05000395/LER-2011-001
| Virgil C. Summer Nuclear Station Unit 1 | |
| Event date: | 05-03-2011 |
|---|---|
| Report date: | 07-01-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3952011001R00 - NRC Website | |
PLANT IDENTIFICATION
Westinghouse - Pressurized Water Reactor EQUIPMENT IDENTIFICATION: XSW1DB, 1DB Switchgear
IDENTIFICATION OF EVENT
On May 3, 2011 at 0514 hours0.00595 days <br />0.143 hours <br />8.498677e-4 weeks <br />1.95577e-4 months <br />, VCSNS personnel identified a violation of 10 CFR 50 Appendix R, Fire Protection Program for Nuclear Power Facilities, Sections III.G.a and III.G.3 during circuit analysis review for transitioning the Fire Protection Program to NFPA 805. Specifically, VCSNS failed to maintain one train of systems free of fire damage, which are necessary to achieve and maintain Hot Shutdown conditions for postulated fires in the Main Control Room (MCR) or Cable Spreading Room (CSR).
EVENT DATE
May 03, 2011 Condition Report CR-11-02299 was generated to address this violation.
REPORT DATE
July 01, 2011
CONDITIONS PRIOR TO EVENT
De-Fueled, 0% Power
DESCRIPTION OF EVENT
On May 3, 2011 at 0514 hours0.00595 days <br />0.143 hours <br />8.498677e-4 weeks <br />1.95577e-4 months <br />, VCSNS personnel identified a violation of 10 CFR 50 Appendix R during circuit analysis review for transitioning the Fire Protection Program to NFPA 805. VCSNS determined that a fire in the MCR, CSR or Control Building (CB) 412 North Chase could result in a hot short that actuates a relay and trips all incoming breakers to the B-train 7.2KVessential electrical bus (XSW1DB), rendering this dedicated safety-related bus inoperable.
The original Appendix R circuit analysis (1982) and re-analysis (1984) identified circuit ESM301XB as being required for safe shutdown, This circuit connects a set of sensing current transformers (CTs) in XSW1DB to an ammeter on the Main Control Board (MCB), and provides over-current sensing for Relay 51BN-1DB (XSW1DB Bus Neutral Over-current Relay). It was recognized that a fire-induced open circuit in this CT circuit could result in damage to, or a fire in, the 1DB switchgear. Therefore, steps were taken to protect the CTs from this open circuit condition as part of the Appendix R re-analysis. However, this analysis and resolution failed to consider the hot-short-to-ground failure mode.
As part of the transition to NFPA 805, a re-analysis of the circuits required to support NFPA 805 performance goals was performed. From this review, it was determined that a fire in CB-15 (Upper Cable Spread Room), CB-17.01 (Control Room) or CB-01.01 (CB412 North Chase) could result in a spurious actuation of Relay 51BN-1DB. This relay actuates Relay 51BX-1DB (XSW1DB Bus Over-current Lockout Relay), which trips and locks-out all incoming breakers to the 1DB switchgear and the feeder breaker for the 480V XSW1DB1 and XSW1DB2 transformers. Tripping of the feeder breaker also trips the incoming main breakers in 480V busses XSW1DB1 and XSW1DB2. The lockout relay also prevents the 'IV EDC incoming breaker from closing and powering the 'IV ESF Train. This condition compromises the ability to use local controls for maintaining power to Train 'If switchgear in the event of a fire in these fire zones, as credited in the Appendix R analysis.
CAUSE OF EVENT
A root cause analysis was conducted to identify the reason a fire in either the MCR, CSR or CB412 North Chase could result in a hot short rendering the B-train essential electrical bus (Bus 1DB) inoperable. The result of this analysis has determined the most likely cause to be human error in Appendix R reviews performed by the vendor. The review of Appendix R circuits was in large part a manual process of evaluation that was prone to human error. Failure to consider a possible hot-short-to-ground for circuit ESM301XB during the original Appendix R analysis and the re-analysis in the early 1980s is indicative of human error. Due to the elapsed time since these errors were made, the exact cause could not be determined.
