05000346/LER-2004-002

LER-2004-002, Reactor Trip During Reactor Trip Breaker Testing Due To Fuse Failure

Docket Number
Event date:	08-04-2004
Report date:	10-04-2004
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3462004002R00 - NRC Website

DESCRIPTION OF OCCURRENCE:

On August 4, 2004, with the plant in Mode 1 operating at approximately 100 percent power, quarterly surveillance testing was in progress on the Reactor Trip Breakers and Reactor Protection System (RPS) [JC] utilizing procedure DB-MI-03011, "Channel Functional Test of Reactor Trip Breaker B, RPS Channel 1 Reactor Trip Module Logic and ARTS Channel 1 Output Logic." In accordance with the surveillance test procedure, technicians placed Reactor Trip Module (RTM) Reactor Protective Channel Switches A and B to the "SIMULATE TRIP" position. When the second switch was placed in the "SIMULATE TRIP" position, a reactor trip occurred at 1023 hours0.0118 days <br />0.284 hours <br />0.00169 weeks <br />3.892515e-4 months <br />.

Unit response to the reactor trip was as designed. Plant parameters stabilized within their normal post­ trip values. One Main Steam Safety Valve (MSSV) [SB-RV], SP17A9 continued to lift following the reactor trip at a value less than its setpoint of 1100 psig. Operators lowered Steam Generator 2 pressure to approximately 970 psig in order to allow SP17A9 to reseat.

Initial notification of this automatic reactor trip was made to the NRC at 1114 hours0.0129 days <br />0.309 hours <br />0.00184 weeks <br />4.23877e-4 months <br /> on August 4, 2004, in accordance with the four-hour reporting requirement of 10CFR50.72(b)(2)(iv)(B) (Event Number 40921). This report is being submitted in accordance with the 10CFR50.73(a)(2)(iv)(A) as an event that resulted in actuation of the Reactor Protection System.

APPARENT CAUSE OF OCCURRENCE:

Electric power to the Control Rod Drives (CRDs) and Motors [AA] is supplied by redundant power channels, and the Reactor Trip Breakers are arranged such that breaker "A" or "C" AND "B" or "D" must be open to remove power from the CRDs. When Reactor Trip Breaker "B" was opened as part of the surveillance test, Reactor Trip Breaker "A" also opened unexpectedly removing power to the CRDs allowing the Control Rods to insert into the Reactor Core.

Reactor Trip Breaker "A" opened due to a fuse failure in a circuit designed to open the breaker when an undervoltage condition is detected on either redundant CRD System power channel. The circuit, part of the Source Interruption Device (SID) installed in both Reactor Trip Breakers "A" and "B", receives undervoltage detection input from both redundant CRD power channels. Refer to Figure 1 for a simplified functional diagram of the CRD circuitry. The fuse in the Reactor Trip Breaker "A" power channel undervoltage detection input that receives power from Reactor Trip Breakers "A" and "C" had failed prior to the test, leaving the Reactor Trip Breaker "A" Source Interruption Device in a half-tripped condition. When Reactor Trip Breaker "B" was opened as intended during the test, input from Reactor Trip Breaker "B" supplied power channel into the Reactor Trip Breaker "A" Source Interruption Device also depowered, indicating an undervoltage condition. With both inputs indicating an undervoltage condition, the Source Interruption Device responded as designed, initiating a shunt trip of Reactor Trip Breaker "A", which resulted in a trip of the reactor due to the depowering of the remaining CRD power source.

While the Reactor Trip Breaker Source Interruption Device fuses are equipped with failed fuse indicator lights that can be observed when the Reactor Trip Breaker cubicle is opened, surveillance test procedure DB-MI-03011 contained no procedural check to determine the status of these fuses prior to test performance. The fuse in Reactor Trip Breaker "A" Source Interruption Device was determined to have failed due to age and fatigue.

ANALYSIS OF OCCURRENCE:

There were no safety concerns identified during or as a result of this event. When the Control Rod Drive Trip Breakers opened, all Control Rods inserted. The Steam Generator outlet pressure increased due to the closing of the Main Turbine Stop Valves [TA-ISV]. The Turbine Bypass Valves (TBVs) [SB- PCV] and the Atmospheric Vent Valves (AWs) [SB-PCV] opened and the MSSVs lifted in response to the increasing secondary system pressure. The MSSVs (except for SP17A9 as described above) and the AWs closed as Steam Generator outlet pressure decreased. The Safety Features Actuation System [JE] was not challenged during this event, and there were no significant deviations in Reactor Coolant System pressure, temperature, or inventory control; or in Steam Generator pressure or inventory control.

CORRECTIVE ACTIONS:

The failed fuse in Reactor Trip Breaker "A" was replaced, along with the remaining fuses associated with the Source Interruption Device supply circuits as a preventative measure for age/weakening.

The Surveillance Test procedures that perform the Channel Functional Test of all four Reactor Trip Breakers were revised to require verification of fuse indication prior to performing the test. Additional procedures that may open the Reactor Trip Breakers will be identified and revised as applicable by December 5, 2004, to require verification of acceptable fuse status prior to opening the Reactor Trip Breakers.

Preventive maintenance activities will be developed by November 15, 2004, to replace the fuses associated with the Control Rod Drive System Source Interruption Devices on a periodic basis to preclude failure due to age and/or weakening due to long term cycling.

FAILURE DATA:

There have been no Licensee Event Reports at the Davis-Besse Nuclear Power Station involving a reactor trip caused by a fuse failure in the previous three years. While the discovery of failed open fuses is a relatively common event, there has been no actuation of engineered safety feature equipment at the Davis-Besse Nuclear Power Station as a result of a failed fuse in the past three years.

Energy Industry Identification System (EIIS) codes are identified in the text as [XX].

NP-33-04-002-00 CR 2004-04927 FIGURE 1 Simplified Functional Diagram of the CRD Circuitry

UNDESIRED OPENED BY

BREAKER TRIP DB-MI-03011BF211 BE211FROM SID FUSE

FAILURE

r SIDMG SET CRD TRIP 4 i II� BKR B 0 TRIP � I I—r- CRD TRIP 0BKR A CRD TRIP

BKR D

0 CRD TRIP FUSE BKR C Volt Reg FAILED CY- C1P-1 (Thr-v-Y-1 /-**N