05000263/LER-2001-010
| Docket Number | |
| Event date: | 04-11-2001 |
|---|---|
| Report date: | 06-08-2001 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2632001000R00 - NRC Website | |
Description At approximately 0450 on April 9,2001, Standby Gas Treatment System (SBGTS)1 Train A was removed from service for routine periodic maintenance and testing. During this period the plant was operating normally at full rated power. No other structures, systems, or components were inoperable which contributed to this event.
On April 11, 2001, following completion of the planned maintenance activities, Surveillance Test 0147, Standby Gas Treatment System Filter Tests, was initiated to satisfy the requirements for post-maintenance and periodic leakage testing of the Train A SBGTS particulate and charcoal filter elements. At 1715, during performance of the in-place halogenated hydrocarbon (freon) charcoal filter' leakage portion of the test, it was found that two of the seven freon leakage measurements exceeded the Technical Specification acceptance criterion of less than 1% penetration. Measured penetration values were 0.92, 0.81, 1.07, 0.75, 0.96, 0.92, and 1.02 %.
The charcoal adsorber assembly was inspected to determine the cause of the unacceptable test measurements. During this inspection it was found that the filter frame3 was curved slightly in two locations. This resulted in slight gaps in the seating surface of three charcoal trays.
The Train A SBGTS charcoal filter assembly was repaired using silicon lubricant. Train A was satisfactorily retested following repairs and declared operable at approximately 1700 on April 13, 2001. The measured penetration following repairs was 0.04% based on individual measurements of 0.021, 0.032, 0.031, 0.034, and 0.035 %.
Train A charcoal filter leakage measurements from previous tests were reviewed, and while acceptable, were found to be significantly higher than the corresponding measurements made on SBGTS Train B following replacement of the charcoal filter cells in December, 1996.
The higher Train A charcoal adsorber leakage measurements were previously noted in 1997.
1EIIS System Code: � BH 2EIIS Component Code: FLT 3EIIS Component Code: FRM
Event Analysis
Analysis of Reportability 10 CFR 50.73(a)(2)(i)(B) requires reporting of any operation or condition which was prohibited by the plant's Technical Specifications.
Train A charcoal filter leak tightness is believed to have degraded during the period following the previous successful filter leakage test. The Technical Specification Limiting Condition for Operation upper limit on charcoal adsorber leakage of 1% penetration was therefore exceeded for more than seven days. This is a violation of Technical Specification 3.7.B.1.a which requires the plant to be placed in a condition not requiring SBGTS operability if one train remains inoperable for longer than seven days.
Safety Significance
We believe that this event has a low safety significance.
The estimated adsorber penetration at the 95% confidence level was less than 0.26% over the Technical Specification limit of 1%.
A conservative assessment of the impact on control room and offsite doses following a loss of coolant accident found that:
- Control room thyroid dose would increase less than 0.5% over the value reported in the Monticello USAR and would remain well below the 10 CFR Part 50, Appendix A, GDC 19 guideline.
- Offsite thyroid doses would increase less than 2% over the values reported in the Monticello USAR and would remain well below the 10 CFR Part 100 guidelines.
Cause
The charcoal adsorber assembly was inspected to determine the cause of the unacceptable test measurements. During this inspection it was found that the filter frame3 was curved slightly in two locations. This resulted in slight gaps in the seating surface of three charcoal trays. These small gaps caused by the distortion in the Train A filter frame were not adequately sealed when the charcoal adsorber was replaced in December, 1996.
Lack of adequate guidance in positioning of upstream and downstream freon test probes in Surveillance Test 0147 also contributed to this event. Variations in probe location from test to test adversely affect the accuracy and reproducibility of leakage measurements. This procedural deficiency may have prevented earlier detection and correction of the excessive charcoal filter leakage.
The procedure for Surveillance Test 0147 was also found deficient in that it did not provide an action level, below the Technical Specification limit, at which point corrective action should be taken to correct filter leakage.
Corrective Actions
The filter assembly was repaired using silicone lubricant to seal the gaps and irregularities between the tray gaskets and the filter frame. The material used is compatible with the post- accident environmental and radiation dose conditions under which the SBGTS would be subjected.
Surveillance Test Procedure 0147 will be revised prior to its next use to:
- Standardize the location of upstream and downstream test probe locations
- Establish an action level for freon penetrations measurements, below the Technical Specification limit, at which corrective action is required Failed Component Identification None Similar Events No similar reportable events.