05000271/LER-2001-001
| Event date: | |
|---|---|
| Report date: | |
| 2712001001R00 - NRC Website | |
FACILITY NAME (1) DOCKET SEQUENTIA REVISION L NUMBER NUMBER YEAR PAGE (3)
DESCRIPTION
On 03/19/01, VY was operating at rated thermal power with the "A" RHR (El IS=B0) subsystem removed from service for routine maintenance.
Event Timeline At 1227 , Instrument and Control Technicians were performing an APRM-A System Functional Test. Per the procedure (VY Operating Procedure 4302), the technicians had placed the APRM-A logic (El IS=IE) in the "bypass" condition, blocking any associated protective signals from the RPS (EIIS=JC) system.
At 1232, 1. An APRM-A high signal was inserted into the bypassed APRM-A for testing.
2. With APRM-A in bypass, the balance of in-service APRM's provide the required protection system outputs.
3. The APRM-A was restored to normal per the test procedure, admitting the APRM-A output to the RPS logic.
4. The licensed reactor operator at the control board observed control rods inserting.
5. There was no indication that a full reactor scram signal was present.
At 1233, 1. The licensed reactor operator initiated a manual scram using the manual scram push buttons.
2. The rapid power reduction resulted in a momentary lowering of reactor water level (shrink), causing the expected Primary Containment Isolation System (PCIS, EIIS=JM) actuation, isolating process flow lines in the following:
- Liquid and Gaseous Radioactive Waste Systems (EIIS=WF)
- Primary Containment Atmosphere Control and Sampling Systems (EIIS=VB)
- Reactor Water Cleanup System (EllS=CE)
- Containment spray and cooling features in the Residual Heat Removal System At 1234, Reactor pressure was being controlled using the Main Turbine Mechanical-Hydraulic Control System (El IS=J1) to control the discharge of decay heat to the main condenser via a main turbine bypass valve.
At 1235, following the shrink and the subsequent restoration of reactor water level, the "C" Reactor Feedwater Pump (EIIS=SJ, P) tripped on high reactor water level.
At 1241 through 1342, during this period the operating crew:
1. Reset the scram signal 2. Established reactor water level control using the "C" Reactor Feedwater Pump and manual control of the low flow feed water regulating valve (EllS=LCV) 3. Reset containment isolation signals and returned affected systems to their normal shutdown plant alignment, as applicable 4. Aligned other balance of plant systems to support shutdown plant operations At 1405, the operating crew verified that the appropriate steps of applicable plant procedures had been performed.
At 1430, VY Instrument and Control personnel discovered an auxiliary contact (EIIS=CNTR) in a back-up scram valve (El IS=V) control circuit out of its normal (open) position. The contact is designed to close upon a signal from the "B" RPS logic and reopen by spring tension when the "B" RPS logic trip signal is reset. The auxiliary contact armature (plunger) was found broken. This condition caused binding that prevented spring force from returning the contact to the open position. The auxiliary contact was in control circuitry that had been tested previously on 03/19/01 and may have become bound in the closed condition during that testing. The failed component is a General Electric model CR105X200P auxiliary contact.
CAUSE
1. The root cause for the binding of the auxiliary contact plunger was normal wear.
2. The reason that the plunger failure was not detected until it resulted in a plant trip is that the circuit design does not provide for positive contact position indication.
ANALYSIS
The RPS System initiates a rapid, automatic shutdown (scram) of the reactor. This action is taken to prevent excessive fuel cladding damage and any nuclear system process barrier damage following abnormal operational transients. The RPS System overrides operator and process controls.
The Control Rod Drive (CRD, EIIS=AA) System inserts the negative reactivity necessary to shut down the reactor when a scram is initiated by the RPS System. When a scram signal is received, high pressure water forces each control rod rapidly into the core.
The RPS and CRD systems function together to initiate an insertion of control rods when plant process parameters (and/or manual operator input) indicate that it is necessary to rapidly insert all 89 control rods. The insertion of the control rods is affected by depressurizing the pneumatics that hold 89 sets of scram valves (one set for each control rod) closed. The primary means for depressurizing the pneumatics to an individual set of scram valves is by repositioning a set of Scram Solenoid Pilot Valves (SSPV's, EIIS=20). As a back-up to the repositioning of the SSPV's, the CRD system employs two back-up scram valves. Repositioning either back-up scram valve will depressurize the pneumatic header supplying all 89 sets of SSPV's, causing a rapid shutdown of the reactor. Repositioning either back-up scram valve requires a scram signal from both the "A" and "B" RPS trip systems.
Prior to the automatic shutdown of the VY plant on 03/19/01, one of the auxiliary contacts providing a "B" RPS trip system input to a single back-up scram had failed in the closed (tripped) position. This, in effect, sealed in a "B" RPS trip signal to that individual back-up scram valve. Design of the control circuitry precluded readily identifying this "invisible half-scram." During the 03/19/01 testing an "A" RPS trip signal was generated and admitted to the back-up valve control circuitry. This step, combined with the failed-closed auxiliary contact in the "B" trip system, completed the logic necessary to reposition a single back-up scram valve and begin depressurizing the related pneumatics.
The broken auxiliary contact failed in the safe (tripped) position. Therefore it presented no challenge to either the RPS or CRD performing their safety objectives.
If the broken plunger in the auxiliary contact had caused the device to bind in the open (non-tripped) position, redundancy within the related circuitry would have ensured that both back-up scram valves actuated as designed, causing the scram air header to depressurize and control rods to insert.
SEQUENTIA REVISION
FACILITY NAME (1) DOCKET YEAR L NUMBER NUMBER PAGE (3) An additional assessment was performed to determine the safety implications of the stuck auxil ary contact possibly inhibiting the operation of its main contact. To perform this assessment it was conservatively assumed that a stuck auxiliary contact would prevent repositioning of the main contactor. The assessment confirmed that the resultant failure of the main contactor could not prevent the RPS system from accomplishing its safety objective. This is consistent with the single failure design requirements of the VY RPS system and Engineered Safety Feature control circuitry. Additionally, it was identified that the results of disabling the main contactor by a stuck auxiliary contact would be readily identified during routine testing and/or normal plant operation. Plant procedures would then direct the crew to take appropriate actions to address the degraded equipment.
The required plant safety systems operated as designed, and the plant staff operated systems in accordance with procedures bringing the plant to a stable shutdown condition. Therefore, this event caused no significant increase in risk to public health and safety.
CORRECTIVE ACTIONS
1. The failed auxiliary contact was replaced and tested satisfactorily. This action was completed on 03/19/01.
2. A Temporary Modification was implemented on 03/20/01.
a. Voltmeters were installed in the two RPS Back-up Scram Valve circuits. The Temporary Modification facilitates voltage readings needed to confirm reset of the auxiliary contacts after scram signals are reset.
b. The voltmeters are being used to take measurements required by interim administrative controls.
3 VY technical personnel identified and inspected similar protective circuits and contacts to determine if additional contacts should be replaced and/or monitored.
a. The assessment concluded it to be prudent to replace a second auxiliary contact in the affected back up scram valve control circuit.
Vermont Yankee is evaluating the feasibility of several long-term corrective actions.
ADDITIONAL INFORMATION
Vermont Yankee has not reported any similar events to the USNRC.