05000237/LER-2013-007
| Dresden Nuclear Power Station, Unit 2 | |
| Event date: | 11-16-2013 |
|---|---|
| Report date: | 01-15-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 2372013007R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Dresden Nuclear Power Station (DNPS) Units 2 and 3 are General Electric Company Boiling Water Reactors 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: 11-16-2013 Reactor Mode: 5 Mode Name: Refueling Unit: 03 Event Date: 11-16-2013 Reactor Mode: 1 Mode Name: Power Operation Event Time: 1019 hours0.0118 days <br />0.283 hours <br />0.00168 weeks <br />3.877295e-4 months <br /> CST Power Level: 000 percent Event Time: 1019 hours0.0118 days <br />0.283 hours <br />0.00168 weeks <br />3.877295e-4 months <br /> CST Power Level: 100 percent
B. Description of Event:
At 10:19:02 CST on November 16, 2013, indication was received in the Control Room that two Secondary Containment [NG] doors, in one access opening, were open simultaneously. A worker, staged in the interlock, reported that while opening the Reactor Building side door the Turbine Building side door opened several inches. The staged worker immediately secured the doors.
An initial troubleshooting walkdown of the interlock doors was performed by Engineering, Maintenance, and Operations. The troubleshooting walkdown did not identify any degraded system components. The interlock was operated approximately fifty cycles and was excessively challenged.
An interlock logic failure could not be recreated and the interlock doors performed as designed.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(C), any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to control the release of radioactive material.
C. Cause of Event:
A review of the electrical schematic diagram was performed to determine the possible failure modes.
In order for the event to occur as explained by the staged worker, an inadvertent loss of voltage to the latching magnets had to have occurred. Simultaneous pushing of the buttons and limit switch interference was verified to have not caused this event. A loss of power to the latching magnets could have been caused by a wiring short, inadvertent opening of the emergency switches, a fuse failure, or an intermittent malfunction of a relay.
The failure was determined to not have been caused a wiring short, inadvertent opening of the emergency switches or a fuse failure. All of these components are passive components and are less likely to fail intermittently than a component that requires actuation. The exposed wiring terminals were visually inspected and show no obvious signs of degradation. The emergency switches were visually inspected and were verified to not have been pressed throughout this event. Additionally, the emergency switch contacts are normally closed and require manual action to break the contacts.
Therefore, it was determined that the failure of the interlock to function properly was do to a relay malfunction within the interlock circuit.
D. Safety Analysis:
As discussed in NUREG-1465, "Accident Source Terms for Light-Water Nuclear Power Plants," the coolant activity phase of a loss of coolant accident begins with a postulated pipe rupture and ends when the first fuel rod has been estimated to fail. For BWRs, the coolant activity phase starts at the initiation of the accident and continues for the first two minutes following the accident. During this phase, the only activity released to the containment atmosphere is that associated with very small amounts of radioactivity dissolved in the coolant itself. The amount of radioactivity in the coolant itself is limited by Standard Technical Specifications 3.4.7, "RCS Specific Activity." The gap activity release phase begins when fuel cladding failure commences. This phase involves the release of that radioactivity that has collected in the gap between the fuel pellet and cladding. This process releases to containment a few percent of the total inventory of the more volatile radionuclides, particularly noble gases, iodine, and cesium. During this phase, the bulk of the fission products continue to be retained in the fuel itself.
As discussed in NRC Regulatory Guide 1.183, "Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors," which is based upon NUREG-1465, the gap release phase for BWRs starts two minutes following initiation of the accident, and continues until the early in-vessel release phase which starts 30 minutes following initiation of the accident.
The Bases for Standard Technical Specifications 3.6.4.1, "Secondary Containment," states that there are two principal accidents for which credit is taken for secondary containment operability. These are a loss of coolant accident (LOCA) and a fuel handling accident involving handling recently irradiated fuel.
Based on the information above, for BWR plants that have adopted an alternative source term in accordance with 10 CFR 50.67, "Accident source term," using the methodology described in NRC Regulatory Guide 1.183, no activity releases are assumed to occur for the first two minutes following initiation of the LOCA.
The Secondary Containment interlock doors were open for approximately five seconds. Based upon the short duration of the secondary containment doors being opened simultaneously and that the Secondary Containment differential pressure remained negative during the course of this event, this event is of low safety significance.
E. Corrective Actions:
As a result of the secondary containment events, site engineering performed an apparent cause evaluation that identified the need to develop a modification to improve the interlock door circuit.
Dresden engineering has completed the engineering change, and the modification is currently awaiting installation.
F. Previous Occurrences:
A search was performed to determine the number of previous occurrences for this event over the past two years. The following table presents these results for the Unit 2 Interlock:
IR Date Sort Description 1305358 12/21/2011 Unit 2 Reactor Building/Turbine Building Interlock Failure 1381599 6/25/2012 Unit 2 Reactor Building/Turbine Building Interlock Failure 1387354 7/11/2012 Unit 2 Reactor Building/Turbine Building Interlock Failure 1581320 11/5/2013 Unit 2 Reactor Building/Turbine Building Interlock Failure The search revealed that there were 4 prior occurrences, within the past two years, that specifically relate to a failure of the Unit 2 Interlock.
G. Component Failure Data:
N/A