | |
Text
SUBJECT:
VIRGIL C. SUMMER - NRC INSPECTION REPORT 05000395/2026092; PRELIMINARY WHITE FINDING
Dear Eric S. Carr:
The enclosed inspection report documents a finding with an associated violation that the U.S. Nuclear Regulatory Commission (NRC) has preliminarily determined to be of low safety significance (i.e., a White finding). The finding involved a failure to properly pre-plan and perform maintenance on the turbine-driven emergency feedwater (TDEFW) pump governor valve linkage. We assessed the significance of the finding using NRCs significance determination process (SDP) and the best available information. The attachment to the inspection report contains a detailed risk evaluation with the basis of our preliminary significance determination. The finding is also an apparent violation of NRC requirements and is being considered for escalated enforcement action in accordance with the Enforcement Policy, which can be found on the NRCs Web site at http://www.nrc.gov/about-nrc/regulatory/enforcement/enforce-pol.html.
In accordance with NRC Inspection Manual Chapter 0609, we intend to complete our evaluation using the best available information and issue our final determination of safety significance within 90 days of the date of this letter. The NRCs SDP is designed to encourage an open dialogue between your staff and the NRC; however, neither the dialogue nor the written information you provide should affect the timeliness of our final determination.
Before we make a final decision on this matter, we are providing you with an opportunity to (1) attend a Regulatory Conference where you can present to the NRC your perspective on the facts and assumptions the NRC used to arrive at the finding and assess its significance, or (2) submit your position on the finding to the NRC in writing. If you request a Regulatory Conference, it should be held within 40 days of receipt of this letter, and we encourage you to submit supporting documentation at least one week prior to the conference in an effort to make the conference more efficient and effective. The focus of the Regulatory Conference is to discuss the significance of the finding and not necessarily the root cause(s) or corrective action(s) associated with the finding. If a Regulatory Conference is held, it will be open for public observation. If you decide to submit only a written response, such submittal should be sent to the NRC within 40 days of your receipt of this letter. If you decline to request a Regulatory March 31, 2026 Conference or to submit a written response, you relinquish your right to appeal the final SDP determination, in that by not doing either, you fail to meet the appeal requirements stated in the Prerequisite and Limitation sections of Attachment 2 of NRC Inspection Manual Chapter 0609.
If you choose to send a response, it should be clearly marked as a "Response to Apparent Violation; (EAF-RII-2026-0008)" and should include for the apparent violation: (1) the reason for the apparent violation or, if contested, the basis for disputing the apparent violation; (2) the corrective steps that have been taken and the results achieved; (3) the corrective steps that will be taken; and (4) the date when full compliance will be achieved. Your response should be submitted under oath or affirmation and may reference or include previously docketed correspondence, if the correspondence adequately addresses the required response.
Additionally, your response should be sent to the U.S. Nuclear Regulatory Commission, ATTN:
Document Control Center, Washington, DC 20555-0001 with a copy to Matthew Fannon, Chief, Projects Branch 2, U.S. Nuclear Regulatory Commission, Region 2, 245 Peachtree Center Avenue N.E, Suite 1200, Atlanta, GA 30303-1200 within 40 days of the date of this letter. If an adequate response is not received within the time specified or an extension of time has not been granted by the NRC, the NRC will proceed with its enforcement decision or schedule a Regulatory Conference.
Please contact Adam Wilson at 404-997-4574 and in writing within 10 days of the issue date of this letter to notify the NRC of your intentions. If we have not heard from you within 10 days, we will continue with our significance determination and enforcement decision. The final resolution of this matter will be conveyed in separate correspondence.
Because the NRC has not made a final determination in this matter, no Notice of Violation is being issued for this inspection finding at this time. In addition, please be advised that the number and characterization of the apparent violation described in the enclosed inspection report may change as a result of further NRC review.
The NRC has received your initial engineering evaluation of the TDEFW pumps functionality during a steam generator injection event which concluded that the TDEFW pump would likely have started successfully despite the degraded governor linkage observed in November 2025.
This conclusion was based on the expectation that higher brake horsepower and steam demand during an injection event would hold the governor valve more open and reduce the chance of an overspeed trip. The pumps successful actuation during the October 14, 2025, transient was also cited as supporting evidence.
