05000254/LER-2010-002
| Docket Number Sequential Revmonth Day Year Year Month Day Year N/A N/Anumber No. | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2542010002R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 2957 Megawatts Thermal Rated Core Power Energy Industry Identification System (EllS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
Unit 1 Reactor Scram Due to Turbine Trip from Low Condenser Vacuum During Main Condenser Flow Reversal
A. CONDITION PRIOR TO EVENT
�Unit: 1 Event Date: August 12, 2010�Event Time: 0358 hours0.00414 days <br />0.0994 hours <br />5.919312e-4 weeks <br />1.36219e-4 months <br /> � Reactor Mode: 1 Mode Name: Power Operation Power Level: 100%
B. DESCRIPTION OF EVENT
On August 12, 2010, a Unit 1 main condenser circulating water flow reversal was scheduled per procedure, Circulating Water System Flow Reversal. The circulating water flow was to be reversed from the south direction to the north direction. This activity received a pre-job brief. This pre-job brief included a discussion of the possibility that the valves would not operate as expected, and the actions to be taken if that occurred. These actions included taking the flow reversal switch [HS] back to the original position, dropping load using the reactor recirculation pumps [P], and stationing an Equipment Operator at motor control center (MCC) [BU] 16-3 to reset thermal overload (TOL) devices if required. A contingency manual scram value of 6.5 inches Hg backpressure was established in accordance with the guidance of procedure, Loss of Condenser Vacuum. Per procedural guidance, the initial backpressure was verified to be below 4.5 inches Hg, and low enough that it was not expected to exceed 5.5 inches Hg during the reversal. Initial backpressure was 4.2 inches Hg.
At approximately 0355 hours0.00411 days <br />0.0986 hours <br />5.869709e-4 weeks <br />1.350775e-4 months <br />, the operator moved the circ water flow selector switch from the south position to the north position. The first half of the flow reversal (the west side of the condenser) was automatically completed as expected. Approximately 60 seconds after the main condenser flow reversal was initiated (approximately 30 seconds for valve movement on the first half, and a 30 second pause built into the logic), the second half of the reversal process started (the east side of the condenser). After approximately 45 seconds, an incomplete sequence alarm was received in the control room. The Control Room Operator identified that motor operated valve (MOV) [20] 1-4402D had failed to open as required. At this point, the west side of the condenser had full flow in the north direction, and the east side did not have any flow with both inlet valves closed. In accordance with procedural guidance and contingency actions discussed in the pre-job brief, the Control Room Operator then placed the circ water flow selector switch back to south in an attempt to restore circulating water flow to the east side of the main condenser and recover its heat transfer capability.
During the first half (west side) of this emergency flow reversal (returning- flow to its original condition), MOV 1-4403B failed to completely open, and both indicating lights (open and closed) went off. Since the incomplete sequence alarm was received during the repositioning of a valve on the west side (the first side to reposition on a flow reversal), the logic automatically stopped the sequence and the valves on the east side of the condenser (MOV 1-4402C and MOV 1-4403D) did not receive a signal to reposition. At this point the main condenser still had no flow through its east side, and the flow through its west side was now in the south direction but throttled by an outlet valve in mid-position.
� Operators attempted an emergency reactor/turbine power reduction in order to reduce the steam flow into the main condenser and preserve vacuum. However, at approximately 0358 hours0.00414 days <br />0.0994 hours <br />5.919312e-4 weeks <br />1.36219e-4 months <br /> the Unit 1 main turbine automatically tripped due to low main condenser vacuum (indicated 6.3 inches Hg condenser backpressure), and the Unit 1 reactor scrammed due to closure of the Unit 1 main turbine stop valves. Approximately 45 seconds after the second (emergency) circulating water flow reversal (return to the south direction) was initiated, a second incomplete sequence alarm was received in the control room.
This LER is being submitted in accordance with 10 CFR 50.73(a)(2)(iv)(A), which requires the reporting of any event or condition that resulted in manual or automatic actuation of the RPS, including reactor scram or reactor trip.
C. CAUSE OF EVENT
The root cause for the turbine trip during main condenser flow reversal was determined to be foreign material (FM) introduced during the manufacturing process for the auxiliary contact unit used in the breaker associated with MOV 1 4402D. The auxiliary contact failure caused the MOV 1-4402D to fail to open.
The potential for introduction of the FM during the purchasing, transportation, storing, or installation of the auxiliary contacts, or during operation with the auxiliary contacts installed, was also examined. It was concluded that the sealed nature of the unit made these other handling periods much less likely than the introduction of the FM during manufacturing.
