05000254/LER-2010-002, Regarding Reactor Scram Due to Turbine Trip from Low Condenser Vacuum During Main Condenser Flow Reversal
| ML102861821 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  (DPR-029) |
| Issue date: | 10/06/2010 |
| From: | William Gideon Exelon Generation Co, Exelon Nuclear |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| SVP-10-066 LER 10-002-00 | |
| Download: ML102861821 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2542010002R00 - NRC Website | |
text
Exeln Exelon Generation Company, LLC www.exeloncorp.com Nuclear Quad Cities Nuclear Power Station 22710 206th Avenue North Cordova, IL 61242-9740 October 6, 2010 10 CFR 50.73 SVP-10-066 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555 Quad Cities Nuclear Power Station, Unit 1 Renewed Facility Operating License No. DPR-29 NRC Docket No. 50-254
Subject:
Licensee Event Report 254/2010-002-00, "Unit 1 Reactor Scram Due to Turbine Trip from Low Condenser Vacuum During Main Condenser Flow Reversal" Enclosed is Licensee Event Report (LER) 254/2010-002-00, "Unit 1 Reactor Scram Due to Turbine Trip from Low Condenser Vacuum During Main Condenser Flow Reversal," for Quad Cities Nuclear Power Station, Unit 1.
This report is submitted in accordance with the requirements of the Code of Federal Regulations, Title 10, Part 50.73(a)(2)(iv)(A), which requires the reporting of any event or condition that resulted in manual or automatic actuation of the reactor protection system (RPS) including reactor scram or reactor trip.
There are no regulatory commitments contained in this letter.
Should you have any questions concerning this report, please contact Mr. W. J. Beck at (309) 227-2800.
Respectfullr William R. Gideon Site Vice President Quad Cities Nuclear Power Station cc:
Regional Administrator - NRC Region III NRC Senior Resident Inspector - Quad Cities Nuclear Power Station
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2010 (9-2007)
, the NRC may digits/characters for each block) not conduct or sponsor, and a person is not required to respond to, the information collection.
- 13. PAGE Quad Cities Nuclear Power Station Unit 1 05000254 1 OF 5
- 4. TITLE Unit 1 Reactor Scram Due to Turbine Trip from Low Condenser Vacuum During Main Condenser Flow Reversal
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED T
I FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR SEQUENTIAL REV MONTH DAY YEAR N/A N/A INUM BER NO.I FACILITY NAME DOCKET NUMBER 08 12 10 2010 002 00 10 06 2010 N/A N/A 9.OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply)
[l 20.2201(b)
El 20.2203(a)(3)(i)
[I 50.73(a)(2)(i)(C)
El 50.73(a)(2)(vii),
1 [1 20.2201(d)
[I 20.2203(a)(3)(ii)
[I 50.73(a)(2)(ii)(A)
El 50.73(a)(2)(viii)(A)
El 20.2203(a)(1)
El 20.2203(a)(4)
El 50.73(a)(2)(ii)(B)
El 50.73(a)(2)(viii)(B)
__ 20.2203(a)(2)(i)
[I 50.36(c)(1)(i)(A)
[I 50.73(a)(2)(iii)
El 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL [I 20.2203(a)(2)(ii) 0l 50.36(c)(1)(ii)(A)
U 50.73(a)(2)(iv)(A)
[I 50.73(a)(2)(x)
El 20.2203(a)(2)(iii)
El 50.36(c)(2)
El 50.73(a)(2)(v)(A)
El 73.71(a)(4) 100%
El 20.2203(a)(2)(iv)
El 50.46(a)(3)(ii)
El 50.73(a)(2)(v)(B)
El 73.71(a)(5)
El 20.2203(a)(2)(v)
El 50.73(a)(2)(i)(A)
El 50.73(a)(2)(v)(C)
[I OTHER El 20.2203(a)(2)(vi)
El 50.73(a)(2)(i)(B)
El 50.73(a)(2)(v)(D)
Specify in Abstract below or in 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 TOL replaced and tested.
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.
SEPIX/ 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.PRINTED ON RECYCLED PAPERPRINTED ON RECYCLED PAPER