The NRC preliminarily determined that the initial evaluation did not sufficiently demonstrate pump functionality due to several uncertainties, including unquantified governor valve travel and overspeed margin, inability to characterize early-stroke binding or stick-slip behavior, and limited as-found information to support alternative probabilistic treatment. As a result, the NRC could not validate the conclusion that the pump would have been functional during an actual transient.
The NRC has received your follow-up response addressing concerns with the initial assessment. The NRC will review the updated information you provided, along with any additional submittals, before reaching a final determination regarding the significance of this finding. For administrative purposes, this inspection report provides an update to the apparent violation documented in NRC inspection report 05000395/2025004, dated February 3, 2026, and accessible at http://www.nrc.gov/reading-rm/adams.html via Accession Number ML26030A039.
This letter, its enclosure, and your response (if any) will be made available for public inspection and copying at http://www.nrc.gov/reading-rm/adams.html and at the NRC Public Document Room in accordance with Title 10 of the Code of Federal Regulations 2.390, Public Inspections, Exemptions, Requests for Withholding.
Sincerely, Mark E. Franke, Director Division of Operating Reactors Safety Docket No. 05000395 License No. NPF-12
Enclosure:
Inspection Report No.05000395/20260092
Inspection Report
Docket Number:
05000395
License Number:
Report Number:
Enterprise Identifier:
I-2026-092-0000
Licensee:
Dominion Energy South Carolina
Facility:
Virgil C. Summer Nuclear Plant
Location:
Jenkinsville, SC
Inspection Dates:
November 12, 2025, to March 18, 2026
Inspectors:
K. Dials, Resident Inspector
M. Read, Senior Resident Inspector
S. Sandal, Senior Reactor Analyst
Approved By:
Mark E. Franke, Director,
Division of Operating Reactor Safety
SUMMARY
The U.S. Nuclear Regulatory Commission (NRC) continued monitoring the licensees
performance by conducting an NRC inspection at Virgil C. Summer Nuclear Plant, in
accordance with the Reactor Oversight Process. The Reactor Oversight Process is the NRCs
program for overseeing the safe operation of commercial nuclear power reactors. Refer to
https://www.nrc.gov/reactors/operating/oversight.html for more information.
List of Findings and Violations
Inadequate Maintenance Strategy Resulting in Inoperability of the Turbine-Driven Emergency
Feedwater Pump
Cornerstone
Significance
Cross-Cutting
Aspect
Report
Section
Mitigating
Systems
Preliminary White
Open
EAF-RII-2026-0008
None (NPP)
A self-revealed apparent violation of Technical Specifications (TS) 6.8.1, Procedures and
Programs, was identified when the licensee failed to schedule and implement a preventive
maintenance procedure appropriate to the circumstances for the governor linkage for the
turbine-driven emergency feedwater (TDEFW) pump, which resulted in the inoperability and
unplanned unavailability of the pump.
Additional Tracking Items
None.
INSPECTION RESULTS
Inadequate Maintenance Strategy Resulting in Inoperability of the Turbine-Driven Emergency
Feedwater Pump
Cornerstone
Significance
Cross-Cutting
Aspect
Report
Section
Mitigating
Systems
Preliminary White
Open
EAF-RII-2026-0008
None (NPP)
A self-revealed apparent violation of TS 6.8.1, Procedures and Programs, was identified
when the licensee failed to schedule and implement a preventive maintenance procedure
appropriate to the circumstances for the governor linkage for the TDEFW pump, which
resulted in the inoperability and unplanned unavailability of the pump.
Description: On November 12, 2025, during routine surveillance testing of the TDEFW pump,
the turbine tripped during its initial start. During troubleshooting, the licensee identified worn
components on the turbine speed control governor valve assembly, specifically, the valve
stem (burr at a bushing), valve stem connector (shoulder worn and pin end worn), cam crank
(gouge from valve stem connector shoulder), and cam crank support (anti-rotation slot very
worn and rough) were degraded and resulted in binding while closing the governor
valve. Inspectors assessed the degradation of each subcomponent of the assembly to
estimate the impact on the governor operation.
