05000333/LER-2015-005
| James A. Fitzpatrick Nuclear Power Plant | |
| Event date: | 9-18-2015 |
|---|---|
| Report date: | 11-17-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| Initial Reporting | |
| ENS 51409 | 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3332015005R00 - NRC Website | |
Background
The Secondary Containment [El IS identifier: NG] is a structure comprised of the Reactor Building that surrounds the primary containment and refuel equipment. Its safety function is designed to provide containment for postulated accident scenarios: loss-of-coolant accident and refueling accident. This structure forms a control volume that serves to hold up and dilute the fission products. Since pressure may increase in Secondary Containment relative to the environmental pressure, it was designed to include a differential pressure vacuum such that external atmosphere would leak into containment rather than fission products leak out.
The systems which maintain a differential pressure vacuum inside Secondary Containment include the normal Reactor Building Ventilation (RBV) system [VA] and the safety-related Standby Gas Treatment (SBGT) system [BH]. During a postulated accident scenario, the normal RBV isolates by closing two intake isolation valves (66A0V-100A and 66A0V-100B) and two exhaust isolation valves (66A0V-101A and 66A0V-101B). At the same time, SBGT initiates in order to filter the Secondary Containment atmosphere prior to releasing to the environment. SBGT has the capacity to maintain the differential pressure vacuum.
During normal operation, the RBV operates with one of two exhaust fans from the refuel floor in service.
Downstream of this exhaust fan is a damper consisting of two interconnected damper sections that are represented by a single designation; 66A0D-106A. Each damper section has an air operator; upper 66A0D- 106A1(0P) and lower 66A0D-106A2(0P). Both air operators must function or 66A0D-106A will not open fully.
A solenoid valve controls the position of 66A0D-106A. When the control circuit for 66A0D-106A receives an open signal, the solenoid valve allows compressed air into the air cylinder of 66A0D-106A1(0P) and 66A0D- 106A2(0P); air pressure overcomes internal spring pressure to open 66A0D-106A. When the solenoid valve gets a signal to close, the solenoid valve releases the air from the air cylinders of 66A0D-106A1(0P) and 66A0D-106A2(0P), allowing the damper to close by force of the internal spring.
66FN-13A is interlocked with 66A0D-106A such that the fan starts only after the damper is full-open. If the damper does not open within a certain time limit, the timer interlock closes the damper. If the damper comes off its open seat (not full-open) then the fan stops and the standby fan receives a start signal.
Damper 66A0D-106A position is provided by two position switches. 66PNS-106A1 is used to determine whether or not the damper is full-open and, 66PNS-106A2 for full-closed. Only 66PNS-106A1 provides the permissive signal for 66FN-13A to start and continue to run. When the Reactor Building is isolated, power is removed from the solenoid valve; this removes air pressure from the damper operators for 66A0D-106A.
Once air pressure is removed, spring pressure will force the air operator in the closed direction and thus close the damper. If the damper is not full-open then the position switch 66PNS-106A1 provides a signal to secure the associated fan 66FN-13A.
Either 66FN-13A or 66FN-13B can be in service, with other fan available in standby to start upon the operating fan tripping off line.
This Licensee Event Report (LER) addresses a malfunction with the 'A' refuel floor damper full-open position switch 66PNS-106A1 concurrent with lower air operator 66A0D-106A2(0P) diaphragm failure.
Event Description
At 1408 on September 18, 2015, when JAF was at 100 percent power, Secondary Containment Refuel Floor exhaust flow from 66FN-13A was degraded and caused Secondary Containment differential pressure to go positive for approximately three minutes. The required differential pressure was restored when alternate exhaust fan was placed in service. A notification to the NRC was made by ENS 51409.
The RBV maintains a fresh air supply in the building using intake and exhaust fans. If the exhaust fan becomes blocked or partially blocked, as it did during this event, then the intake air supply begins to overfill the building. The extra air increases building pressure relative to the outside environment.
The change in differential pressure exceeded the Technical Specification (TS) Surveillance Requirement (SR) the SR the TS Limiting Condition of Operation (LCO) was not met. Therefore, Secondary Containment was Inoperable. Restoration of the LCO was completed within the allowed action completion time of the TS. This Licensee Event Report (LER) is being submitted per 10 CFR 50.73(a)(2)(v)(C) as a condition that could have prevented the fulfillment of safety function to control the release of radioactive material.
