05000263/LER-2013-001
| Monticello Nuclear Generating Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i)(C), 50.54(x) TS Deviation 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 2632013001R00 - NRC Website | |
EVENT DESCRIPTION
When the condition was discovered, the Monticello Nuclear Generating Plant [MNGP] was in a refueling outage and defueled.
On April 12, 2013 at 2200, during investigation into the inability to complete a Safety Relief Valve (SRV) discharge line excess flow check valve test, a relief valve discharge monitoring instrument valve [RTV], MS-44-2B, was discovered to have been inappropriately closed. The closed valve isolated two differential pressure switches [PDIS] that impact operation of E Low-Low Set (LLS) Valve [RV]. The valve found closed was returned to its normal open position.
EVENT ANALYSIS
This event is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by Technical Specifications and 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition. This event was not considered a safety system functional failure.
SAFETY SIGNIFICANCE
The function of the SRV LLS System is to minimize the possibility of a SRV reopening while an elevated water leg is in its discharge line. The elevated water leg occurs after an SRV closes. The condensing steam in the SRV discharge line creates a vacuum which draws Torus water into the discharge line. If an SRV reopens while an elevated water leg is in the discharge line, the resulting water thrust load on primary containment could exceed its design strength. The system also reduces subsequent SRV actuations during plant transients and improves overall SRV performance.
The discovered condition would not have prevented E SRV from opening for LLS or on mechanical setpoint.
The purpose of the signal from the instruments associated with valve MS-44-2B is to provide a 10 second interlock following actuation of the E SRV, before the E SRV can be opened again. The maximum expected discharge line water leg clearing time is 5.75 seconds.
The LLS protective function is fully automatic and requires no operator action. The LLS SRVs have lower opening setpoints than the mechanical setpoints. These lower setpoints are for electrical/pneumatic actuation. In the worst case transients, two LLS SRVs are required. The third is provided for redundancy. The H and G SRVs were not affected by the isolated pressure switches and would have been capable of functioning as designed. The E SRV actuates at 1072 psig for LLS. As this is the highest set point for an SRV in the LLS sequence, it is less likely that E SRV would be required to actuate. Additionally, the Automatic Depressurization System is completely independent of LLS and was available to depressurize the Reactor.
CAUSE
Following the 2011 Refueling Outage, the E SRV tailpipe temperatures were elevated. On June 25, 2011, the E SRV main stage body valve and SRV pilot assembly were replaced. Per procedural restoration step, valve MS-44-2B was closed. This closure step was performed as part of a restoration activity to reinstall a vent cap.
A procedure change had been made in 2005, including venting steps, which did not reflect differences in tailpipe configurations on the SRVs.
CORRECTIVE ACTION
The immediate corrective action was to restore the valve MS-44-2B to open. The long-term corrective action is to revise the procedure to incorporate the difference in SRV tailpipe vent cap configuration.
PREVIOUS SIMILAR EVENTS
There were no previous similar licensee event reports in the past three years.
ADDITIONAL INFORMATION
Energy industry identification system (EllS) codes are identified in the text within brackets [xx].