The governor valve stem is connected to the valve stem connector and kept aligned through
two carbon bushings. The valve stem was found to have a minor burr which contributed to
binding during full open to full closed travel. Inspectors determined that this was a
contributing cause to the responsiveness of the governor, but the output of the governor
should have been able to overcome the resistance absent additional issues. The licensee
deburred the valve stem and restored the smooth operation through the bushing.
The valve stem connector is attached to the stem and rides in the cam plate using a cam
follower, large washer, and lock nut. The valve stem connector has a cylindrical end pin that
extends into the fixed cam crank support anti-rotation slot. The alignment of the cam crank,
valve stem connector, washer, and anti-rotation slot keep the valve stem from rotating and
keep the valve stem connector perpendicular to the cam crank during valve operation. The
squared shoulder of the valve stem connector was worn from rotating into the cam crank
during years of operation. The cam plate had gouges from the rotation of the valve stem
connector shoulder which were located near the full open position of the cam plate slot.
Inspectors determined that the gouging was the most significant cause of binding during
pump start on November 12, 2025. The licensee replaced the valve stem connector and cam
crank.
The rotation of the valve stem connector degraded both the valve stem connector and cam
crank, and inspectors determined that this rotation was caused by the worn anti-rotation slot
of the cam crank support and worn valve stem pin end. The bottom of the anti-rotation slot
was worn significantly worse than the top of the slot due to the force of the governor
assembly output.
Inspectors reviewed historical maintenance activities on the linkage between the governor
and governor valve. Licensee procedure MMP-300.015, Turbine Maintenance, Emergency
Feedwater Pump TPP0008, Revision 19, Section 7.4, required during governor valve
maintenance, Examine linkage for wear and proper operation by manually stroking linkage to
full open. Additionally, step 7.4.2 specifically required examining the radius faces of cam
crank clevis for wear and even contact with surfaces. There were no additional procedure
requirements to inspect linkage subcomponents. Section 7.17 for governor linkage
adjustment did not require inspection for excessive play or degraded subcomponents.
Inspectors noted that the last governor valve preventive maintenance using Section 7.4 was
performed in May 2017 (Work Order 88101005471). Inspectors noted that Section 7.4 for
valve maintenance was performed in 2008, 2011, and 2017, and was scheduled for 2023, but
the 2023 work order was deferred to 2027. The inspectors determined that the licensee did
not provide procedural steps to identify excessive play or worn subcomponents in the
governor valve linkage except during governor valve maintenance activities. Further, the
inspectors determined that governor valve maintenance was last performed in 2017 and
therefore inspections for degradation of the governor valve linkage were not scheduled
appropriate to the circumstances.
Corrective Actions: The licensee tuned the governor, replaced components on the governor
valve and linkage, and smoothed edges of the anti-rotation slot.
Corrective Action References: Condition Report (CR) 1306541 and CR1306755
Performance Assessment:
Performance Deficiency: The failure to properly pre-plan and perform maintenance that can
affect the performance of safety-related equipment in accordance with written procedures,
documented instructions, or drawings appropriate for the circumstances, was a performance
deficiency. Specifically, the licensees failure to perform preventive maintenance appropriate
to the circumstances on the governor valve linkage of the TDEFW pump was a performance
deficiency that was reasonably within the licensees ability to foresee and correct.
Screening: The inspectors determined the performance deficiency was more than minor
because it was associated with the Equipment Performance attribute of the Mitigating
Systems cornerstone and adversely affected the cornerstone objective to ensure the
availability, reliability, and capability of systems that respond to initiating events to prevent
undesirable consequences. Specifically, the TDEFW pump tripped during testing, rendering
the pump inoperable.
Significance: The inspectors assessed the significance of the finding using IMC 0609,
Appendix AProperty "Inspection Manual Chapter" (as page type) with input value "NRC Inspection Manual 0609,</br></br>Appendix A" contains invalid characters or is incomplete and therefore can cause unexpected results during a query or annotation process., The Significance Determination Process for Findings At-Power. The affected
cornerstone was Mitigating Systems, as determined by IMC 0609, Attachment 4, Initial
Characterization of Findings. The inspectors screened the performance deficiency using
Exhibit 2 of Appendix A and determined a detailed risk evaluation was required because the
degraded condition represented a loss of the Probabilistic Risk Assessment (PRA) function of
one train of a multi-train TS system for greater than its TS allowed outage time.
