05000237/LER-2015-002
| Dresden Nuclear Power Station, Unit 2 | |
| 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 |
| 2372015002R02 - NRC Website | |
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PLANT AND SYSTEM IDENTIFICATION
Dresden Nuclear Power Station (DNPS), Unit 2, 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: 02 Event Date: 02-07-2015 Reactor Mode: 4 Mode Name: Cold Shutdown Event Time: 12:29 CST Power Level: 000 percent
B. Description of Event:
On February 7, 2015, with the reactor in mode 4, an Electromatic Relief Valve (ERV) [RV] actuator failed to open during the performance of an extent of condition testing. The testing involved an operator manually actuating the ERV from the main control room with operators and engineers staged in the field. However, when the demand signal was given, the 2C actuator plunger did not move and the valve did not open.
Corrective actions involved replacement of all four Unit 2 ERVs with an upgraded design. This completes the replacement of all ERVs for Dresden Unit 2 and 3.
This failure has been determined to be of very low safety significance.
Based upon a review of relevant information (i.e., the probable cause of the failure), 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.
Exact valve failure time cannot be determined, However, Based on the actuator design vulnerability and weakness in relief valve maintenance practices due to procedure deficiency, 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. Therefore, this event is also being reported 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.
C. Cause of Event:
The failed ERV actuator sub-components were inspected in the field and immediately following removal. Based upon the in-field inspection, the 2C ERV actuator's binding point was identified to be at the top of the guide post below the top of the top guide post bushing. PowerLabs performed a failure Dresden Nuclear Power Station, Unit 2 05000237 analysis of the 2C ERV that confirmed that the binding point was located at the top of the spring guide post and near the top of the bushing. Additionally, the report identified some additional wear and an additional potential binding point on the plunger arm. PowerLabs was not able to replicate the failure, however, it was noted that the ERV appeared to have been shock loaded at some point during transportation.
The failure mode was evaluated to determine the failure mechanism of mechanical binding. The mechanical binding was determined to have been caused by preferential wear between the guide post and bushing due to an alignment issue. The 2C ERV actuator had measurable material loss on the bushing. The ERV actuator is normally open and de-energized. The bushings on the 2C were last replaced in 2005 per WO 636642. The wear due to actuation has been determined to be insignificant and is not related to the wear mechanism. The basis for the failure mechanism was determined by engineering inspection and a Powerlabs autopsy.
The preferential mechanical wear between ERV actuator sub components was determined to have been caused by the guide posts being in constant contact with the bushings during operation. The rigid guide post was noted to have been slightly angled away from the solenoid centerline, from the base of the post, with all base bolting completely intact and torqued, and interfering with the inside diameter of the bushing. These guide posts have sometimes been found slightly angled in previous Was, requiring additional maintenance effort to bend the post into straight, concentric alignment with the bushing.
Without this additional maintenance attention, the guide post and bushing would have had constant contact for the duration of cycle operation. The valve would have passed all as-left testing because the wear mechanism had not yet occurred.
The guide posts being in constant contact with the bushings during operation was determined to have been caused by the guide posts not having optimal alignment performed during maintenance. The only opportunities to inspect, repair if necessary, and verify that the guide posts are aligned with the bushings is during maintenance inspections performed during each refuel outage. During the inspection following the 2C failure, the posts were found to be in a non-optimal alignment. The posts were slightly angled away from the solenoid centerline, from the base of the post, with all base bolting completely intact and torqued, and interfering with the inside diameter of the bushing. Per vendor guidance, the posts should be aligned concentric with the bushings.
The guide posts not having optimal alignment performed during maintenance was determined to have been caused by a lack of procedural guidance to specify how to align the guide posts. The root cause of failure is that there is a lack of maintenance procedural guidance to align, determine excessive wear, and characterize wear of the ERV actuator sub-components.
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.
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:
- Dresden replaced the four Unit 2 ERV actuators with an improved design
- Implement the procedure revision to provide specific guidance on how to align all components during re-assembly, provide specific component replacement guidance based on wear measured, and provide specific procedural guidance on how to characterize the wear observed on the ERV actuator sub-components.
E. Previous Occurrences:
A similar event occurred in 2003 where, during D3M10, 3E ERV differences were noted. An issue report noted that the multiple actuator differences (i.e., Post and Bushing) caused the post to be slightly below the top of the upper bracket bushing and slight wear allows the bracket to ride (hang up) on top of the spring post when attempting to actuate with slight finger pressure.
The actuator was found to be degraded, but it was verified to have performed its design function during a bench test. No failure was identified by the station, and the degraded sub-components were replaced as a conservative measure.
This event was determined to be similar to the 2C ERV failure, however, since a failure did not occur during the 2003 event, there was insufficient information obtained that could have reasonably been used to foresee the failure in February 2015.
G. Component Failure Data:
Manufacturer Model S/N Type Dresser Industries 1525VX-3-0S108 DA39300 Relief Valve