05000483/LER-2003-009

LER-2003-009,

Callaway Plant Unit 1
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown
Initial Reporting
ENS 40263	10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown
4832003009R00 - NRC Website

I. � DESCRIPTION OF THE REPORTABLE EVENT

A. REPORTABLE EVENT CLASSIFICATION

This event is reportable per 10CFR50.73(a)(2)(i)(A), a plant shutdown required by Technical Specifications.

B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT

Callaway Plant was in Mode 1 at 100 percent power.

START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT

Electrical power inverter NN11 failed on 10/20/03 and repairs were not completed prior to exceeding Technical Specification 3.8.7 Action completion time limits which required a plant shutdown.

a NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES At 0721, 10/20103, while Callaway was operating at 100 percent power, electrical inverter NN11 [Model SV12075 manufactured by Solidstate Controls, Inc.! failed causing a momentary loss of voltage on safety-related 120 VAC bus NN01. Normally, the transfer between power supplies is "bumpless" where there is no interruption of power to the downstream loads and is accomplished using a static transfer switch. In this event, a zener diode failed in the static transfer switch circuit and prevented a bumpless transfer from occurring.

When the static transfer switch circuit failed, a momentary loss of power lasting approximately four seconds occurred. As a result of the momentary power loss, the following plant perturbations were experienced:

• BNLCV0112D,the Refueling Water Storage Tank (RWST) suction supply valve to "K Centrifugal Charging Pump (CCP). opened,

• "Am and "B" Reactor Coolant Pump (RCP) Thermal Barrier Heat Exchanger component cooling water isolation valves BBHVO013 and BBHVO014 isolated,

• EFHVO043, "A' Essential Service Water to Service Air Compressor 'A", closed, and when NNO1 re-energized:

• ABPIC0001A, "A Steam Generator Atmospheric Steam Dump (SIG ASD) controller, returned to service in the MANUAL control mode with a 50 percent OPEN signal,

• Pressurizer master Pressure Controller BBPK0455A, did nat respond to increasing Reactor Coolant System (RCS) pressure.

When the CCP suction swapped to the RWST, the Control Room staff isolated normal letdown and maintained charging through the RCP seals. In response to BBPK0455A not operating as expected, a Reactor Operator took manual control of BBPK0455A, restored Pressurizer pressure to its normal band.

returned BBPK0455A to automatic, and then verified that it operated correctly. After the initial response, the Control Room staff used plant off-normal procedure OTO-NN-0001 to respond to the event and complete required immediate actions. The plant was stabilized and power remained at 100 percent.

TIS 3.8.7 Action A, for Inverters — Operating, was entered at 0721, 10/20/03. This allowed 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> to restore the faulted inverter to Operable or enter Action B which required entering Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. Troubleshooting efforts had not identified the failure mechanism by the end of the 24-hour Action A time limit. In anticipation of non-compliance with TIS Action A, a controlled plant shutdown was commenced at 0100, 10/21/03 and ENS Event Notification 40263 was made at 0420 (Central Time), 10/21/03.

Investigation of the NN11 failure mechanism ultimately identified the failure to be the result of a failed zener diode in the static transfer switch circuitry. Replacement of the static transfer switch electronic circuit board corrected the fault and NN11 was declared Operable at 2202, 10/21/03. A reactor startup was performed on 10/24103 and normal power operation resumed.

During the troubleshooting process to determine the failure mechanism of NN11, additional investigations were conducted to determine if the observed plant perturbations were to be expected for a momentary loss of power. It was determined that all alarms and automatic actions were expected results for a loss of power to NN01, except for the automatic closure of BBHVOO13 and BBHVOOI4. A definitive cause for closure could not be established, however, research concluded the most probable cause for their operation was due to a momentary spike in the power supply for the valve flow control circuitry when power was

• restored to NNO1.

E. METHOD OF DISCOVERY OF EACH COMPONENT, SYSTEM FAILURE, OR PROCEDURAL ERROR

The discovery of the failure of NN11 was identified through simultaneous alarm annunciators and plant responses. The failure mechanism was identified through subsequent troubleshooting efforts.

II. � EVENT DRIVEN INFORMATION

A. SAFETY SYSTEMS THAT RESPONDED

No safety systems were actuated as a result of this event.

B. DURATION OF SAFETY SYSTEM INOPERABILITY

Inverter NN11 was inoperable for 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br />, 41 minutes.

C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT.

This event did not result in a significant challenge to nuclear safety or a significant risk to the health and safety of the public. With the exception of the events immediately surrounding the momentary loss of power and the restoration of power, the components powered by NNO1 remained functional. Additionally, the redundant train remained operable throughout this event No unanticipated alarms or automatic actions were caused by this event. An approved procedure providing specific guidance for a loss of power to NNOI was available and was implemented. Plant operations were conducted in compliance with Technical Specification requirements.

Therefore, this event did not result in a significant challenge to nuclear safety.

III. � CAUSE OF THE EVENT The cause of the event was the failure of the static transfer switch in NN11. An extensive review of internal and external Operating Experience (OE) concluded that this particular failure was an uncommon, random failure and not documented as occurring previously.

a IV, � CORRECTIVE ACTIONS static transfer switch circuit board.

was written to document this event. Procedure OTO-NN-0001 actions identified by this event.

review of past CARs in an effort to identify similar past that were applicable to the static transfer switch failure.

was performed and no similar LERs were identified.

below are from the IEEE Standard 805-1984 and IEEE Standard Corrective actions were to replace the faulted Callaway Action Request (CAR) 200307636 will be reviewed for potential revision to include

V. PREVIOUS SIMILAR EVENTS

CAR 200307636 documents a historical occurrences. No past CARs were identified A review of past LERs from 2000 to present

VI. ADDITIONAL INFORMATION

The system and component codes listed 803A-1984 respectively.

System: � EF Component: � INVT - � — � - � -