05000395/LER-2014-004
| V.C. Summer Nuclear Station, Unit 1 | |
| Event date: | 7-22-2014 |
|---|---|
| Report date: | 9-19-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3952014004R00 - NRC Website | |
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2.0 EVENT DESCRIPTION On 7/22/2014 at 0414, while increasing in power following a planned maintenance outage, VC Summer Unit 1 automatically tripped due to low water level in "C" Steam Generator. Reactor power was 48% at the time of the turbine trip. The trip occurred when the Condensate Demineralization (WI) System Condensate Bypass Valve, XVB09210-WI, failed to open when taken to OPEN at the Filter/Demineralizer Control Panel (XPN0087). Due to procedural deficiencies, Operators at the local panel did not verify that valve XVB09210-WI actuated open. When the Condensate Polishing Demineralizer flow control valves were closed in later procedure steps, condensate flow through the polishing skid was isolated resulting in a loss of Condensate (CO) flow to the Deaerator. The loss of CO flow caused the Deaerator Storage Tank (DAST) to reach the low-low level setpoint, which tripped all Main Feedwater Pumps.
The loss of Feedwater led to an automatic reactor trip on Lo-Lo Steam Generator level in "C" Steam Generator.
3.0 EVENT ANALYSIS The CO System and the Feedwater System form the portion of the secondary plant cycle that returns water to the Steam Generators.
The CO System collects condensed steam in the main and auxiliary condensers and pumps this condensate to the DAST where it becomes Feedwater in the Feedwater System.
The WI System is operated at low power levels during plant startup to clean up the CO System. CO polishing is provided by aligning the Condensate Pumps to the WI System, forcing flow through the powdered resin type Condensate Polishing Demineralizers at up to 50% of maximum condensate flow. Each demineralizer vessel can process 25% of full condensate flow. Three Condensate Polishing Demineralizers (two in service with one in regen or standby) are filled with ion exchange resin which produces pure water by removing chemical impurities and any particles suspended in the condensate water.
XVB09210-WI, Condensate Bypass Valve, is a non-safety 18 inch Jamesbury air operated butterfly valve that failed to open during power ascension due to a failed solenoid valve assembly. The solenoid operated valve (SOV) failed due to an aged elastomeric component. The type of solenoid operated valve (SOV) that failed is only associated with the WI System.
While performing the system operating procedure to remove the WI System from service, the operator at the Filter/Demineralizer Control Panel did not verify that there was a flow path through XVB09210-WI prior to isolating the flow through the Condensate Polishing Demineralizers. This resulted in the loss of all condensate flow.
The station's Reactor Protection System performed as designed and within their functional limits. All Emergency Feedwater pumps automatically started on Lo-Lo Steam Generator level and all control rods inserted fully.
4.0 SAFETY SIGNIFICANCE A PRA sensitivity analysis was performed on the addition of a reactor trip. The initiating event frequency (IEF) is based on plant data, with the reactor trip value currently calculated as five events in 10.8 years, or 0.463/year. Therefore the new IEF, with the additional reactor trip, can he estimated as six events in 10.8 years, or 0.556/year.
The current core damage frequency (CDF) is 4.2239E-06 and the post event CDF is estimated at 4.2929E-06 (with one additional reactor trip).
The resultant change in core damage frequency (,CDF) is 6.9E-08, which is below the risk significance threshold. This delta equates to an increase of approximately 1.63% in CDF.
5.0 PREVIOUS OCCURRENCE There have been no previous occurrences of loss of condensate flow resulting in a reactor trip in the last three years.
6.0 CORRECTIVE ACTIONS The solenoid actuator for XVB09210-WI was replaced and the valve was returned to functional status. The system operating procedure for the WI system has been revised to direct the operator to verify that XVB09210-WI has actuated open before reducing the flow through the condensate polisher vessels.
Additional corrective actions being evaluated and tracked by the station's corrective action program are: Enhanced training on the WI system, additional operating procedure enhancements, and increased preventative maintenance.