05000352/LER-2011-001
| Limerick Generating Station, Unit 1 | |
| Event date: | 05-09-2011 |
|---|---|
| Report date: | 07-07-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3522011001R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On Friday, April 15, 2011, a charcoal sample was removed from "1 B" Reactor Enclosure Recirculation System (RERS) Subsystem (EIIS:VA) during performance of the 24-month "B" Reactor Enclosure Recirc System Charcoal Analysis surveillance test (ST-4-076-807-1).
The sample was shipped to a qualified vendor for analysis. The analysis results became available on Monday, May 9, 2011, at 1057 hours0.0122 days <br />0.294 hours <br />0.00175 weeks <br />4.021885e-4 months <br />, which was 24 days after the sample was taken. The analysis results failed the test acceptance criteria that required the methyl iodide penetration to be less than 2.5 percent. The test results indicated that methyl iodide penetration was 7.258 percent. The "1 B" RERS subsystem was declared inoperable. The "1 B" RERS subsystem charcoal bed (EIIS:ADS) was replaced and the subsystem was returned to operable status on Saturday, May 14, 2011 at 0600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br />.
At the time the sample was removed there was no firm evidence that "1 B" RERS was inoperable. The sample analysis failure on May 9, 2011, provided firm evidence that the "1 B" RERS subsystem charcoal had exceeded the Technical Specification (TS) 3.6.5.4 RERS surveillance requirement for methyl iodide penetration to be less than 2.5 percent on April 15, 2011, when the sample was removed. The "1 B" RERS subsystem charcoal was restored to less than 2.5 percent methyl iodide penetration on Saturday May 14, 2011. This resulted in a period of 29 days that the "1 B" RERS subsystem was inoperable since it could not satisfy the surveillance requirement. TS 3.6.5.4 Action "a" requires restoration of an inoperable RERS subsystem within 7 days. This requirement was not satisfied. However, the subsystem was restored within 5 days of discovery of the condition.
This event resulted in the nuclear power plant being in a condition prohibited by Technical Specifications. This LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(i)(B).
Analysis of the Event
There were no actual safety consequences associated with this event. The potential safety consequences of this event were minimal. The "lA" RERS subsystem was operable during this period. In addition the "1 B" RERS subsystem was capable of operating at near design capacity. The potential increase in accident dose to the licensed operators in the main control room was determined to be minimal.
The TS surveillance requirement 4.6.5.4.c states the following:
After every 720 hours0.00833 days <br />0.2 hours <br />0.00119 weeks <br />2.7396e-4 months <br /> of charcoal adsorber operation by verifying within 31 days after removal that a laboratory analysis of a representative carbon sample obtained in accordance with Regulatory Position C.6.b of Regulatory Guide 1.52, Revision 2, March 1978, shows, the methyl iodide penetration of less than 2.5% when tested in accordance with ASTM D3803-1989 at a temperature of 30°C (86°F) and a relative humidity of 70%.
UFSAR section 6.5.1.3.1.e.4 describes the design bases for the RERS charcoal adsorber.
It states the following:
The charcoal adsorber is rated 95.0% for trapping of radioactive iodine as elemental iodine (12) and 95.0% trapping of radioactive iodine as methyl iodide (CH31) when passing through charcoal (2 inch bed depth) at 70% relative humidity. The air residence time in the 2 inch charcoal bed is not less than 0.25 seconds.
The vendor test in accordance with ASTM D3803-1989 specifies an acceptance criterion of 97.500 percent efficiency, which correlates to 2.5 percent methyl iodide penetration. The plant design basis assumes the charcoal is 95 percent efficient. Therefore, a successful analysis ensures that a safety margin factor of 2 is maintained.
The actual analysis determined the methyl iodide penetration was 7.258 (+/- 0.018) percent or an efficiency of 92.742 (+/- 0.018) percent. This resulted in the charcoal efficiency being 2.258 percent below the required 95 percent plant design basis efficiency.
Cause of the Event
The cause of the event was unplanned inoperability of the "16" RERS subsystem due to an increase in charcoal methyl iodide penetration. The charcoal bed was originally scheduled to be changed in July 2010 based on an increasing trend in penetration and the age of the bed. The rescheduling of the charcoal bed replacement was due to resource and charcoal replacement equipment issues. The age of the charcoal bed may have been a contributing cause. The TS allowed outage time was exceeded during the period when the routine sample was being analyzed by the vendor. Operations management was notified of the unsatisfactory analysis results when they became available but the seven day TS allowed outage time had been exceeded prior to the notification.
Corrective Action Completed The "16" RERS subsystem charcoal bed was replaced and tested.
Corrective Actions Planned A long term charcoal replacement strategy independent of test analysis and age will be developed.
Previous Similar Occurrences There was one previous similar event during the last three years where charcoal sample analysis results rendered the "A" Control Room Emergency Fresh Air Supply (CREFAS) subsystem inoperable. This event was reported in LER 2009-002. The corrective action for this event was to add charcoal bed age and penetration data as inputs to bed performance monitoring and replacement decision making. This corrective action was implemented and resulted in a request by the System Manager on July 13, 2009 to change the 1B RERS charcoal bed based on age. However, it did not prevent recurrence due to unanticipated resource and charcoal replacement equipment issues.
Component data:
System: Reactor Enclosure Recirculation System Component: ADS Adsorber Component Number: 1B-F261 Manufacturer: C780 CVI Corp