05000424/LER-2012-005
| Vogtle Electric Generating Plant - Unit 1 | |
| Event date: | 10-08-2012 |
|---|---|
| Report date: | 12-3-2012 |
| 4242012005R00 - NRC Website | |
A. REQUIREMENT FOR REPORT
This report is being voluntarily submitted.
B. UNIT STATUS AT TIME OF EVENT
At the time of discovery the unit was in Mode 1. Other than described herein, there was no other inoperable equipment that contributed to the event.
C. DESCRIPTION OF EVENT
Vogtle Unit 1 experienced a mismatch in secondary steam flow during power ascension from 2% to 10% reactor power (Mode 2 to Mode 1). The control room operators confirmed the mismatch by observing divergence in reactor coolant system loop differential temperatures, secondary steam pressures, and secondary steam flows between loops one and four and loops two and three. At this time, the reactor was manually shutdown. Subsequent investigation determined the outboard main steam isolation valve on loops two and three were indicating open in the Main Control Room when, in fact, the valves were closed.
Control room main steam isolation valve (MSIV) haridswitch position indication showed 1HV-3016B (loop 2 outboard) and 1HV-3026B (loop 3 outboard) MSIVs open, which was not supported by corresponding loop steam line pressure indications. Implementation of a trouble shooting plan determined 1HV-3016B (loop 2 outboard) and 1 HV-3026B (loop 3 outboard) were closed with control room handswitches indicating open. Follow up ultrasonic testing (UT) identified stem length discrepancies on both 1HV- 3016B and 1HV-3026B. Following valve disassembly, it was identified the valve stems were sheared above the T-head.
Westinghouse representatives were consulted and conveyed to the site the material used for the MSIV stems, ASME SA564 Gr. 630 condition H1075, is susceptible to embrittlement when exposed to temperatures above 500°F. Metallurgy analysis was performed on the sheared stems validating that thermal embrittlement was the failure mechanism. The failure analysis concluded the stem fracture was a sudden failure which occurred during the opening of the valves. UT and visual inspections were performed on the remaining (6) MSIV stems for Unit 1 with no significant stem fractures detected.
D. CAUSE OF EVENT
Main steam isolation valve stem thermal embrittlement due to long term exposure to temperatures greater than 500 degrees.
E. SAFETY ASSESSMENT
The MSIV systems isolate steam flow from the secondary side of the steam generators following a high energy line break thus terminating flow from the unaffected steam generators. The failure mode results in closure of the affected MSIV thereby performing the required safety function.
F. CORRECTIVE ACTION
The valve stems for the affected MSIVs were replaced. Long range corrective actions are to replace the valves stems on all MSIVs on both units with a material resistant to thermal embrittlement at temperatures within the normal system operating range.
G. ADDITIONAL INFORMATION
1) Failed Components:
Rockwell International A-290 Main Steam Isolation Valve Actuator 2) Previous Similar Events:
None 3) Energy Industry Identification System Code: [SB] — Main / Reheat Steam 4) Other Systems Affected: None 5) Commitment Information: This report does not create any new permanent licensing commitments.