05000237/LER-2013-004
| Dresden Nuclear Power Station, Unit 2 | |
| Event date: | 11-05-2013 |
|---|---|
| Report date: | 01-06-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 2372013004R00 - 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: 11-05-2013 Reactor Mode: 1 Mode Name: Power Operation Unit: 03 Event Date: 11-05-2013 Reactor Mode: 1 Mode Name: Power Operation Event Time: 0842 hours0.00975 days <br />0.234 hours <br />0.00139 weeks <br />3.20381e-4 months <br /> CDT Power Level: 092 percent Event Time: 0842 hours0.00975 days <br />0.234 hours <br />0.00139 weeks <br />3.20381e-4 months <br /> CDT Power Level: 100 percent
B. Description of Event:
On 11/05/2013, Operators received a control room alarm indicating that the Unit 2 Reactor Building [NG] to Turbine Building [NM] interlock doors had been opened simultaneously. A door operator was staged in the interlock at the time of the event and verified both doors had momentarily opened simultaneously (i.e., about one second). At the time of the event, a door operator pressed the turbine side door hand switch allowing personnel traffic to enter the interlock. Simultaneously, the reactor building side door swung open and immediately returned back to the secure position.
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:
An initial troubleshooting walk-down of the interlock doors was performed by Engineering, Maintenance, and Operations. The walk-down identified that the reactor building side door magnets had lost their latching strength and that the door could be easily opened with the magnets energized.
Upon further evaluation, the magnets were not making solid contact with the door causing this symptom. The magnets are adjustable and are designed to float horizontally within the frame to allow solid contact with door. The magnets were adjusted to allow more horizontal float in order to make full contact with the door thus eliminating the weak latching symptom.
Further engineering troubleshooting identified the magnets had not come out of adjustment, but instead a newly installed limit switch was physically interfering with the door. Maintenance was performed that replaced the LS-2 limit switch. The newly installed limit switch is approximately 0.150 inch wider than the previously installed mechanically deformed switch. Due to the design configuration of the limit switch, this size difference caused the door to make physical contact with limit switch before making flush contact with the magnets in the door frame. The distance between the magnets and the door caused inadequate latching strength.
A known design vulnerability exists in the U2(3) 517' RB-TB interlock door control logic. The fundamental design of the interlock circuit does not incorporate an exclusive-or logic (XOR) scheme.
This means if both doors receive a signal to open at precisely the same time, the logic could allow both doors to open. Simultaneous pushing of the buttons was verified, in both events, to have not occurred by the door operator staged in the interlock. Although this was not an apparent cause of these events, this is a system vulnerability that could lead to an interlock door failure.
Upon review of the interlock circuit logic design and system performance, engineering has determined a more robust interlock circuit is required to obtain desired performance. A modification is required to remove system single point vulnerabilities and incorporate redundant mechanical latching mechanisms.
D. Safety Analysis:
The Secondary Containment interlock doors were open for approximately one second. 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 a modification is currently in process.
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 The search revealed that there were 3 prior occurrences, within the past two years, that specifically relate to a failure of the Unit 2 Interlock.
G. Component Failure Data:
N/A