ANALYSIS OF EVENT
A fire in certain areas of the Control Building could cause a hot-short-to-ground in circuit ESM301XB, which would actuate Relay 51BN-1DB. This relay would subsequently actuate Relay 51BX-1DB to trip all incoming breakers to XSW1DB (offsite power and `13' EDG breaker when the diesel is in the test start mode), which trips the feeder breaker to the 480VAC XSW1DA1 and XSW1DA2 transformers. Tripping of the feeder breaker also trips the incoming main breakers in 480V busses XSW1DB1 and XSW1DB2. This would result in the complete loss of power to the 'IV ESF train until the problem was diagnosed, the relays were reset, and breakers reclosed. Because Fire Emergency Procedures (FEPs) rely on 'B' Train equipment to shut down the plant in the event of a fire in the MCR or CSR, this would have an adverse impact on the ability to safely shut down the plant. The condition is less significant for a postulated fire in the CB412 North Chase because the FEPs rely on 'A' Train equipment.
VCSNS has enforcement discretion for fire issues identified during the transition to NFPA 805 as long as the risk impact of these issues is not 'RED' (Change in Core Damage Frequency (CDF)>1.0E-04/yr) per the guidance in NRC Inspection Manual Chapter 0609, "Significance Determination Process." A PRA Evaluation was conducted to assess the risk significance of both this Appendix R violation and that associated with LER-2011-002-00 to determine if enforcement discretion remains applicable to VCSNS. The results of this evaluation determined that the total CDF increase for both events is below the 'RED' threshold of 1.0E-04/yr, which complies with the requirements for enforcement discretion.
CORRECTIVE ACTIONS
Corrective actions include: 1) roving fire watch patrols were established in the fire zones through which the impacted circuits are routed, and 2) The FEPs were revised to provide guidance for preventing and/or responding to the bus lockout condition. The guidance consists of disabling the XSW1DB Neutral Over-Current relay, confirming that none of the XSW1DB Phase Over-Current Relays have tripped, and resetting and/or verifying the 51BX-1DB relay is not tripped. 3) Extent of Condition Evaluation was performed and identified no other similar conditions. VCSNS will develop a permanent hardware change as part of NFPA 805 implementation.
PRIOR OCCURRENCES
LER-2010-002, "Unanalyzed Condition Due to Wiring Discrepancy in the "B" Emergency Diesel Generator (EDG) Appendix R Isolation Circuitry," identified a conductor that was not removed during the implementation of a design change in 1983. The cause of this deficiency was a design modification implemented in 1983 that did not contain sufficient information to remove the conductor. The circumstances surrounding this deficiency are not similar to the condition described in this LER.
10 CFR PART 21 REPORTING REQUIREMENTS The following information is provided to meet the requirements of 10 CFR 21.21(d)(4)(i)-(viii). The hot short scenario was not considered during the Appendix R analysis and is considered to be a defect in the circuit design. The lockout of XSW1DB has been determined to be a Significant Safety Hazard (SSH) since it would adversely impact the ability to safely shut down the plant in the event of a fire in any of the three affected areas.
(i) Name and address of the individual or individuals informing the Commission.
Dan Gatlin, V.P. Nuclear Operations SCE&G Co.
HWY 215 Bradham Blvd.
PO Box 88 Jenkinsville, SC 29065 (ii) Identification of the facility and the activity supplied for such facility or such activity within the United States which fails to comply or contains a defect.
Activity which contains a defect:
Appendix R design analysis pertaining to the unanalyzed condition described in this Licensee Event Report.
(iii) Identification of the firm supplying the activity which fails to comply or contains a defect:
Gilbert & Associates, Inc. (now Worley Parsons) supplied the described activity.
(iv) Nature of the defect or failure to comply and the safety hazard which is created or could be created by such defect or failure to comply.
See Description of Event section of this event notification.
(v) The date on which the information of such defect or failure to comply was obtained:
SCE&G completed the Part 21 evaluation on June 27, 2011.
(vi) In the case of a basic component which contains a defect or fails to comply, the number and location of all such components in use at, supplied for, or being supplied for one or more facilities or activities subject to the regulations in this part.
V. C. Summer is a single unit facility. Worley/Parsons is investigating the potential for similar Appendix R analyses performed for other licensees.
(vii) The corrective action which has been, is being, or will be taken; the name of the individual or organization responsible for the action; and the length of time that has been or will be taken to complete the action.
See Corrective Actions section above.
(viii) Any advice related to the defect or failure to comply about the facility, activity, or basic component that has been, is being, or will be given to purchasers or licensees.
Licensees whose Appendix R analyses were performed by Gilbert/Commonwealth are advised to contact WorleyParsons concerning the existence of a similar condition.