05000374/LER-2004-001
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| Initial Reporting | |
| ENS 40495 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3742004001R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 3489 Megawatts Thermal Rated Core Power
A. CONDITION PRIOR TO EVENT
Unit(s): 2 Event Date: 2/1/04 Event Time: 2334 Reactor Mode(s): 1 Power Level(s): 100 Mode(s) Name: Run
B. DESCRIPTION OF EVENT
At 2334 hours0.027 days <br />0.648 hours <br />0.00386 weeks <br />8.88087e-4 months <br /> on 2/1/04, the 2B Reactor Protection System (RPS) Motor-Generator (MG)[EF] Set output breaker (EPMA D) tripped, resulting in a loss of the 1B' RPS bus. The loss of the 'B' RPS bus resulted in a half-scram and 'B' side Primary Containment Isolation System (PCIS)(JM] Group 1 (except Main Steam Isolation Valves), 2, 3, 4, 5, 6, 7 and 10 isolations. The Instrument Nitrogen (IN)[LE] and Reactor Water Clean Up (RT)(CE] systems were isolated when their containment isolation valves closed on Group 10 and Group 5 isolation signals.
The event required entry into multiple abnormal procedures, including LOA-RP-201, "Unit 2 Loss of Reactor Protection System Power," LOA-PC-201,"Primary/Secondary Containment Trouble," LOA-IN-201 "Loss of Drywell Pneumatic Air Supply," and LOA- RT-201 "Loss of Reactor Water Clean Up System." Operator response to this event was in accordance with expectations.
The RPS bus was reenergized using its alternate supply and the affected containment isolation valves were restored to their normal positions by 0045 on 2/2/04, with the exception of the IN system.
The Group 10 isolation isolated the IN compressor and caused the rupture disk in the IN compressor discharge line to rupture. When the IN compressor is unavailable, nitrogen pressure to the Automatic Depressurization System (ADS)[SB] accumulators is provided by a backup compressed gas bottle system. This system maintains the ADS accumulator supply header nitrogen pressure greater than or equal to 150 psig, as required by Technical Specification (TS) Surveillance Requirement (SR) 3.5.1.3.
Pressure slowly decreased in the accumulator supply header following the loss of the compressor while the rupture disk was being replaced. By 0920 on 2/2/04, accumulator nitrogen pressure had dropped below 150 psig on the 'C', 'R', 'U' and `E' ADS valves. Due to the inoperability of two or more ADS valves, TS Required Action (RA) G.1 and G.2 had been entered at 0715 to place the plant in Mode 3 with in 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and to reduce reactor steam dome pressure to less than or equal to 150 psig with 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
At 1000 on 2/2/04, the IN system was restarted following replacement of the rupture disk. At 1015, pressures in all four accumulators were verified to be greater than 150 psig, and the ADS valves were declared operable.
The loss of RPS bus 'B' is reportable under 10 CFR 50.72(b)(3)(iv)(A) and 50.73(a)(2)(iv)(A) as an event or condition that resulted in the manual or automatic actuation of any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B), specifically, containment isolation valves in more than one system. An ENS call (EN 40495) was made at 0144 CST on 2/2/04.
The inoperability of multiple ADS valves is reportable under 10 CFR 50.73(a)(2)(vii) as an event where a single cause or condition caused two or more trains to become inoperable in a single system designed to mitigate the consequences of an accident.
C. CAUSE OF EVENT
The cause of the 2B RPS MG set output breaker trip was determined to be an erratic output from the MG set voltage regulator gain potentiometer. The potentiometer is a wire-wound, open faced, carbon tipped adjustable resistor.
The erratic output was due to oxidation, and was eliminated by cycling the potentiometer from its maximum to minimum setting several times to clear high resistance spots ("wiping").
A contributing problem was high resistance contacts on the 2K relay. The relay contacts are in series with the voltage regulator and the neutral bus, and carry the field current. On an overvoltage condition the 2K contacts open. Upon examination, it appeared that the silver plating on the contacts had been removed by past maintenance practices, resulting in inconsistent contact resistance and fluctuating voltage. The contacts appeared to have been filed rather than burnished.
The cause of the inoperability of multiple ADS valves was that regulator 2IN035 responds sluggishly under low demand conditions, such as are experienced following the loss of the IN compressor with no ADS valve position changes.
D. SAFETY ANALYSIS
The safety significance of this event was minimal. Upon the loss of the RPS bus, the supplied systems de-energized to their safe position. All systems powered by the 'B' RPS responded as designed following the loss of electrical power. Had a design basis event occurred, the reactor would have scrammed, and the systems affected by the loss of the RPS bus would have already been in the required configuration to mitigate the consequences of the accident.
The Emergency Core Cooling System (ECCS) safety analysis assumes one actuation of ADS valves to depressurize the reactor for operation of the low pressure ECCS systems. The 150 psig requirement is based on two cycles of the ADS valves. The lowest pressure observed on the four ADS accumulators during the event was approximately 149 psig. Engineering judgment is that at least one cycle was available, and that the safety function would have been met..
Another function of ADS is to remain open for long-term decay heat removal.
Eighty-eight psid is required to hold the ADS valves open, and the design containment pressure is 45 psig; therefore, the minimum accumulator pressure required to keep the ADS valves open is 133 psig. Maintenance history of the pressure regulator shows that under low demand the regulator opens at 137.5 psig the first time, after which it controls acceptably at 150 psig.
E. CORRECTIVE ACTIONS
1. The voltage regulator and the 2K relay for the 2B RPS MG set were replaced (Complete).
2. The potentiometers on the 1A, 1B, and 2A RPS MG set voltage regulators will be wiped, and the voltage regulators calibrated (AT#198850-24) .
3. Procedure LES-RP-101 "Inspection of Reactor Protection Motor-Generator Sets" will be revised to wipe the voltage regulator potentiometers prior to performing the four year load test (AT#198850-25).
4. A Training Request will be issued to evaluate whether a knowledge deficiency exists in Maintenance regarding the difference between burnishing and filing relay contacts (AT# 198850-26).
5. Plant Engineering will evaluate upgrades to the IN pressure regulator to prevent future occurrences (AT# 198871-04).
F. PREVIOUS OCCURRENCES
Nitrogen Backup Pressure Regulator Failure This LER documented a previous occurrence of the IN regulator IN035 failing to maintain 150 psig in the ADS accumulator header. Corrective actions were to inspect the regulator and to replace the stem assembly. Additionally, a surveillance procedure was developed to cycle the regulator on a refuel basis.
Until this event, there have been no reportable events of this nature in the past 16 years.
G. COMPONENT FAILURE DATA
General Electric, Voltage Regulator Circuit Board, Model# 3300A03B0061 General Electric CR120A Relay Target Rock, Pressure Regulator, Model# 77U401