05000263/LER-2002-005
| Monticello Nuclear Generating Plant | |
| Event date: | 07-22-2002 |
|---|---|
| Report date: | 09-20-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2632002005R00 - NRC Website | |
Description On July 22, 2002 Monticello Nuclear Generating plant was operating at 100% power and in the process of performing the ESW Valve and Pump Test on the 14 ESW pump. This test required bypassing the temperature control valve' on the B-division CRV2 making the B- division CRV inoperable. During the test, the A-division CRV compressor' tripped on low cooling water flow. The A-division CRV was declared inoperable at 1630. At this time both CRV divisions were inoperable. T.S. 3.17.A.3.a requires entering an action statement to restore one CRV division to operable status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. The B-division CRV was returned to an operable status at 1701, resulting in both divisions being inoperable for 31 minutes. After restoring B-division CRV, the station exited the 24-hour action statement and entered a 30-day action statement for the A-division CRV per T.S. 3.17.A.2.a.
Troubleshooting was performed on A-division CRV and personnel determined that the setpoint of the differential pressure switch4 DPS-4029A had drifted to approximately 24 psi (the desired setpoint is 13 psi). The differential pressure switch (DPS-4029A) measures Service Water dP to ensure there is adequate cooling water flow through the condenser for V-EAC-14A. The switch trips the compressor if adequate cooling water flow cannot be maintained. When DPS-4029A drifted high, V-EAC-14A tripped on low service water flow and could not be restarted. The switch was adjusted into acceptable tolerances and no other problems were noted. Following recalibration of DPS-4029A, the A-division CRV was declared operable. The A-division CRV was inoperable for approximately 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
Event Analysis
Analysis of Reportability The event is reportable under 10 CFR 50.73(a)(2)(v) (D), because both trains of a system designed to control the consequences of an accident were unavailable.
The event is a safety system functional failure.
1 EIIS Component ID - TCV 2 EIIS System Code VI 3 EIIS Component ID - CMP "EIIS Component ID - PDIS Safety SignificanCe The safety significance of the event is considered low. Cooling to the control room was only lost for approximately 31 minutes. Significant room heat up was not encountered. The highest Control Room temperature recorded 2030-A (HOURLY CONTROL ROOM LOGS AND WATT HOUR METER READINGS) for 7/22/02 was 77 degrees Fahrenheit. The peak temperature was not recorded, but the comments section states that 78 degrees F was exceeded while attempts were being made to restore cooling. The time period above 78 degrees F was short.
The PRA group reviewed the event and concluded the event was of low safety significance.
The basis of this conclusion was: the threat of a chemical or radiological air contamination issue affecting control room habitability was very small, significant radiological habitability issues for the main control room would only occur following some core damage scenarios and therefore Core Damage Frequency (CDF) was unaffected by this event, and the potential increase in Large Early Release Frequency (LERF) was limited due to the short duration of the loss of Emergency Filtration capability (approximately 31 minutes).
Cause
The cause of the event was excessive drift of the differential pressure switch, DPS-4029A.
The station reviewed the calibration history for DPS-4029B and found no history of calibration drift.
Corrective Actions
A new model switch will be installed in accordance with the station process and 10CFR50.59, due to a previous occurrence of setpoint drift of DPS-4029A (discussion below in previous events section). The existing model switch is obsolete and no longer available.
As an interim action, the existing switch will be recalibrated at six-month intervals, instead of the normal interval of two years. ..
Failed Component Identification DPS-4029A Manufacturer:
Model:
Range:
Type:
UTE - UNITED TECHNOLOGIES J21KD MODEL 150 0 to 40 PSID Differential Pressure Switch ..- .....„ Previous Similar.Events On 06/22/01 a condition report was written for DPS-4029A out of tolerance. Station investigation found the primary cause of the out of tolerance to be setpoint drift of the switch. No trend of excessive drift was identified. In addition, a contributing cause of the out of tolerance was that micro switch terminal screws were loose. The switch was calibrated and the terminal screws were tightened.