A Region II Senior Reactor Analyst performed a detailed risk evaluation. The finding was
preliminarily determined to be of low safety significance (White). The preliminary risk estimate
was obtained by performing a conditional failure analysis of the turbine-driven emergency
feedwater pump using a 27.32-day exposure period. The dominant sequences were
associated with control building fire initiating events accompanied by loss of power to the
7.2kV buses and transfer of the control room when TDEFW pump function was not
available. See Attachment, TURBINE-DRIVEN EMERGENCY FEEDWATER DETAILED
RISK EVALAUTION, for a summary of the preliminary risk determination analysis.
Cross-Cutting Aspect: Not Present Performance. No cross-cutting aspect was assigned to
this finding because the inspectors determined the finding did not reflect present licensee
performance.
Enforcement:
Violation: TS 6.8.1, Procedures and Programs, requires, in part, that written procedures
shall be established, implemented, and maintained covering the activities referenced in the
applicable procedures recommended in Appendix A of Regulatory Guide (RG) 1.33, Quality
Assurance Program Requirements (Operation), Revision 2. Section 9 of RG 1.33,
Procedures for Performing Maintenance, requires, in part, that maintenance that can affect
the performance of safety-related equipment should be properly pre-planned and performed
in accordance with written procedures, documented instructions, or drawings appropriate for
the circumstances.
Contrary to the above, since 2017, the licensee failed to pre-plan or perform preventive
maintenance on the TDEFW pump governor valve linkage using written procedures,
documented instructions, or drawings appropriate for the circumstances, which affected the
performance of this safety-related equipment. Specifically, the licensee did not provide
procedural steps to identify excessive play or worn subcomponents in the governor valve
linkage except during governor valve maintenance activities. Governor valve maintenance
was last performed in 2017 and therefore inspections for degradation of the governor valve
linkage were not scheduled appropriate to the circumstances. As a result, during surveillance
testing on November 12, 2025, the degradation caused sufficient binding to cause an
overspeed trip of the TDEFW pump.
Enforcement Action: This violation is being treated as an apparent violation pending a final
significance (enforcement) determination.
ATTACHMENT: TURBINE-DRIVEN EMERGENCY FEEDWATER DETAILED RISK
EVALAUTION
OVERALL RISK SUMMARY
The V.C. Summer turbine-driven emergency feedwater (TDEFW) pump was rendered
inoperable due to tripping on an overspeed condition caused by wear and binding of the
governor valve mechanical linkage mechanism. A risk evaluation used a 27.32-day exposure
period and estimated an increase in delta-CDF of 1.80E-06/year (consistent with a White
finding). Fire sequences were strongly dominant and made up approximately 92% of the
estimated risk increase. The dominant sequences included control building fire scenarios
accompanied by loss of power to the 7.2kV buses and transfer of the control room when
TDEFW pump function was not available.
EXPOSURE TIME
The last successful operation of the TDEFW pump for a surveillance test occurred on August
21, 2025. The TDEFW pump also successfully started and ran in response to a plant transient
that occurred on October 14, 2025. Following the plant transient the licensee shut down the
plant to modes four and five for a period of 5.68 days to perform repairs on unrelated systems.
The analyst considered the condition exposure start time to be the last successful operation of
the pump that occurred on October 14, 2025. The TDEFW pump was returned to service on
November 16, 2025, following the November 12, 2025, failure. The total time the pump was
inoperable was 27.32 days (including four days of repair time with 5.68 days of exposure time
removed to account for the time the plant was in modes four and five). Since the governor valve
linkage mechanism for the TDEFW pump did not wear while the pump was on standby,
degradation of the linkage mechanism was only expected to occur during pump operation.
Therefore, the total exposure time of 27.32 days was used for the analysis consistent with the
guidance discussed in Section 2.3 of Volume 1 of the Risk Assessment Standardization Project
handbook (ADAMS ML17348A149).
RISK ANALYSIS/CONSIDERATIONS
1.
The TDEFW failure was modeled as fail-to-start (FTS) due to the overspeed trip of the pump
during its start sequence.
2.
