05000249/LER-2012-001
| Docket Numbersequential Revmonth Day Year Year Month Day Year 05000Number No. N/A | |
| Event date: | 07-10-2012 |
|---|---|
| Report date: | 09-10-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| Initial Reporting | |
| 2492012001R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Dresden Nuclear Power Station (DNPS) Units 2 and 3 are General Electric Company Boiling Water Reactors with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditions Prior to Event:
Unit: 03 Event Date: 7-10-2012 Event Time: 0236 hours0.00273 days <br />0.0656 hours <br />3.902116e-4 weeks <br />8.9798e-5 months <br /> CDT Reactor Mode: 1 Mode Name: Power Operation Power Level: 100 percent
B. Description of Event:
On July 10, 2012, instrument maintenance personnel prepared to perform instrument calibration for the high flow isolation logic associated with the Isolation Condenser [BL] system. Instrumentation for both the high steam flow and high return flow were to be calibrated and functionally tested during the maintenance. The high flow instruments are arranged in series and if a high flow signal is received, it causes relay 0595-115A or 0595-115B to de-energize. These relays in turn provide inputs into the isolation trip logic for the Isolation Condenser. In order to prevent trip logic actuation during testing, test toggle switches (TTSs) are installed in parallel with contacts from both the 0595-115 relays that input into the trip logic. In accordance with plant procedures, maintenance personnel installed four TTSs to accommodate testing of both divisions of the high flow instrumentation.
Using plant procedures, maintenance personnel commenced testing of differential pressure indicating switch 3-1350A, Isolation Condenser high steam flow. The toggle switches for TTS TS-A and TS-AA were placed in the "ON" positions. At approximately 0236 hours0.00273 days <br />0.0656 hours <br />3.902116e-4 weeks <br />8.9798e-5 months <br />, a high flow signal was simulated and the 0595-115A relay de-energized as expected. However, the main control room received an unexpected annunciator indicating an isolation condenser line break due to trip logic relay 0595-116A also de-energizing as relay 0595-115A changed state. This occurrence indicated that TS-A, which had been installed in parallel to contacts 3 and 4 of relay 0595-115A, had not maintained electrical continuity.
At approximately 0236 hours0.00273 days <br />0.0656 hours <br />3.902116e-4 weeks <br />8.9798e-5 months <br />, the Isolation Condenser system was declared inoperable. Operations personnel entered and performed the required actions specified by Technical Specification 3.5.3.
This condition resulted in an inboard isolation signal to the Isolation Condenser. All of the appropriate valves completed their isolation function, as designed.
Following the unexpected isolation, walkdowns were performed to verify that the proper configuration had been used. No anomalies were identified. Voltage at the termination points of TS-A was tested and confirmed that the installed TTS had failed to maintain electrical continuity.
The Isolation Condenser system was restored to the standby condition in accordance with plant procedures and the system was declared operable at approximately 1055 hours0.0122 days <br />0.293 hours <br />0.00174 weeks <br />4.014275e-4 months <br /> on July 10, 2012.
The unplanned isolation of the Isolation Condenser system is reportable in accordance with 10 CFR 50.73(a)(2)(v)(B), "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to remove residual heat.
C. Cause of Event:
The suspect TTS was sent for failure analysis. The failure analysis indicated that the most probable cause was grease on the TTS internal stationary contacts. The TTSs were assembled by plant instrument maintenance personnel between 1998 and 2000. Even though the TTS test leads are periodically replaced, there have been no preventative maintenance activities to identify age-related degradation with regard to TTS internal operations.
D. Safety Analysis:
The safety significance of this condition is low. The High Pressure Coolant Injection system [BJ] was available to provide pressure control and makeup to the reactor vessel inventory in the event of an accident or transient. Therefore, health and safety of the public were not compromised as a result of this condition.
E. Corrective Actions:
All TTSs were replaced. A preventative maintenance activity is being generated to inspect or refurbish all TTSs assembled by the instrument maintenance personnel on a periodic basis.
F. Previous Occurrences:
A review of DNPS Licensee Event Reports (LERs) for the last three years did not reveal any reportable conditions related to Isolation Condenser isolations.
G. Component Failure Data:
Manufacturer � Component Type � Model N/A � Toggle Switch � N/A Test Toggle Switch