05000249/LER-2014-001
| Dresden Nuclear Power Station, Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2492014001R02 - NRC Website | |
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PLANT AND SYSTEM IDENTIFICATION
Dresden Nuclear Power Station (DNPS), Unit 3, is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditions Prior to Event:
Unit: 03 Reactor Mode: 5 Event Date: 11-06-2014 Event Time: 09:12 CST Mode Name: Refuel Power Level: 000 percent
B. Description of Event:
On November 6, 2014, with the reactor in mode 5, Operations identified that an electromatic relief valve [RV] actuator failed to actuate during the performance of scheduled surveillance testing. The surveillance involves an operator manually actuating the ERV from the main control room with operators staged in the field. The operators in the field reported an audible click when the manual actuation was initiated. However, when the demand signal was given, the actuator plunger did not move and the valve did not open.
Exact valve failure time cannot be determined, however, based on actuator design vulnerability and the visual inspection and measurements of failure, it is estimated that the valve would not have been able to perform its function some time during the previous fuel cycle. Additionally, a review of the equipment history showed that there were occasions where, during surveillance testing, an additional ERV would have been inoperable. Having two of the five relief valves inoperable could challenge the ability of the ERVs to provide pressure control.
This event is being reported under 10 CFR 50.73(a)(2)(i)(B) "Any operation or condition which was prohibited by the plant's Technical Specifications," and under 10 CFR 50.73(a)(2)(v)(D), "Any event or condition that could have prevented the fulfilment of the safety function of structures or systems that are needed to mitigate the consequence of an accident as discussed in the Tech Spec basis.
Dresden Unit 3 has four main steam [SB] lines (MSL). Each main steam line has a different combination of ERVs, one Target Rock Relief valve [RV], and Safety Relief Valves [RV] (SRV). The MSL contains the Target Rock and two SRVs. The '13' MSL contains two ERVs, the 3B and 3E, and two SRVs. The remaining MSLs each contain one ERV and two SRVs.
The ERV actuators are inspected each outage to perform proactive replacement of worn sub- components.
C. Cause of Event:
The Apparent Cause of failure was determined to be that actuator design is susceptible to vibration induced wear in conjunction with the vibration on the '6' Main Steam Line near the 3E ERV. Of the remaining ERVs, degradation was discovered on the 3B ERV which is located on the same Main Steam Line.
D. Safety Analysis:
The ERVs are used, in conjunction with Low Pressure Coolant Injection (LPCI) [BO], as a back-up to the High Pressure Coolant Injection (HPCI) [BJ] system during a small area break loss of coolant accident. The ERVs are used with the SRVs and Target Rock for overpressure protection during Anticipated Transient Without Scram (ATWS) scenarios. Based upon the Dresden PSA notebook, success criteria can be met with one valve failing to operate. Degradation occurred on the 3E and 3B ERV following guide post replacement during the previous outage. Expected mechanical wear occurred on the 3C and 3D ERVs over the previous two cycles when the previous guide post replacement had been performed. 3B ERV, though degraded, passed surveillance testing during D3R23.
As HPCI, the Isolation Condenser [BL], the remaining three ERVs, the Target Rock relief valve, and eight safety relief valves were available; this failure is of very low safety significance.
E. Corrective Actions:
- The failed ERV was replaced with a hardened design during the Refueling Outage.
- The remaining Unit 3 actuators were replaced with the hardened design.
- Unit 2 actuators are scheduled to be replaced during the upcoming refuel outage.
- Detailed inspections of the supporting structure for the 3B MSL.
- Further investigation into the source of the increased vibrations on the 3B MSL will be performed.
- The new actuators will be inspected for wear during the next Unit 3 refuel outage.
Previous Occurrences:
3E failed during logic testing during the 2012 refueling outage. Failure was attributed to inadequate tightening of hardware. Inadequate tensioning of hardware was not an attribute of the current failure mechanism.
G. Component Failure Data:
Manufacturer Model S/N Type Dresser Industries 1525VX-3-0S108 BK7082 Relief Valve