05000390/LER-2014-002
| Watts Bar Nuclear Plant, Unit 1 | |
| Event date: | 02-11-2014 |
|---|---|
| Report date: | 04-11-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 49818 | 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor, 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident, 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| LER closed by | |
| IR 05000390/2015000 (30 April 2015) | |
| 3902014002R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
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Watts Bar Nuclear Plant, Unit 1 05000390
I. PLANT CONDITIONS
At the time of discovery, Watts Bar Nuclear Plant (WBN) Unit 1 was operating in Mode 1 at 100 percent rated thermal power (RTP).
II. DESCRIPTION OF EVENT
A. Event
On February 11, 2014, Watts Bar Nuclear Plant (WBN) engineering and operations personnel discovered that non-conservative operator manual action (OMA) completion times were incorporated in Appendix R procedures. Preliminary Westinghouse transient analysis calculations of WBN Unit 1 fire protection features revealed that there is less time than previously credited to perform certain OMAs to prevent pressurizer overfill during certain Appendix R fire scenarios.
Specifically, an assumed 10 CFR 50 Appendix R fire in rooms 713.0-A6, 713.0-A28, 737.0-A1A, 757.0-A2, 757.0-A5, 757.0-A9, 772.0-A1, 772.0-A2, or 772.0-A5 could result in spurious operation of multiple components in the normal and emergency charging flow paths. Westinghouse's analysis demonstrated the required time to isolate the normal charging path, secure the second charging pump and isolate the emergency charging path is approximately 12.5 minutes. Watts Bar Unit 1 procedures are non-conservative in that they allow these actions to be completed in 18 minutes.
This event is reportable under 10 CFR 50.73(a)(2)(ii)(B).
B. Inoperable Structures, Components, or Systems that Contributed to the Event None C. Dates and Approximate Times of Occurrences Date Time Event (EDT) 01/14/92 N/A Calculation WBN-OSG4-031 Appendix B for Isolation of Excess charging Flow developed.
02/11/14 0748 Westinghouse provides preliminary analysis results to WA.
02/11/14 1800 TVA confirms analysis applicability to WBN Unit 1.
02/11/14 1928 TVA reports this event in accordance with 10 CFR 50.72 (EN#49818) D. Manufacturer and Model Number of Components that Failed.
There were no failed components associated with this event.
E. Other Systems or Secondary Functions Affected
There were no systems or secondary functions associated with this event.
F. Method of discovery of each Component or System Failure or Procedural Error There were no failed components associated with this event. However, there was a latent design error in a WBN1 fire protection program design calculation that was discovered during a detailed design review of Appendix R scenarios completed for WBN Unit 2 as part of plant licensing.
G. Failure Mode and Effect of Each Failed Component There were no failed components associated with this event.
H. Operator Actions
Operator actions are noted in section II.0 above.
I. Automatically and Manually Initiated Safety System Responses There were no automatic or manually initiated safety system responses associated with this event.
III. CAUSE OF THE EVENT
A. The cause of each component or system failure or personnel error, if known There were no failed components or systems associated with this event. The cause of this event was determined to be a latent design error in an Appendix R related calculation that occurred during the initial design of WBN Unit 1.
B. The cause(s) and circumstances for each human performance related root cause N/A
IV. ANALYSIS OF THE EVENT
Prior to licensing, WBN1 had performed an Appendix R analysis of spurious equipment operation where the consequence of the event is pressurizer overfill. For the rooms identified in Section II above, a fire is assumed to occur in one of the rooms, which then results in equipment failures. The fire is assumed to short equipment control cables routed within the room, resulting in spurious equipment operation.
Electrical cables associated with the charging pumps, the reactor coolant letdown isolation valve, and the emergency core cooling flow path isolation valves from the charging pumps to the reactor coolant system (also called the Boron Injection Tank [BIT] valves) are located in the listed rooms. For the rooms identified, it is assumed that the fire will result in the start of a second charging pump, the isolation of reactor coolant letdown, and the opening of a BIT isolation valves. The net result is that more water will be injected into the reactor coolant system than is being removed, and the pressurizer will begin to fill.
