05000278/LER-2009-007
| Docket Number | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2782009007R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 3 was in Mode 3 at 0% power when this event was discovered on 9/14/09. Unit 3 had just shutdown on 9/13/09 for its 17th Refueling Outage (P3R17). There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On 9/18/09, an Engineering evaluation determined that the ouboard Main Steam Isolation Valve (MSIV) A0-3-01A-086A did not meet its required Technical Specification (TS) minimum closure time of >/= 3 seconds. This determination was based on MSIV (EMS: ISV) stroke time testing performed on 9/14/09 at approximately 1215 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.623075e-4 months <br />. As a result of this testing, it was identified that the A0-3-01A-086A had a closure stroke time of 2.22 seconds. This time measurement was based on the observed change in open and closed indicator lights. Although the measurement of the valve's closure time using the indicator lights is conservative with respect to the actual closure time of the valve, the Engineering evaluation determined that the valve's closure time did not meet the required >/= 3 seconds. It is estimated that the actual valve closure time was 2.67 seconds.
This report is being submitted pursuant to:
10CFR 50.73(a)(2)(i)(B) — Condition Prohibited by TS — This occurrence is reportable under this criterion since there was evidence that the MSIV fast closure condition existed during plant operation. TS Limiting Condition for Operation (LCO) 3.6.1.3, Primary Containment Isolation Valves (PCIVs) requires that PCIVs be operable during operational modes 1, 2, and 3. Condition A of TS 3.6.1.3 requires that the associated penetration be isolated within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> if an MSIV is inoperable. Because the AO-3-01A 086A MSIV was inoperable during its mode of applicability for a time period greater than 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />, it was determined that a condition prohibited by TS occurred.
Repairs were made to the A0-3-01A-086A during P3R17 and the valve was returned to an operable status prior to plant startup on 10/11/09 for cycle 18 operations.
Analysis of the Event
There were no actual safety consequences associated with this event.
The purpose of the MSIVs is to isolate the four main steam lines to mitigate postulated design events. Each of the four main steam lines contains an inboard valve (inside primary containment) and an outboard valve (outside primary containment).
� Analysis of the Event, continued The safety objective of the MSIVs is to close automatically to:
- Prevent damage to the fuel barrier by limiting the loss of reactor coolant in case of a major leak from the steam piping outside the primary containment
- Limit release of radioactive materials by closing the nuclear system process barrier in case of gross release of radioactive materials from the reactor fuel to the reactor cooling water and steam
- Limit release of radioactive materials by closing the primary containment barrier in case of a major leak from the nuclear system inside the primary containment.
Each valve is a "Y"-shaped, 26-in globe valve. The main disc, or poppet, is attached to the lower end of the stem and moves in guides at a 45° angle from the inlet pipe. The valve stem travel is approximately 10 inches. An air cylinder with spring assist is part of the valve assembly and provides the motive capability for the valve. The speed of the valve closure is controlled by an oil dashpot which can be adjusted by a 3/4" needle valve located on the oil dashpot.
In accordance with TS 3.6.1.3, the isolation time of an MSIV is to be >1= 3 seconds and seconds. The closure speed of analysis to limit the release of reactor coolant or radioactive materials. The closure speed of >1= 3 seconds is to ensure that the MSIV would close at a speed slow enough so that simultaneous closure of all steam lines will not induce a more severe over-pressure transient on the nuclear system than closure of the turbine stop valves while the turbine bypass valves remain closed.
An analysis was performed that evaluated the as-found closure speeds of the Unit 3 MSIVs to determine if there were any adverse affects on the overpressure safety analysis. It was concluded that this condition did not have a significant affect on the safety analysis and the plant never operated outside of the safety analysis.
The event was considered not to be risk significant.
Cause of the Event
The cause of the event was due to not requiring preventive maintenance for the MSIV oil dashpot (EIIS: DPT) needle control valve (EIIS: FCV). Based on troubleshooting during P3R17, it was determined that when the oil dashpot stroked, a small amount of oil would leak from the o-ring seal around the stem of the 3/4" needle control valve. Over time, this leakage had resulted in approximately 6 pints of oil leakage loss out of the 1.4 gallon capacity oil dashpot. The dashpot was verified to be full during the previous refueling outage (P3R16).
However, it was determined that subsequent operations of the valve resulted in oil leakage past the needle valve stem o-ring such that the oil dashpot contained insufficient inventory of oil.
Insufficient oil in the dashpot resulted in inadequate dampening of the MSIV motion.
� Cause of the Event, continued The needle control valve had previously been determined to not require periodic preventive maintenance. The needle control valve had been in service for an extended period of time at PBAPS.
The needle control valve manufacturer is Auto-Ponent. The model number is NT-750S-C4.
Corrective Actions
The leaking MSIV oil dashpot needle control valve was replaced. Testing was performed to assure no other leaks from the oil dashpot. Other Unit 3 MSIVs were inspected for similar leakage and no similar concerns were found.
The preventive maintenance program will be upgraded to ensure that the Units 2 and 3 MSIV needle control valves receive appropriate preventive maintenance in future outages.
Previous Similar Occurrences There were no previous LERs identified concerning conditions prohibited by TS involving short MSIV stroke times.
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