05000335/LER-2013-001
| St. Lucie Unit 1 | |
| Event date: | 3-12-2013 |
|---|---|
| Report date: | 5-10-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3352013001R00 - NRC Website | |
Description of the Event
On March 12, 2013 at 1451 EDT,. St. Lucie Unit 1 was in Mode 1 at 100W reactor power when the spurious closure of KV-08-1B, 1B Main Steam Isolation Valve (MSIV), resulted in an automatic reactor trip on Thermal Margin/Low Pressure (TM/LP). Upon troubleshooting and valve disassembly, it was discovered that interference between the internal tail link and the valve body prevented the valve disc from fully opening until the spindle was on the back seat. This allowed unintentional loading of internal parts resulting in the failure of the valve's lower shear pin. The .pin failure led to spindle/disc separation and inadvertent closure of the valve.
Cause
A root cause evaluation determined that the internal interference was caused by an oversized tail link.
The dimensional nonconformance that caused interference with the valve body and prevented the valve from opening to the backseat was not identified during manufacturer certification or 'receipt inspection.
Analysis of the Event
The Unit 1 MSIVs, HCV-08-1A and HCV-08-1B, were upgraded as part of the recent extended power uprate (EPU) to ensure they could withstand impact stresses associated with spurious closures (no reverse flow} at the higher steam velocities of EPU conditions. This upgrade included replacing internal valve components with larger ones made of stronger materials and replacing the pneumatic actuator with an electro- hydraulic actuator. The Unit 1 MSIVs were originally manufactured by Schutte & Koerting (S&K). The replacement valve actuator is manufactured by Enertech. The new valve internal components replaced in SL1-24 were manufactured by Flowserve based on Kalsi Engineering analysis.
To resolve the interference contact of the tail link and valve body, metal was removed from the oversized tail link by grinding. Critical valve and internal dimensions and clearances were verified to be in accordance with design criteria. A thorough valve, piping, and support analysis was performed. This included evaluating reuse of the valve body and specific internal parts. It also included analysis of the piping and supports. Walkdowns of the piping and supports were performed both inside and outside of containment. Extensive inspections and non-destructive exams (NDE) were performed on the valve body, valve internals, and supports.
The 1A MSIV, HCV-08-1A, was also dissembled. An oversized tail link was also found.
The tail link was contacting the valve body just like the 1B MSIV and would have initially prevented the valve from reaching the backseat. However, the spindle tab had partially torn allowing the valve to backseat. The partial tear prevented an in- service failure. The 1A MSIV tail link was reworked to eliminate the body interference and the spindle was replaced.
The spare tail link in stores was inspected, and dimensional checks confirmed it was also oversized. The spare tail link was reworked. The other installed MSIV parts were inspected, and no installation or other dimensional discrepancies were identified.
During post modification testing, stroke length measurements for HCV-08-1A and 1B did not meet the acceptance criterion defined by the work order instructions. This was an opportunity to identify a problem with internal interference in HCV-08-1B when the limit switches could not be set as specified. The resulting actions focused on limit switch setpoint adjustment and limit switch mounting changes to address the condition.
Safety Significance
The Conditional Core Damage Probability (CCDP) and Conditional Large Early Release Probability (CLERP) values were evaluated for the stated event and were found to be significantly below the thresholds required by RG-1.174 for the risk to be "Small" where CCDP is below 1.0E-06 and CLERP is below 1.0E-07. Given the response of the plant and the insignificant CCDP and CLERP values, the health and safety of the public were not affected by this event.
This reactor trip event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A) as an automatic actuation of the reactor protection system (RPS). This event had no significant safety consequence since the Reactor Protection System successfully performed its intended safety function upon the failure of the 1B Main Steam Isolation Valve.
Corrective Actions
The corrective actions listed below are entered into the site corrective action program. Any changes to the actions will be managed under the corrective action program.
1. The tail links (including the spare) were reworked and full spindle travel to the back seat were verified for HCV-08-1A and HCV-08-1B.
2. The Station will revise the maintenance procedure (and associated vendor manual) for Main Steam Isolation Valve disassembly, inspection, repair and reassembly to include steps to ensure that the valve opens completely until the spindle is on the back seat. This will also include a caution that that the H-Link pins can be overloaded by the actuator if the valve does not open to the back seat.
Similar Events A search of the corrective action database was performed and identified no similar issues that were related to the valve failures in this report.
Failed Component(s) Main Steam isolation Valve Manufacturer The Unit 1 MSIVs were originally manufactured by Schutte & Koerting (S&K).
The replacement valve actuator is manufactured by Enertech.
The new valve internal components replaced in SL1-24 were manufactured by Flowserve based on Kalsi Engineering analysis.