05000457/LER-2015-001
| Braidwood Station, Unit 2 | |
| Event date: | 01-26-2015 |
|---|---|
| Report date: | 03-27-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| LER closed by | |
| IR 05000456/2001500 (4 August 2015) | |
| 4572015001R00 - NRC Website | |
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A. Plant Condition Prior to Event Event Date/Time: January 26, 2015 / 0246 hours0.00285 days <br />0.0683 hours <br />4.06746e-4 weeks <br />9.3603e-5 months <br /> CST Unit 2: Mode 1 - Power Operations, Reactor Power 100 percent Reactor Coolant System [AB]: Normal operating temperature and pressure. There was no inoperable equipment that contributed to this event.
B. Description of Event
On January 26, 2015, while performing a quarterly valve stroke surveillance the 2C steam generator power operated relief valve (SG PORV) manual isolation valve, 2MS019C, failed to close.
2MS019C, also known as a 'block valve' per the technical specification (TS) 3.7.4 bases, is an 8 inch Anchor/Darling Valve Company manual isolation gate valve for the Main Steam (MS) system. The SG PORV manual block valves must be capable of closing to isolate a failed open SG PORV valve.
2MS019C is manually operated using a 20 inch hand wheel. The hand wheel shaft drives a spur gear which ultimately drives a beveled gear. This bevel gear drives a larger 49 tooth bevel gear that rotates the valve stem nut. The large 49 tooth bevel gear is orientated vertically within the 2MS019C valve actuator gear box.
On January 26, 2015, Equipment Operators (EO) attempted to close 2MS019C, but the stem of the valve did not move. The EO then attempted to open 2MS019C, and the valve stem moved approximately one inch. The EO reattempted to close 2MS019C, but the stem moved approximately one-half inch before the hand wheel began to spin freely with little resistance.
Based upon equipment failure analysis, the 2MS019C actuator failure was caused by severe corrosion and pitting in two consecutive teeth on the 49 tooth bevel gear. The bevel gear corrosion/pitting patterns indicated there was exposure to water within the gear box. The degradation removed enough material to cause the remaining intact cross- sections to fail by overloading during an attempt to close the valve. Significant corrosion and pitting degradation were also observed on other teeth for approximately two-thirds of the bevel gear circumference. Based on the magnitude of the corrosion and pitting, it is likely that moisture had been present in the gear box for an extended period of time. The most likely source of moisture was packing leakage that condensed on the underside of the yoke, dripped onto the stem and flowed downward into the actuator gear box.
There is the potential that the valve would not have been capable of closing since the last quarterly surveillance. The degraded 2MS019C actuator condition potentially existed longer than the 30 day completion time associated with TS 3.7.4, "SG PORV," which would constitute a condition prohibited by technical specifications.
Braidwood Station, Unit 2 05000457 Event Timeline:
On August 14, 2007, a small packing leak (water was leaking into the valve yoke) was identified on 2MS019C. The packing leak stopped without any maintenance action.
On November 12, 2008, 2MS019C valve actuator was rebuilt with new parts, including the large 49 tooth bevel gear.
During this maintenance activity, water was found inside the actuator gear box. No valve or valve packing work was performed.
On April 14, 2014, a packing leak of approximately 10 drops per minute was identified on 2MS019C.
On September 17, 2014, 2MS019C packing was adjusted to resolve the packing leak.
On October 27, 2014, the 2MS019C quarterly main steam system valve stroke surveillance was successfully completed by stroking 2MS019C closed and opened 5 times.
On January 26, 2015, while performing quarterly MS system valve stroke surveillance, 2MS019C failed to close.
C. Cause of Event
2MS019C failure analysis determined the large 49 tooth bevel gear failure was caused by excessive corrosion. The bevel gear corrosion was attributed to water intrusion into the actuator gear box resulting from a historic valve packing leak. The valve packing leak identified on April 2014 was characterized as a "steady drip/small stream...steam is condensing as it exits the packing leading to approximately 10 drops per minute." 2MS019C is orientated horizontal which allows the condensing steam to be directed into the valve actuator gear box. The actuator gear box accumulates condensed steam and water, which provided the corrosion mechanism for the failed spur gear. The spur gear corrosion was localized to the portion of spur gear directly impacted by the accumulated water in the actuator gear box.
D. Safety Significance
The function of the SG PORV block valve is to isolate a failed open SG PORV. One SG PORV line for each of the four steam generators is provided. Each SG PORV line consists of one SG PORV and an associated block valve. The SG PORVs are provided with upstream block valves to permit testing at power, and to provide an alternate means of isolation.
The failure of the 2C steam generator power operated relief valve block valve, 2MS019C, to close is not considered an event or condition that could have prevented fulfillment of a safety function. This condition did not create any actual plant or safety consequences because the 2MS018C was always operable and capable of operating on demand.
E. Corrective Actions
2MS019C valve actuator was repaired on January 29, 2015. Damaged components, including the large 49 tooth spur gear were replaced.
An extent of condition review was performed verifying no current or historical SG PORV block valve packing leaks existed that could cause water intrusion into the other Unit 1 and Unit 2 SG PORV block valve actuator gear boxes.
F. Previous Occurrences
No previous similar events are known.
3. Component Failure Data The failed component was the 2C SG PORV block valve, 2MS019C actuator, model number 93-14799 manufactured by Anchor/Darling Valve Company.
This event has been reported to ICES (315341).