05000237/LER-2014-003
| Dresden Nuclear Power Station, Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2372014003R02 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
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: 05-03-2014 Reactor Mode: 1 Mode Name: Power Operation Event Time: 1209 hours0.014 days <br />0.336 hours <br />0.002 weeks <br />4.600245e-4 months <br /> CDT Power Level: 100 percent
B. Description of Event:
On 04/29/14 at approximately 0943 hours0.0109 days <br />0.262 hours <br />0.00156 weeks <br />3.588115e-4 months <br />, U2 experienced an electrical transient which resulted in a generator excitation step change and an increase from 180 MVAR to 350 MVAR. A team was assembled to perform complex troubleshooting of Automatic Voltage Regulator [EL] (AVR) components. Troubleshooting was performed based on vendor recommendations and the data was used to hypothesize the component failure. Based on the troubleshooting completed, it was identified that AVR Channel 2 was not functioning properly. In addition, a 3 percent mismatch was identified between Channel 1 and Channel 2 AVR output.
The decision was made to make Channel 1 match Channel 2 by raising the low limiter set-point and then force the channel swap. The transfer procedure was reviewed and approved by Dresden and seven revisions were issued to address comments. Several Technical calls were conducted to validate the recommended AVR channel swap actions. The troubleshooting team had determined a phase was missing in the AC power to the AVR converter bridge it was determined the channel transfer should only be performed if all the fuses for the 480 Volt input sources were intact.
During the channel transfer, generator excitation voltage started to decrease. When the AVR was switched from Channel 2 to Channel 1, it automatically swapped back to channel 2 and transferred into manual control. Main Generator MVARS dropped to -200 MVARs and began swinging from -400 MVARs to -200 MVARs.
Operations unsuccessfully attempted to manually raise MVARS. The Main Generator Out-Of-Step Relay actuated resulting in a Main Generator trip and reactor Scram. Subsequent review identified that a logic block feedback circuit was not accounted for when developing the channel transfer procedure.
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:
The Root Cause of this event was determined to be attributed to insufficient validation of vendor products, following major equipment upgrades, to ensure reliability of the newly installed equipment.
Vendor equipment design details require high reliance on vendor support to understand, operate and troubleshoot equipment anomalies.
Site personnel could not perform an independent failure analysis because the proprietary information was not readily available. In addition, vendor supplied simulator equipment was not designed to replicate actual conditions that existed during troubleshooting. Therefore, the cause of some anomalies could not be validated, resulting in additional vendor reliance.
The root cause also identified four contributing cause:
1. Insufficient risk factor assessment for designs which are highly reliant on vendor expertise or vendor provided complex digital technology.
2. The Unitrol 6000 Dresden Unit 2 Channel Transfer Procedure provided by the vendor was inadequate.
3. Automatic Voltage Regulator did not meet all design requirements.
4. Less-than-adequate Automatic Voltage Regulator Factory Acceptance Test and Site Acceptance Test.
D. Safety Analysis:
The Conditional Core Damage Probability (CCDP) associated with the automatic reactor scram was minimal because the key mitigating systems were available. In this event, mitigating systems were not impacted and were available during the reactor shutdown. Therefore, this event was considered non- risk significant and the safety significance of this event was considered minimal.
E. Corrective Actions:
Affected components were replaced during the force outage. The Automatic Voltage Regulator was successfully tested and it has been operating since May of 2014.
The Corrective Action to Prevent Recurrence (CAPR) is to revise HU-AA-1212, titled Technical Task Risk/Rigor Assessment, Pre-Job Brief, Independent Third Party Review and Post-Job Brief, to address risks associated with vendor / sub-vendor design / technical products including risks associated with digital technology.
A Corrective Action has been created to generate procedure guidance for performing a channel swap with a degraded AVR.
F. Previous Occurrences:
There have been no similar events at the same plant. This event is isolated to the ABB Unitrol 6000 equipment. The equipment involved in this event was installed in November of 2013 and therefore no other similar events have been recorded.
G. Component Failure Data:
Manufacturer Model S/N Type ABB Unitrol 6080 101681-842-1 -