05000261/FIN-2010009-11
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Description | Valve FCV-626 is located in the combined CCW return from the three RCP thermal barrier heat exchangers. In its normal open position it allows CCW flow to pass through the thermal barrier heat exchangers, providing backup cooling to the RCP seals in the event of a loss of the primary cooling flow (seal injection) from the charging pumps. There are two close functions for FCV-626: 1) closes on high flow in the return line which is indicative of a rupture of a thermal barrier heat exchanger (RCS to CCW system leak) and 2) closes in response to a Phase B containment isolation signal. The valve has no automatic opening functions. The valve closed when power to safety-related 480 volt Bus E-2 was transferred to the emergency diesel generator. The valve remained closed for approximately 39 minutes before the operators recognized the condition, reopened FCV-626, and restored CCW cooling to the RCP thermal barrier heat exchangers. Plant staff knew that FCV-626, a motor operated valve, was powered from Bus E-2 via MCC 6. However, plant staff, including operators, was unaware that FCV-626 would close on a momentary loss of power. Additionally, the simulator was modeled such that FCV-626 remained open when power to Bus E-2 was momentarily interrupted. The high flow closure function of FCV-626 is accomplished using flow orifice FE-626, which is located in the thermal barrier return line, and provides flow switch FIC-626 with a hydraulic input to operate high and low flow contacts. When the high flow contact opens, relay FIC-626X is de-energized and closes a contact in the closing circuit for the motor operator of FCV-626, thereby closing FCV-626. Plant procedure EDP-008, Instrument Buses, incorrectly indicated that the power source for the flow switch FIC-626 control circuit was from Instrument Bus 1. The power for the FIC-626 control circuit is from Instrument Bus 4. Instrument Bus 4 is fed from MCC-6, which also feeds motor operated valve FCV-626. When Bus E-2 transferred to the EDG, both valve FCV-626 and relay FIC-626X lost power for approximately 10 seconds. During this time interval, relay FIC-626X repositioned to its de-energized state, which closed a contact in the close circuit of valve FCV-626. When Bus E-2 reenergized, valve FCV-626 immediately began to close, which sealed in contacts to completely close the valve. The close circuit was sealed in before relay FIC-626X reset to its energized state. The team concluded that the most likely cause of the time delay for the relay to reset was a constant voltage transformer located between the relay and MCC-6. The safety significance of inadvertent shutoff of RCP thermal barrier cooling water is discussed in Section 1.1 of this report. The team reviewed historical data associated with the control circuit for valve FCV-626 and determined the licensee had at least two potential opportunities to discover the behavior of FCV-626 on a loss of power. The first opportunity was in 2005 while implementing Engineering Change (EC) 59456. While performing the EC, workers encountered wiring issues as documented in NCR168221. The licensee subsequently determined that flow switch FIC-626, which was previously thought to be powered from Instrument Bus 1, required no power to operate. As part of that investigation, it was noted that relay FIC-626X was powered from Instrument Bus 4. Wiring discrepancies existed in some of the associated drawings, but were either not noted or not pursued. 27 Enclosure 1 Additionally, the licensee did not update EDP-008, which continued to show Instrument Bus 1 Breaker 16 as the power source for the flow switch FIC-626 control circuit. The licensee has written NCR 391995 to correct these deficiencies. The second opportunity occurred in 2008 during the performance of OST-163, Safety Injection Test and Emergency Diesel Generator Auto Start on Loss of Power and Safety Injection (refueling). As part of the test, 480V safety-related Bus E-2 was transferred to the B EDG. Data recovered by the licensee indicated that FCV-626 closed at approximately the same time the B EDG re-energized Bus E-2. However, during the test, CCW system flow, which was being provided by the opposite train of power, was stable and RCPs were not running. Thus sufficient information was available to recognize that flow perturbations were not the cause for FCV-626 closing. About two hours later, the valve was reopened. The licensee either did not identify or did not pursue the unexpected behavior of FCV-626. The ERT entered the problems described in this section into the corrective action system. One corrective action will clarify the drawings associated with the control circuitry of FCV-626 and make some minor corrections to current drawings. Another corrective action will implement a modification to prevent inadvertent closure of valve FCV-626 for a momentary loss of power. The licensee stated the modification would be implemented prior to restart of the plant from the refueling outage. Additional review by the NRC will be needed to determine whether the design of FCV- 626, which caused the valve to close during a momentary loss of power, represents a performance deficiency. An Unresolved Item will be opened pending completion of this review. This issue is identified as URI 05000261/2010009-11, FCV 626, RCP Thermal Barrier Outlet Isolation CCW Valve, Unexpected Closure. In order to better understand the reason FCV-626 closed during a momentary loss of power, the team reviewed the licensing bases for the CCW system, including FCV-626. The team reviewed correspondence between the licensee and the NRC regarding NUREG 0737, Clarification of TMI Action Plan Requirements, Item II.K.3.25, Power on Pump Seals. This TMI item required the licensee to determine the consequences of a loss of RCP cooling due to a loss of offsite power lasting two hours. In their correspondence, the licensee stated that no modifications were necessary because the CCW system is still operable during a loss of offsite power (powered from the emergency buses) and provides flow to the RCP thermal barrier heat exchangers. They also stated that the B and C CCW pumps are automatically (requiring no operator action) started by a station blackout signal during a loss of offsite power event. Additional review by the NRC will be needed to determine if the behavior of RCP seal cooling following a loss of offsite power event is consistent with the description provided by the licensee in NUREG 0737 correspondence and if any differences represent a violation. An Unresolved Item will be opened pending completion of this review. The issue is identified as URI 05000261/2010009-12, NUREG 0737 Response From Licensee to the NRC Describing the Behavior of RCP Seal Cooling Following a Loss of Offsite Power Event. |
Site: | Robinson |
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Report | IR 05000261/2010009 Section 4OA5 |
Date counted | Jun 30, 2010 (2010Q2) |
Type: | URI: |
cornerstone | Mitigating Systems |
Identified by: | NRC identified |
Inspection Procedure: | IP 93800 |
Inspectors (proximate) | J Hanna J Hickey L Miller L Wert P Fillion P Pieringer R Monk S Currie F Ehrhardta Nielsenc Kontz D Bollock D Mills G Laska G Skinner J Beavers J Brady J Hickey J Worosilo M Bates P Braxton R Musserr Monkf Ehrhardt J Hanna L Miller J Hickey P Fillion L Wert P Pieringer S Currie |
INPO aspect | |
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Finding - Robinson - IR 05000261/2010009 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Robinson) @ 2010Q2
Self-Identified List (Robinson)
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