The recorder traces for condenser vacuum were also examined and it was determined that the failure of MOV 1 4402D was the primary driver for the event.
Failure of MOV 1-4403B was identified as a contributing cause of the event. The complex troubleshooter for MOV 1 4403B identified that the valve failed approximately halfway through its open stroke due to the TOL tripping. The TOL was replaced and taken to the shop and bench-tested. It was determined that the TOL was appropriately set per the existing setpoint guidance.
For a typical MOV, the TOL setting would be such that repeat operation of the valve in a short time (within a few minutes) would increase the likelihood that the TOL would trip. Heating of the TOL occurs during the operation of the valve, and operating it quickly again does not provide sufficient time for the TOL to cool down, causing it to trip sooner on the second and each subsequent operation.
During the time of the event, the area around the breaker cubicle had an elevated temperature due to the higher outside ambient temperatures. Also, the specific failures involved in this event resulted in all three circulating water pumps operating such that the only path of circulating water available through the condenser was through the MOV 1 4403B. The complex troubleshooter concluded that it is reasonable and likely that the TOL would trip in this condition.
As a result, given that operating MOV 1-4403B a second time within a few minutes is a required function of the valve, it was determined that the cause of the TOL trip for MOV 1-4403B was the setpoint of the TOL did not anticipate the specific conditions of this event, and is hence considered a contributing cause for this event.
Tests were also performed on the MOV 1-4403B valve and actuator [84], and it was determined that neither had evidence of degradation.
�
D. SAFETY ANALYSIS
The safety significance of this event was minimal. The reactor and turbine responded as designed to the low condenser vacuum. There were no complications during the turbine trip and reactor scram, and all systems functioned as required.
Although the turbine trip occurred prior to the expected vacuum level, it was determined that this is due to the location of the instrument taps in the condenser for the turbine trip being at a higher elevation than the location of the instrument taps for the RPS and main control room indication. Specifically, there are two sets of instrument taps in the condenser. The instrument taps that feed the turbine trips are located near the top of the condenser at the skirt.
The instrument taps that feed the RPS Instrumentation are lower in the condenser. Due to differences in elevation and event characteristics, the turbine trip in response to the low vacuum in this event occurred at a higher indicated vacuum (lower indicated backpressure) than anticipated by the operators. This issue, however, did not contribute to the scram.
E. CORRECTIVE ACTIONS
Immediate:
- MOV 1-4402D auxiliary contacts replaced
- MOV 1-4403B valve and actuator tested for degradation.
- All Unit 1 and Unit 2 main condenser reversing valve auxiliary contacts tested.
- On-shift training was provided to the Operators concerning main condenser vacuum readings.
- Temporary procedure change made to lower contingency manual scram criteria from 6.5 to 6.0 inches Hg backpressure.
Follow-up:
- Implemented testing of auxiliary contact units prior to installation in main condenser reversing valve breakers.
- Procedure change was implemented to require verifying contact integrity prior to reversing flow.
- The thermal overloads for the main condenser reversing valves will be reviewed for potential changes to setpoints or design.
- Training and procedure changes will be implemented to communicate the potential disparity between the turbine trip vacuum reading and the RPS vacuum reading.
- Review for potential modification of the RPS and turbine trip vacuum taps such that they read the same.
- Replace all Unit 1 and Unit 2 main condenser reversing valve auxiliary contacts.
F. PREVIOUS OCCURRENCES
The Station Events Database, EPIX, NPRDS, and LERs were reviewed for similar events. This event was caused by FM from the manufacturing process identified in the auxiliary contact unit for the breaker associated with the valve MOV 1-4402D. There were no prior incidents identified involving a turbine trip or reactor scram due to low vacuum during main condenser flow reversal at Quad Cities.
- Station Events Database — Quad Cities Investigation Report (IR) 106653 - MO 1-4402B Condenser Flow Reversing Valve Stuck Mid-Position (5/3/02) — The apparent cause was determined to be "extreme wear of the ring gear in the Limitorque SMB-3 actuator." Corrective Actions included generating preventive maintenances (PMs) to replace the flow reversing valve actuators as a task every four years, and PMs to obtain current trace data on flow reversing valve actuators annually. Corrective actions were, however, applied to minimizing actuator degradation as appropriate, and did not involve the auxiliary contact unit for the breaker as identified in this LER.
- EPIX/ NPRDS — No similar events identified for Quad Cities.
- LERs - No similar events identified for Quad Cities.
G. COMPONENT FAILURE DATA
This event has been reported to EPIX as Failure Report No. 1062.
The component that failed was a General Electric CR305X auxiliary contact unit.
�NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER