05000254/LER-2007-002
| Quad Cities Nuclear Power Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii)(A), Common Cause Inoperability 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| 2542007002R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 2957 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
Safety function not met due to out of tolerance turbine first stage pressure switches due to setpoint drift.
A. CONDITION PRIOR TO EVENT
�Unit: 1� Event Date: November 16, 2007 Event Time: 1230 hours0.0142 days <br />0.342 hours <br />0.00203 weeks <br />4.68015e-4 months <br /> �Reactor Mode: 1� Mode Name: Power Operation Power Level: 97%
B. DESCRIPTION OF EVENT
On November 16, 2007, during full poWer operation on Unit 1, Instrument Maintenancepepartment surveillances determined that both Division I main turbine [TA] first stage low_ pressure-scram-errable-switches-[PS]-were_found_ , out of tolerance high and therefore would haii6 prevented fulfillnYent of a reactor scram safety function during power ascension between 38.5% Rated Thermal Power (RTP) and the as-found switch setpoint. Technical Specifications 3.3.1.1, Reactor Protection System Instrumentation [JD], requires an enable of the turbine stop valve [ISV] closure scram function and an enable of the turbine control valve [FCV] fast closure (trip oil pressure-low) scram function at any reactor power greater than or equal to 38.5% RTP. Technical Specification Table 3.3.1.1-1 describes " 38.5% RTP" as the "Applicable Modes or Other Specified Conditions," for when these two functions apply to plant operations.
All four Unit 1 switches were replaced by a plant modification in May 2007 (refueling outage Q1R19). The original Barksdale model B1T-M12SS-TC switches were replaced with Ashcroft model B450SXCH-400PSI. The new switches were selected due to their improved deadband properties needed to resolve a relay [94] chatter issue, and were procured under a commercial grade dedication process by an approved vendor (Trentec).
The 1-0504A/B/C/D switches sense the main turbine first stage pressure, and are part of the Reactor Protection System (RPS) logic. The purpose of the switches is to provide an enable of the turbine stop valve closure RPS function and as an enable of the turbine control valve fast closure '(trip oil pressure-low) RPS function. Both functions are enabled to provide a reactor scram signal when reactor power is greater than or equal to 38.5% RTP.
The switches are normally on a required calibration frequency of once every three months. When both switches in Division I exceeded their required tolerances during the November 2007 calibration, the trip function enabled by the switches would have been unavailable if the unit was ascending in reactor power through the 38.5% RTP level.
However, since power ascension was not in progress and the Unit was at full power (97%) during this event, Technical Specification requirements were not exceeded. There were no plant or public safety consequences as a result of this event.
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C. CAUSE OF EVENT
A root cause investigation (ongoing) has identified ,that the replacement Ashcroft pressure switches were overpressurized during the commercial grade dedication testing process by Trentec because the Ashcroft switch nameplate proof pressure was in error. The switches were tested to 2400 psig, which was provided as the switch nameplate proof pressure. The Instrumentation, Systems, and Automation Society (ISA) industry standard for nameplate proof pressure is a pressure that would not damage the switch. The testing performed at 2400 psig resulted in damage to the switches and .led to a reduction in the repeatability of calibration to the required switch setpoint. Should the-final results of the root cause investigation significantly impact the information of this LER, a Supplemental LER will be issued.
D. SAFETY ANALYSIS
The safety significance of this event was minimal. The unit was operating at tfull power at the time of the event with the switches enabled for the turbine stop valve closure scram function and enabled for the turbine control valve fast closure (trip oil pressure-low) scram function. Therefore, the turbine stop valve closure and turbine control valve fast closure signals would have properly caused a reactor scram if required. In addition, it was determined that the reset value of the switch (the point where the scram would be bypassed when the power level is being reduced) was well below the enable .value due to the improved -deadband-properties. Unit 1 has no other applications in which this switch is used, and Unit 2 has no applications in which this switch is used. Consequently, plant impact of this condition with the plant operating was minimal.
E. CORRECTIVE ACTIONS
- The switches were recalibrated to within their required as-left setting tolerances on 11/16/07.
- A compensatory action identified in the Operability Determination required a temporary increase in the quarterly calibration frequency -of the switches to a monthly frequency to reduce the likelihood of time related setpoint drift. The as-found values at the first recalibration of all four switches (on 12/07/07) and second recalibration (01/04/08) were satisfactory.
- The switches are anticipated to be replaced like for like, or with a suitable substitute by the end of February 2008, pending the outcome of the ongoing root cause investigation.
- The commercial parts dedicator (Trentec), has been contacted to ensure this issue is entered into their corrective action program.
- Station Procurement is revising procurement requirements for these pressure switches to ensure testing _ is performed to the required pressures.�
F. PREVIOUS OCCURRENCES
One of two Division II switches was found to have exceeded its required tolerance during testing on 10/29/07. This is the only other event where an Ashcroft switch had exceeded the required tolerance that would have prevented fulfillment of a reactor scram safety function during power ascension between 38.5% RTP and the as-found switch setpoint.
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G. COMPONENT FAILURE DATA
The turbine first stage pressure switches are Ashcroft Model B450SXCH-400PSI. The switches are a commercial grade product supplied by Ashcroft and were dedicated for a Safety Related application by a qualified vendor (Trentec). An EPIX/NPRDS search identified no failure reports concerning this Ashcroft model switch, nor any Ifailures of Ashcroft switches with similar model numbers.
_NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER