05000461/LER-2005-001
| Clinton Power Station | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4612005001R00 - NRC Website | |
PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
Unit: 1� Event Date: 5/4/2005� Event Time: 0106 Central Daylight Time Mode: 1 (POWER OPERATION)� Reactor Power: 95 percent
DESCRIPTION OF EVENT
During normal plant operation, either the Division 1 or Division 2 Subsystem of the Main Control Room Ventilation System (VC) [VI] is in operation with the bypass dampers [DMP] open such that airflow is directed around and not through the VC recirculation charcoal filters [FLT]. Each subsystem is operated in the high radiation mode with airflow through the recirculation filters during surveillance testing and to support plant maintenance.
On May 4, 2005, the plant was in Mode 1 with reactor power at about 95 percent. At about 0106 hours0.00123 days <br />0.0294 hours <br />1.752645e-4 weeks <br />4.0333e-5 months <br />, charcoal adsorber [ADS] samples were taken from Division 1 VC recirculation charcoal bed filter OVCO7SA to perform charcoal penetration testing in accordance with the Ventilation Filter Testing Program as required by Control Room Ventilation System Technical Specification (TS) Surveillance Requirement 3.7.3.3. The samples were later sent to a vendor for analysis with results due within 31 days per the TS Bases.
On May 9, 2005, at about 0400 hours0.00463 days <br />0.111 hours <br />6.613757e-4 weeks <br />1.522e-4 months <br />, a planned maintenance outage commenced for the Division 2 Emergency Diesel Generator System (EDG) [EK]. The Division 2 EDG outage continued until May 14, 2005, at about 0530 hours0.00613 days <br />0.147 hours <br />8.763227e-4 weeks <br />2.01665e-4 months <br />. The Division 2 VC Subsystem was considered inoperable during this time without its emergency diesel generator power available.
On May 16, 2005, the charcoal sample analysis results were received from the test vendor, showing the charcoal penetration value was 7.084 percent. After applying a correction factor of 1.153 in accordance with procedures, the penetration value was 8.168 percent. The administrative limit for charcoal penetration per the surveillance procedure is 4.5 percent with an operability limit of 6 percent, thus the sample results indicated unacceptable charcoal in the Division 1 VC Subsystem. Main Control Room operators were notified of the charcoal sample analysis results at about 1800 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.849e-4 months <br /> and they entered Action A.1 requirements of TS 3.7.3 that require restoration of the Division 1 VC Subsystem to an operable status in 7 days. At about 2115 hours0.0245 days <br />0.588 hours <br />0.0035 weeks <br />8.047575e-4 months <br /> operators commenced shifting VC operation from the Division 1 to the Division 2 subsystem to support replacement of the charcoal in Division 1 VC; the VC shift was completed by 2115 hours0.0245 days <br />0.588 hours <br />0.0035 weeks <br />8.047575e-4 months <br />. Additional charcoal samples were taken and analyzed by an independent vendor, validating the unsatisfactory charcoal analysis results.
The degraded charcoal in the Division 1 VC Subsystem was replaced. On May 18, 2005 at about 1700 hours0.0197 days <br />0.472 hours <br />0.00281 weeks <br />6.4685e-4 months <br /> the replacement charcoal was verified to be acceptable in accordance with procedures, and the Division 1 VC subsystem was declared operable.
As a result of the unsatisfactory charcoal, the 7-day Required Action Completion Time to restore an inoperable VC subsystem and the Required Actions to be in Mode 3 (HOT SHUTDOWN) and Mode 4 (COLD SHUTDOWN) per TS 3.7.3 were not met. Additionally, the Division 2 VC Subsystem was inoperable from May 9 to May 14 due to the Division 2 EDG maintenance outage.
With both VC subsystems inoperable during the period of May 9 to May 14, the station was in TS 3.7.3 Required Action D.1, requiring entry into TS Limiting Condition for Operation (LCO) 3.0.3.
Since the sample results were not known during the period of the EDG outage, Action D.1 and LCO 3.0.3 were not entered. When the sample results were known, Action D.1 and LCO 3.0.3 were no longer applicable as the Division 2 EDG and Division 2 VC subsystem were operable.