Pump recovery was also considered for station blackout (SBO) sequences using a beyond
design basis procedure (BDMG-4.0, Manual Operation of Turbine-Driven Emergency
Feedwater Pump (Includes Abnormal Operation), Revision 2), to disconnect the governor
valve mechanical linkage and control TDEFW pump speed with local manual control of the
trip and throttle valve.
3.
No recovery credit was provided in the risk analysis for operator actions to reset the
overspeed trip mechanism prior to implementing BDMG-4.0 procedures.
4.
The analysis utilized V.C. Summer PRA model fire risk estimates (adjusted to reflect
Integrated Human Event Analysis for Event and Condition Assessment (IDHEAS ECA)
Human Error Probability (HEP) values) from a prior TDEFW pump overspeed trip
mechanism failure (ADAMS ML26033A004) by interpolating the relative position between
the upper and lower Standardized Plant Analysis Risk (SPAR) model fire risk estimates. The
analysis also assumed dominant fire PRA sequences would not be significantly different
than for the previous risk analysis.
5.
Flexible Coping (FLEX) mitigating strategies and equipment were credited in the analysis
using 24-hour PRA mission time. FLEX equipment reliability was modeled using information
contained in PWROG-18042-NP, Revision 1, FLEX Equipment Data Collection and
Analysis (ADAMS ML22123A259).
Systems Analysis Program for Hands-On Integrated Reliability Evaluations (SAPHIRE) software
Version 8.2.12 and V.C. Summer SPAR model Version 8.82 were used for the evaluation.
1.
The SPAR model was modified to account for the capability of powering the A-or B-train 7.2
kV engineered safety feature (ESF) bus from the 13.8 kV normally energized section of the
Parr hydro unit switchyard. This offsite power supply is an underground feed via transformer
XTF-5052 that has the capability of providing power to either ESF bus and does not require
manning or start-up of the Parr hydro unit. No credit was provided in the analysis for start-up
and operation of the Parr hydro unit as an alternate source of electrical power. In addition,
the SPAR model was also modified to account for the ability of transformer XTF-31 and
XTF-4 to supply offsite power to the ESF buses. Two new HEP basic events were created to
model operator actions to recover offsite power supplies. The first HEP (ACP-XHE-XM-
PARRHYDRO) estimated the failure to manually align the 13.8 kV offsite source from the
normally energized portion of the Parr hydro switchyard to either the A-or B-train 7.2 kV
ESF bus. The second HEP (ACP-XHE-XA-ALT1DA(B)) estimated the failure to manually
align either XTF-31 to the B-and A-train ESF buses or XTF-4 to the A-and B-train ESF
buses.
2.
Following consultation with Idaho National Labs, the SPAR TDEFW fault tree logic (EFW-
TDP-XPP8) was modified to include basic events representing implementation of operator
procedures to disconnect the governor valve linkage mechanism and recover function of the
pump. No credit was provided in the analysis for an attempt to relatch and start the pump
prior to implementing procedures to disconnect the mechanical linkage.
3.
The TDEFW pump failure likelihood following implementation of procedures to disconnect
the governor valve linkage mechanism from the pump (EFW-TDP-XPP-8-FRESTARTX2)
was estimated using an informed prior Bayesian update based on one failed attempt to start
the pump in one demand. The current data for this update was based on the one demand
and one failure associated with the performance deficiency with any other demands and
failures removed due to the use of the licensee procedure to remove the overspeed trip from
use. The Bayes update yielded an estimate failure likelihood of 8.25E-03 which was used in
the evaluation.
4.
The failure likelihood for operator actions (EFW-TDP-XHE-ABNORMAL) to implement
procedures to disconnect the governor valve linkage mechanism, restart the TDEFW pump
and control steam generator (SG) water level was also estimated using IDHEAS ECA
Version 1.3 and the guidance described in RIL 2024-17. Results were compared with
SPAR-H, which yielded a value of 1.3E-01. The IDHEAS ECA evaluation was considered
best available information due to its increased capability of assessing cognition influencing
factors and timing uncertainty distributions. IDHEAS ECA yielded a failure likelihood of
3.95E-02 which was used in the analysis.
5.
