05000249/LER-2010-001

LER-2010-001, OPRM Power Supply Failure during Maintenance Results in Unit 3 Automatic Reactor Scram

Docket Number
Event date:	10-11-2010
Report date:	12-10-2010
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
2492010001R00 - NRC Website

PLANT AND SYSTEM IDENTIFICATION

Dresden Nuclear Power Station (DNPS) Unit 3 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: 03 Event Date: 10-11-2010 Event Time: 1004 hours0.0116 days <br />0.279 hours <br />0.00166 weeks <br />3.82022e-4 months <br /> Reactor Mode: 1 Mode Name: Power Operation Power Level: 100 percent

B. Description of Event:

On October 11, 2010 at approximately 1004 hours0.0116 days <br />0.279 hours <br />0.00166 weeks <br />3.82022e-4 months <br /> CDT, personnel on Dresden Unit 3 were preparing to perform a test of the Electrical Protection Assembly (EPA) breakers. The EPA breakers are a part of the Reactor Protection System (RPS) [JC]. In order to perform this test, the 3A RPS bus was transferred from its normal power supply to the reserve power supply. This transfer evolution requires a dead-bus transfer. When the RPS bus is de-energized, a half-scram is generated, per design.

Using approved plant procedures, personnel de-energized the 3A RPS bus. The Main Control Room received the expected half-scram on the 'A' RPS. The 3A bus was re-energized from the reserve power supply. Prior to the half-scram being reset, a half-scram signal was generated from the 'B' RPS. The unexpected half-scram signal concurrently with a half-scram signal that was planned during maintenance preparations resulted in a full RPS actuation.

All control rods inserted to their full-in positions. Vessel level was controlled using the condensate [SD] and feedwater [SJ] systems. Vessel pressure control was maintained using the main turbine bypass valves to the main condenser.

Following the reactor scram, the Auxiliary Power System [EB] transferred from it main power source to the reserve power source. During the transfer the Unit 2/3 Emergency Diesel Generator (EDG) [EK] automatically started. The start signal was due to the EDG start logic sensing the contact for the main feed breaker and the contacts for the reserve feed breaker to Bus 33 being open concurrently. Based on historical events, this condition typically exists for approximately 74 milliseconds. The bus never experienced an under-voltage condition during the transfer. Therefore, the automatic start is an invalid actuation of the EDG start logic.

The plant was stabilized and troubleshooting commenced.

C. Cause of Event:

The half-scram signal that occurred on 'B' RPS was generated from the when the 3-0590-107F relay de-energized. This relay is associated with the Nuclear Instrumentation portion of the RPS system.

This relay is associated with Intermediate Range Monitor (IRM) 16, Average Power Range Monitor (APRM) 6 and Oscillation Power Range Monitor (OPRM) 6. With the Mode switch in RUN, the IRM was bypassed and could not generate a trip signal.

The operating procedure that was being used to transfer the RPS bus, DOP 0500-03 Reactor Protection System Power Supply Operation, contained a prerequisite to place APRM 6 in bypass prior to commencing the bus transfer. The OPRM was the only active component in the 107F relay string.

Troubleshooting indicated that OPRM 6 had no power. Further investigation revealed that the input fuse had blown on the OPRM power supply causing it to lose output power.

Initial examination of the circuit board, did not reveal any failures of board components other than the blown input fuse. The board was sent out for additional testing. No defects were identified besides the blown input fuse. The fuse was replaced and the power supply was successfully turned on. All indications were within expected ranges. Following seventy-two hours of operation, no defects were identified. The power supply was sent for a detailed failure analysis.

A failure analysis of the power supply indicates that the OPRM 5 Vdc power supply is susceptible to electrical noise.

The power supply is designed with a circuit (Crowbar Circuit) which senses voltage transients and prevents the voltage excursion from damaging voltage sensitive components downstream of the power supply output. When a voltage spike occurs, the crowbar circuit is activated, which essentially shorts the circuit to ground and blows the input fuse. This results in the power supply being turned off thus terminating the voltage transient on the down stream components.

Efforts have been made to identify the source of the electromagnetic interference. However, the source has not been identified at this time. Investigations and failure analyses are continuing in order to mitigate the affects of electromagnetic interferences on the OPRM power supplies.

D.� Safety Analysis:

Following the scram, the plant response and appropriate Operator actions taken, indicates that the event is of low risk significance. The safety significance of this event is minimal due to minimal consequential impact on the health and safety of the public and reactor safety.

E.� Corrective Actions:

The failed 5 Vdc power supply for OPRM 6 was replaced.

Planned work activities which require transfer of the RPS buses to their reserve power supplies are being moved to times that the OPRM system is not required to be operable. This practice will continue until a more viable resolution can be implemented.

F. Previous Occurrences:

A review of DNPS Licensee Event Reports (LERs) for the last three years did not identify any LERs associated with a failure of Nuclear Instrumentation power supplies.

G. Component Failure Data:

Manufacture Model Component ABB Combustion Engineering 2001731-017 Power Supply