05000237/LER-2014-002
| Dresden Nuclear Power Station, Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2372014002R02 - 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 Reactor Mode: 1 Event Date: 04-12-2014 Event Time: 1012 hours0.0117 days <br />0.281 hours <br />0.00167 weeks <br />3.85066e-4 months <br /> CDT Mode Name: Power Operation Power Level: 100 percent
B. Description of Event:
On April 12, 2014, at 10:12am, without warning, Dresden Unit-2 Main Power Transformer [EL], Siemens Serial # 1731.658, faulted causing a Reactor Scram. The transformer had been in service for 4 years and 5 months. According to the critical parameters historical data, there were no indications warning of this failure. The result of this Main Power Transformer fault was a turbine-trip/reactor-trip.
All control rods [AA] inserted to their full-in position. Following the reactor trip, all systems operated as expected.
The Main Power Transformer failed by flashover from 'A' Phase Low Voltage (LV) Lead to 'A' Phase High Voltage (HV) Coil. The Root Cause investigation was indeterminate, based on Siemens' Final Draft Forensic Analysis Report. Based on a Support Refute analysis of possible failure modes, the "Most Probable" Root Cause was determined to be a combination of insulation issues that appear to be from the manufacturing process.
The transformer was replaced with a transformer of a different design and a different manufacturer. As a further corrective action, improved inspections during transformer manufacturing are to be instituted.
This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A), any event or condition that resulted in manual or automatic actuation of any of the system listed in paragraph (a)(2)(iv)(B).
C. Cause of Event:
Since a root cause was not identified, a "most probable" Root cause was determined. The Root Cause team evaluated the potential failure modes based on the Siemens Forensic Analysis Report, Siemens FMEA, and IEEE C57.125, "IEEE Guide for Investigation of Transformers.
Based upon the evidence from the teardown inspection, the team identified that varnish insulation was missing in some locations and what appeared to be mechanical scratches in the varnish were found under intact paper. Additionally, one of the scratched locations had carbonization, which could have been created by a partial discharge between strands while the transformer was operating. The indications mentioned did not exist on the same location of the intact phases. Therefore, the most probable cause of the transformer failure was determined to be a combination of the various insulation issues.
D. Safety Analysis:
The safety significance of this event was determined to be minimal based upon the availability of the required systems to safely shut down the reactor.
E. Corrective Actions:
Based upon the event and the investigation performed, the following corrective actions were taken or are planned to be taken:
1. The transformer was replaced with a transformer of a different design and from a different manufacturer.
2. Improved transformer manufacturing oversight will be instituted to ensure critical steps regarding physical inspections and lead manipulation/installation inspection are required to ensure quality insulation.
3. Procedures will be revised to ensure critical inspections during manufacturing processes are identified. Inspection plans will specify required holds for inspections by licensee, or designee.
Previous Occurrences:
No previous occurrences of this failure type wereldentified.
Component Failure Data:
Manufacturer Model . S/N Type Siemens ELIN 1731658 TDQ-A27D9K-99