Fire risk was estimated using detailed risk evaluation information from a prior V.C. Summer
TDEFW pump failure due to spurious overspeed trip mechanism actuation (ADAMS
ML26033A004). The analysis utilized the V.C. Summer PRA model fire risk estimates
(adjusted to reflect IDHEAS ECA HEP values) from the prior TDEFW pump failure by
interpolating their relative position between the upper and lower SPAR model fire risk
estimates and adjusting the exposure period from 73 days to 27.32 days. The analyst noted
that although the exposure time was shorter for this analysis (27.32 days vs. 73 days) the
overall risk estimate was higher due to the inability of operators to recover from the failure by
resetting the overspeed trip mechanism before implementing other recovery procedures.
6.
Credit for implementation of procedures to recover the TDEFW pump under SBO plant
conditions were implemented in the SPAR model using event tree post-processing rules to
remove the contribution of recovery terms from cutsets for non-SBO sequences (i.e.,
success of offsite or emergency power sources).
7.
The SPAR model fragilities for the 7.2kV ESF boards were updated with more recent plant-
specific values. Additionally, BIN-4 and BIN-5 seismic events removed from consideration
due to the baseline conditional core damage probability for earthquakes of that magnitude
approaching 1.0.
8.
The following event sequences were used in the evaluation:
Fire
Internal Events
Seismic
Tornado
LARGE EARLY RELEASE FREQUENCY IMPACT
The finding was evaluated in accordance with IMC 0609, Appendix H, Containment Integrity
Significance Determination Process, as a Type A finding. Although the estimated change in
core damage frequency (delta-CDF) was greater than 1E-07/year, the dominant accident
sequences did not involve SG tube rupture or interfacing system loss of coolant accidents.
Therefore, the issue associated with the TDEFW pump would not be expected to be a
significant contributor to an increase in large early release frequency (delta-LERF) risk. Delta-
CDF was determined to be the risk metric of interest for this evaluation.
CALCULATIONS
SAPHIRE condition assessments were performed using the SAPHIRE Events and Conditions
Assessment (ECA) module and setting the TDEFW FTS basic event to true.
Best Estimate:
Model
Event
Sequence
delta-CDP
(27.32 Days)
Fire
1.65E-06*
Internal Events
1.12E-07
Seismic
2.52E-08
High Wind
7.63E-09
Internal Flood
6.59E-11
Tornado
1.78E-11
Total
1.80E-06
- Fire risk was estimated from V.C. Summer fire PRA data by adjusting estimated Incremental
Core Damage Probability (ICDP) using IDHEAS ECA HEP values over the 27.32-day exposure
period and interpolating results from previous risk analysis for overspeed trip mechanism
actuation.
Estimated Fire delta-CDP
Exposure
Period
(days)
Lower
Bound
Best
Estimate
Upper
Bound
Prior Analysis
Estimated Fire Risk
5.80E-07
1.31E-06
1.78E-05
Current Analysis
Interpolated Fire Risk
27.32
9.39E-07
1.80E-06
2.13E-05
Internal Events dominant cutsets included reactor trip accompanied by failure of both motor-
driven emergency feedwater pumps and failure to initiate feed and bleed.
Fire dominant contributors using the V.C. Summer fire PRA model included scenarios with loss
of power to the Class 1E 7.2kV buses accompanied by loss of TDEFW pump function and
relocation of control room function.
Sensitivity 1 - No BDMG TDEFW Pump Recovery Credit
To evaluate the sensitivity of analysis results with respect to credit for implementation of SBO
procedures to uncouple the TDEFW governor valve mechanism, restart and control the TDEFW
pump using the trip and throttle valve, the SPAR model was adjusted to remove recovery credit.
Model
Event
Sequence
No BDMG Credit
delta-CDP (27.32
Days)
Fire
1.65E-05*
Internal Events
2.68E-07
Seismic
2.22E-07
High Wind
2.55E-08
Internal Flood
9.81E-10
Tornado
1.26E-10
Total
1.70E-05
IDHEAS ECA HEP values over the 27.32-day exposure period and interpolating results from
previous risk analysis for overspeed trip mechanism actuation.