Event Analysis
Air operator failure Damper 66A0D-106A needs to be full-open in order for the fan 66FN-13A to exhaust air out of the Reactor Building. The damper is operated by two air operators: 66A0D-106A1(0P) and 66A0D-106A2(0P). Each operator is kept opened by air pressure such, if air supply fails, it will fail shut. A diaphragm retains the air pressure in the operator. Once the diaphragm failed, the operator was forced shut by its internal spring. When 66A0D-106A2(0P) failed, 66A0D-106A did not stay in the full-open position which restricted exhaust flow.
Position Indication Switch Position switch 66PNS-106A1 detects whether 66A0D-106A is full-open. In the event that it is not full-open, the control circuitry should initiate a shutdown of RBV fan 66FN-13A and a start of 66FN-13B. During the events of this LER, the air operator failure and closing of 66A0D-106A did not stop 66FN-13A. The failure to shutdown the 'A' fan when the damper started to close resulted in 66FN-13A continuing to run without exhausting enough air to overcome the amount of air supplied by the Reactor Building Supply.
On August 6, 2014, an automatic swap of the 'A' train to the 'B' train occurred. This was evident when an Operator observed the 66FN-13B exhaust fan running with its control switch Green-flagged and 66FN-13A fan not running with its control switch Red-flagged. The flagged indication represents the manual operation of the fan where Red means running and Green means off. Therefore, at some point after 'A' was manually set to run, it had tripped and the 'B' train initiated to cover the RBV exhaust load. This event doesn't have a control room alarm associated with it. The same event happened again on August 17, 2014.
On August 19, 2014, following the fan trips, troubleshooting involved replacing a portion of the switch assembly for 66PNS-106A1.
Cause
The cause of this event was inadequate position switch setup of 66PNS-106A1 during corrective maintenance that was performed in August 2014. Without the proper position switch setting, the Reactor Building Ventilation damper operator 66A0D-106A2(0P) diaphragm failure and subsequent failure of 66A0D-106A to remain full-open was not automatically detected such that the alternative exhaust fan could be initiated and Secondary Containment differential pressure maintained.
The diaphragm failure of the lower air operator 66A0D-106A2(0P) is the same type of failure which occurred in the upper air operator 66A0D-106A1(0P). The upper operator failure caused the October 28, 2014, Secondary Containment pressure transient, reported in LER-14-002 by letter dated December 19, 2014, JAFP-14-0147. However, in both cases, the position switch 66PNS-106A1 didn't work to swap ventilation exhaust fans. Position switch 66PNS-106A1 had not been identified as a cause in LER-14-002.
External events:
Cooper Nuclear Station: LER 2014-001, Secondary Containment Declared Inoperable due to Rise in Differential Pressure when Operator inadvertently closed exhaust damper (ML14070A363).
Clinton Power Station: LER 2014-001, Damper Solenoid Valve Failure (ML14091A242).
FAILED COMPONENT IDENTIFICATION:
Position Switch Manufacturer: Honeywell Damper Manufacturer: Honeywell Manufacturer Model Number: LSP6B Manufacturer Model Number: MP904B NPRDS Manufacturer Code: H260 NPRDS Manufacturer Code: H260 NPRDS Component Code: ZIS NPRDS Component Code: CDMP FitzPatrick Component ID: 66PNS-106A1 FitzPatrick Component ID: 66A0D-106A2(0P)
Corrective Actions
Completed Actions
- Replaced and adjusted 66PNS-106A1 to indicate and function correctly
- Replace diaphragm in lower operator 66A0D-106A2(0P)
- Replace damper operators for 'B' exhaust train: 66A0D-106B1(0P) and 66A0D-106B2(0P) Future Actions
Safety Significance
There was no radiological consequence during this event.
The potential for a radiological consequence was only applicable during the time period that Secondary Containment did meet the 0.25 inches vacuum water gauge. A higher Reactor Building pressure could allow for the exfiltration of radioactive material during an accident. However, the release would still be detected by radiation monitors and this would initiate a Secondary Containment isolation.
A differential pressure vacuum is maintained within Secondary Containment by two of four RBV exhaust fans or one of two SBGT trains. During a postulated accident scenario, RBV is placed in isolation and the SBGT is used to maintain differential pressure. This event only affected one of four RBV exhaust fans.
The condition does not adversely impact that ability of RBV to isolate or SBGT to initiate and maintain a sufficient differential pressure. Therefore, the capability of Secondary Containment to mitigate the consequence of an accident is unaffected by this deficiency.
References
- Condition Report: CR-JAF-2015-04166, Root Cause Evaluation
- Work Order: WO 00391716-01, August 19, 2014, Replace Position Switch 66PNS-106A1
- Licensee Event Report: JAFP-14-0147, LER-14-002, dated December 19, 2014