An analysis of the condition having both subsystems of VC inoperable at the same time is included in the Analysis of Event section of this report. The analysis concludes that the degraded charcoal in Division 1 VC subsystem was capable of performing its safety function.
Issue report 335698 was initiated to perform a root cause evaluation of the charcoal filter bed penetration test failure after only 17 months of service and to identify corrective action.
No automatic or manually initiated safety system responses were necessary to place the plant in a safe and stable condition. No other inoperable equipment or components directly affected this event
CAUSE OF EVENT
The cause of this event is that charcoal purchasing requirements were inadequate for the unique application at Clinton Power Station (CPS) that involves high airflow velocity, thin charcoal beds, and no airflow heaters. The charcoal installed in the Division 1 VC filter was purchased to ANSI/ASME N509-1980, "Nuclear Power Plant Air Cleaning Units and Components," that has no limit on as-manufactured moisture levels. High moisture levels of 12 percent led to aging and minor caking resulting in reduced adsorption efficiency.
The root cause analysis identified potential failure modes and determined that no poisons or excessive moisture was introduced during storage or operation of the charcoal bed. The three other VC beds had lower moisture, passed the penetration test, and did not have any evidence of the charcoal caking as was observed in the OVCO7SA charcoal. Chemical tests performed on the degraded charcoal from the OVCO7SA bed revealed insignificant levels, 2 percent, volatiles and poisons. The charcoal installed in OVCO7SA had been stored per requirements, and tested while in storage and retested when loaded into the OVCO7SA bed in November 2003 with satisfactory results. The OVCO7SA bed had not been exposed to water, high humidity, or poisons during the seventeen months it was installed prior to the May 4, 2005 sample.
The CPS VC recirculation filters have reduced penetration test margin because they are 2-inch thick beds with high (80 feet per minute) airflow and no air intake heaters. Most VC charcoal beds in the industry operate at 40 feet per minute. High airflow rates in thin beds significantly reduce the residence time and increase the penetration levels, leading to bed performance that is more sensitive to charcoal degradation.
Moisture adversely impacts activated charcoal performance and causes caking. The presence of caking indicates that charcoal grain size increased during operation. Both of these mechanisms led to reduced charcoal bed efficiency.
SAFETY ANALYSIS
This event is reportable under the provisions of 10CFR50.73(a)(2)(i)(B) as a condition prohibited by the plant's technical specifications.
This event had no impact on radiological conditions and presented no risk to control room operators from degraded control room ventilation charcoal bed filter iodine adsorption efficiency under normal and transient conditions. Under accident conditions, the radiological consequences from this event would be negligible because the measured penetration level of 8.168 percent, although greater that the CPS operating limit of 6 percent for the penetration test, was less than the 15 percent limit that is consistent with the NRC approved safety factor in GL 99-02, Laboratory Testing of Nuclear- Grade Activated Charcoal. CPS uses the more demanding ASTM D3803-1989, Standard Test Method for Nuclear-Grade Activated Charcoal, test that is the basis for the safety factor of 2 discussed in GL 99-02. (Clinton has included the GL 99-02 provisions in a proposed change to the operating license for the CPS Ventilation Filter Testing Program.) If a design basis accident had occurred while the unsatisfactory charcoal was installed, control room operators would not have received radiation exposure greater than the analyzed level.
No safety system functional failures occurred during this event.
CORRECTIVE ACTION
New acceptable charcoal was installed in the Division 1 VC Subsystem recirculation charcoal bed filter OVCO7SA and was validated to be acceptable on May 18, 2005.
Other charcoal beds in the VC Subsystems were tested and found acceptable; however, the Division 2 recirculation and make-up charcoal filter beds were replaced.
A requirement to limit moisture to less than or equal to 8 percent by weight will be incorporated into purchasing specifications for charcoal. (IR 335698-15)
PREVIOUS OCCURRENCES
None
COMPONENT FAILURE DATA
None The following table identifies commitments made in this document. (Any other actions discussed in the submittal represent intended or planned actions. They are described to the NRC for the NRC's information and are not regulatory commitments.)
COMMITMENT TYPE
COMMITMENT ONE-TIME ACTION PROGRAMMATIC (Yes/No) (Yes/No) A requirement to limit moisture to less than or equal to 8 percent by weight will be incorporated into purchasing specifications for charcoal. (IR 335698-15) Yes