Operator action outside the control room is required to isolate the BIT flow path and the normal charging flow path and to shutoff the second charging pump before the pressurizer becomes water solid. The original analysis determined that with spurious equipment operation, operations personnel would have 18 minutes to isolate plant features to prevent pressurizer overfill. Watts Bar Unit 1 had demonstrated that the required actions to prevent pressurizer overfill could be completed within 15 minutes, and thus the associated OMAs were acceptable.
While performing design reviews in support of WBN Unit 2 licensing, engineering personnel identified that a Chemical and Volume Control System (CVCS) {EIIS: CB} malfunction event was very similar to certain Appendix R fire sequences where high charging flow could occur concurrent with loss of letdown. Since the time available for this event was somewhat less than the Appendix R analysis indicated, TVA contracted Westinghouse to perform a more detailed analysis of the plant's response to these fire events.
The result of those preliminary analyses demonstrated that less time was available to take operator action than previously calculated. Based on this preliminary analysis, this condition was determined to be reportable pursuant to 10 CFR 50.72.
V. ASSESSMENT OF SAFETY CONSEQUENCES
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event The Appendix R event described involves the simultaneous spurious start of a second charging pump, opening of the safety injection flow path from the charging pumps, and the isolation of letdown.
For this event to occur, a fire would need to start in one of the rooms identified in Section II above.
The combustible loading in each of these rooms ranges from low to moderately severe. The fire would then have to be large enough to cause specific control cables to short but not fail catastrophically. No credit is taken for either the detection or suppression that is present in these rooms. While required for consideration, the simultaneous actuation of the affected components remains unlikely.
Operator action will be taken during this event to open a pressurizer Power Operated Relief Valve (PORV) to prevent over-pressurizing the reactor coolant system. This would result in a release of reactor coolant to the Pressurizer Relief Tank (PRT) until excess charging flow is secured, at which point the PORV would not need to be opened to control reactor coolant pressure. Reactor coolant system inventory would be maintained by seal injection, with the reactor head vent valves being used to provide a letdown flow path.
B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident N/A C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from the discovery of the failure until the train was returned to service N/A
VI. CORRECTIVE ACTIONS
The corrective actions for this issue are being tracked within the WBN Corrective Action Program (CAP) under Problem Evaluation Reports (PERs) 845185, 857207, and 852332.
A. Immediate Corrective Actions
Upon determining that operator actions could not be completed in the required time frame to respond to an Appendix R fire in the rooms identified, compensatory Appendix R fire watches were initiated.
B. Corrective Actions to Prevent Recurrence
Modifications will be completed during the WBN 1 Fall 2015 refueling outage that will reduce the number of fire related failures in the rooms identified such that operator action may be taken prior to a pressurizer overfill condition occurring.
To ensure that the Fire Hazards Analysis (FHA) is properly supported, a detailed review of other supporting analyses and evaluations is being conducted for the Watts Bar plant to address the potential generic implications of this event. A similar review will also be performed for TVA's Sequoyah plant. This event was due to a latent design engineering error.
VII. ADDITIONAL INFORMATION
A. Previous similar events at the same plant
On January 13, 2014, WA submitted LER 390/2013-005, "Postulated Fire Induced Failure of Chemical and Volume Control System Centrifugal Charging Pumps." This LER describes an unanalyzed condition where a potential fire induced failure of both Unit 1 Chemical and Volume Control System Centrifugal Charging Pumps (CCPs) could occur due to a fire in the Auxiliary Building. The event described in LER 390/2013-005 involved the same underlying cause of a latent design engineering error during initial plant licensing as the event being reported in this LER.
Because this event is similar to a previous reportable event, additional corrective actions are described above to address this issue.
B. Additional Information
None
C. Safety System Functional Failure Consideration
While this event could have resulted in a sequence where the pressurizer would overfill, no actual equipment failures (other than those resulting from the assumed fire) are expected to occur. In the event that operator action cannot be taken in time to terminate excess charging flow, one or both of the PORVs will lift to prevent reactor coolant system over-pressurization before the code safety valves lift. Once excess charging flow is isolated, the PORV(s) would reclose, or could be isolated by its associated block valve. Accordingly, this is issue does not represent a safety system functional failure.
D. Scrams with Complications Consideration N/A
VIII. COMMITMENTS
None.