Sensitivity 2 - BDMG-4 Recovery Human Error Probability Increased to 1.0E-01
The sensitivity of analysis results with respect to HEPs for implementation of BDMG-4
procedures to disconnect the governor valve actuator mechanism, restart the TDEFW pump
and throttle flow to prevent SG overfill were evaluated. Although the same HEP was used for
this analysis as for the last overspeed trip mechanism failure (3.95E-02), the analyst noted that
gouges on the cam plate for the governor valve linkage that were associated with the conditions
being evaluated for this performance deficiency could make it difficult for operators to push the
cam plate up while performing steps to gag the governor valve in the full open position and
affect either the timing of the operator action or the ability to perform that step. This could result
in the previous HEP underestimating the failure likelihood. Therefore, this sensitivity considered
higher likelihoods of operator failure at 1.0E-01 and 1.0 to determine how sensitive the analysis
results would be to uncertainties (increases) in the HEP value.
Model
Event
Sequence
3.95E-02 HEP
delta-CDP
1.00E-01 HEP
delta-CDP
1.00 HEP
delta-CDP
Fire
1.65E-06*
1.71E-06*
1.65E-05*
Internal Events
1.12E-07
1.26E-07
2.68E-07
Seismic
2.52E-08
5.10E-08
2.22E-07
High Wind
7.63E-09
9.43E-09
2.55E-08
Internal Flood
6.59E-11
7.44E-11
9.81E-11
Tornado
1.78E-11
3.71E-11
1.26E-10
Total
1.80E-06
1.89E-06
1.70E-05
IDHEAS ECA HEP values over the 27.32-day exposure period and interpolating results from
previous risk analysis for overspeed trip mechanism actuation.
The analyst assessed the results to determine the overall impact on delta-CDP as a function of
HEP value using a curve-fit analysis.
The analyst noted that the overall impact of HEP values was not influential in the overall results
in the range of 3.95E-02 to 1.00E-01 (approximately 2.5x the value used). However, at HEP
values approaching 6.50E-01 (approximately 16x the value used) the estimated delta-CDP
could approach the 1.0E-05 threshold and begin changing overall conclusions of the analysis.
The analyst determined that additional human reliability analysis to account for uncertainties
would not be expected to increase the HEP enough to alter overall conclusions of this analysis.
Sensitivity 3 - SPAR Model Estimated Fire Risk
SPAR model ECA condition assessments for fire sequences were performed with a 27.32-day
exposure time both with and without credit for alternate sources of AC power to the 7.2kV buses
from the 13.8kV, 115kV and 230kV transformers. Although these sources of power could be
available to mitigate failure of the TDEFW pump, there was uncertainty regarding whether the
SPAR model would correctly account for necessary cables that could be damaged during
specific fire scenarios. This sensitivity established an upper (no credit for alternate AC sources)
and lower (full credit for alternate AC sources) bound for SPAR model fire risk. The SPAR
model produced results at 7.93E-07/year when this offsite power credit was applied and 2.11E-
05/year when it was not applied. Therefore, SPAR model fire risk was highly sensitive to the
application of this credit. The analyst noted that the V.C. Summer fire PRA model estimated
delta-CDP value of 1.65E-06 for a 27.32-day exposure period which was within the SPAR
models upper and lower bounds of fire risk. Additionally, the analyst noted that the SPAR model
fire information had been incorporated into the model more than 10 years ago. Given the SPAR
model uncertainties associated cable fire damage and the age of the information used to
develop the SPAR fire model, the analyst concluded that the V.C. Summer peer-reviewed
NFPA-0805 fire PRA model could be considered best available information for the estimation of
fire risk.
Model
Event
Sequence
SPAR lower bound
Fire Sequences
delta-CDP
SPAR upper bound
Fire Sequences
delta-CDP
Fire
7.93E-07
2.11E-05
Internal Events
1.12E-07
1.12E-07
Seismic
2.52E-08
2.52E-08
High Wind
7.63E-09
7.63E-09
Internal Flood
6.59E-11
6.59E-11
Tornado
1.78E-11
1.78E-11
TOTAL
9.39E-07
2.13E-05
Sensitivity 4 - PRA Fire and Electric Power Research Institute (EPRI) Calculator Human Error
Probability Values
The sensitivity of analysis results with respect to preliminary licensee fire PRA values and HEPs
were evaluated to determine the overall impact on analysis results.
The V.C. Summer PRA utilized two separate basic events for the alternative start of the TDEFW
pump per the beyond design basis procedure (BDMG-4) and control of SG level following the
pump start. The NRCs analysis was for one combined HEP event with two critical tasks for both
the alternative start of the TDEFW pump (critical task 1) and control of the SG level (critical task
2). In the ECA analysis of external events, the combined HEP event was used, and credit was
given for SBO sequences.
The two EPRI HRA calculator HEPs and distribution values were used for this sensitivity and
are identified in the following table. The EPRI HEP values were summed to be consistent with
the combination of those two critical tasks into a single HEP using the IDHEAS application.
Fire estimates were obtained from V.C. Summer preliminary values and EPRI HEPs were used
for the remaining SPAR model sequences.
Model
Event
Sequence
delta-CDP
Fire
8.94E-07
Internal Events
1.06E-07
Model
Event
Sequence
delta-CDP
Seismic
1.52E-08
High Wind
6.64E-09
Internal Flood
4.69E-11
Tornado
1.60E-11
Total
1.02E-06
OVERALL RESULTS
SAPHIRE ECA condition assessments were performed by setting the TDEFW pump FTS basic
event to true over an exposure period of 27.32 days. Interpolated V.C. Summer fire PRA model
results were considered best available information for estimating fire risk contribution and were
strongly dominant in the results. The dominant sequences included control building fire
scenarios accompanied by loss of power to the 7.2kV buses and transfer of the control room
when TDEFW pump function was not available.
The results are summarized below:
EVENT
SEQUENCE
Best
Estimate
delta-CDP
No BDMG
Credit
delta-CDP
delta-CDP
SPAR Fire
Lower/Upper Bound
delta-CDP
PRA Fire &
Values
Fire
1.65E-06*
1.65E-05*
1.71E-06*
7.93E-07 to 2.11E-05
8.94E-07
Internal Events
1.12E-07
2.68E-07
1.26E-07
1.12E-07
1.06E-07
Seismic
2.52E-08
2.22E-07
5.10E-08
2.52E-08
1.52E-08
High Wind
7.63E-09
2.55E-08
9.43E-09
7.63E-09
6.64E-09
Internal Flood
6.59E-11
9.81E-10
7.44E-11
6.59E-11
4.69E-11
Tornado
1.78E-11
1.26E-10
3.71E-11
1.78E-11
1.60E-11
TOTAL
1.80E-06
1.70E-05
1.89E-06
9.39E-07 to 2.13E-05
1.02E-06
IDHEAS ECA HEP values over the 27.32-day exposure period and interpolating results from
previous risk analysis for overspeed trip mechanism actuation.
No BDMG TDEFW Pump Recovery Credit - showed a strongly influential impact to estimated
risk when the BDMG procedure for TDEFW pump recovery was not credited.
BDMG-4 Recovery Human Error Probability - demonstrated that there was not an influential
change in overall estimated risk (i.e., overall analysis conclusions were not changed) based on
the use of HEP values less than approximately 6.50E-01.
SPAR Model Estimated Fire Risk - the SPAR model was very sensitive to consideration of the
sources of power to the 7.2kV switchgear for fire scenarios. The V.C. Summer fire PRA model
yielded results that were within the lower and upper bound established by the sensitivity and
was considered best available information for estimation of fire risk.
PRA Fire and EPRI HEP Values - Fire estimates were obtained from V.C. Summer preliminary
values and EPRI HEPs were used for the remaining SPAR model sequences. Although
interpolation of fire results yielded higher risk estimates for the best estimate case and could be
considered conservative, the analyst noted that the V.C. Summer fire risk remained above the
1E-06 threshold when using more favorable HEP values. Accordingly, the analyst concluded
that the use of interpolated fire risk did not alter the overall conclusions of the analysis.
LICENSEE SIGNIFICANCE EVALUATION
The licensee had not provided a documented evaluation of the risk significance of this failure at
the time of this analysis.
EXTERNAL EVENTS CONSIDERATIONS
Initial internal event risk estimates were greater than 1E-07, therefore all other external event
sequences were evaluated in the risk assessment. Fire event sequences were determined to be
significant contributors to the overall estimated risk.
CONCLUSIONS/RECOMMENDATIONS
The estimated risk increase (delta-CDF) over the nominal case for the inoperability of the
TDEFW pump was 1.80E-06/year, which should be considered a finding of low (White)
significance.