05000296/LER-2007-002
| Browns Ferry Unit 3 | |
| Event date: | 07-24-2007 |
|---|---|
| Report date: | 09-24-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2962007002R00 - NRC Website | |
I. PLANT CONDITION(S)
Prior to the event, Unit 3 was operating at approximately 100 percent of rated thermal power (RTP) (3458 megawatts thermal). Unit 1 was operating at approximately 95 percent of RTP (3285 megawatts thermal) and Unit 2 was operating in Mode 1 at 100 percent RTP (3458 megawatts thermal). Units 1 and 2 were unaffected by the event.
II. DESCRIPTION OF EVENT
A. Event:
On July 24, 2007, at 1645 hours0.019 days <br />0.457 hours <br />0.00272 weeks <br />6.259225e-4 months <br /> central daylight time (CDT) the Unit 3, Division II Emergency Core Cooling Systems (ECCS) [AD] Analog Trip Unit (ATU) Inverter [EJ] failed due to a cleared fuse. The inverter provides 120 VAC instrument power to the High Pressure Coolant Injection (HPCI) [BJ] system pump discharge flow controller. WA subsequently declared HPCI inoperable. After verifying the Reactor Core Isolation Cooling [BN] (RCIC) system was operable, a fourteen day Technical Specification (TS) Limiting Condition for an Operation (LCO) for inoperable HPCI was entered. Channel B of the reactor feed pump turbine [SJ] and main turbine high water level trip [TA] was also declared inoperable, entering a seven day TS LCO.
The drywell normal pressure indicator and the bulk suppression pool water temperature indicator was also declared inoperable, entering a thirty day LCO.
At approximately 2318 hours0.0268 days <br />0.644 hours <br />0.00383 weeks <br />8.81999e-4 months <br /> CDT, following fuse replacement, the affected TS instruments were returned to normal. The ECCS inverter output voltage and frequency was monitored for HPCI was then declared operable and the TS LCO exited. The other LCOs previously entered were exited.
Because HPCI was inoperable during the timeframe the inverter was out of service, TVA is submitting this report in accordance with 10 CFR 50.73(a)(2)(v)(B) and (D) as; any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to: remove residual heat; and to mitigate the consequences of an accident.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Major Occurrences:
July 24, 2007 at 1645 hours0.019 days <br />0.457 hours <br />0.00272 weeks <br />6.259225e-4 months <br /> CDT � Unit 3 Division II ECCS ATU Inverter failed due to cleared fuse.
July 24, 2007, at 2105 hours0.0244 days <br />0.585 hours <br />0.00348 weeks <br />8.009525e-4 months <br /> CDT � TVA made an eight hour non-emergency report per 10 CFR 50.72(b)(3)(v)(B) and 10 CFR 50.72(b)(3)(v)(D).
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
The event was immediately apparent to the control room staff through numerous alarms and indications.
F. Operator Actions
None.
G. Safety System Responses
No operational transient was introduced by the failure of the Unit 3 Division II ECCS ATU Inverter.
Ill. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause of the loss of the Unit 3 Division II ECCS ATU Inverter is the input fuse cleared during the transfer of the 250 VDC RMOV Board from the alternate to the normal power supply.
B. Root Cause
The probable cause for the fuse clearing was a voltage transient on the ECCS inverter during the 250 VDC RMOV Board power supply transfer that resulted in a higher than normal inrush current across the input fuse.
C. Contributing Factors
None.
IV. ANALYSIS OF THE EVENT
The purpose of the safety-related 250 VDC system is to provide a highly reliable power source to various plant loads without the dependence on AC power. In addition to the DC loads directly supplied by the 250 VDC systems, safety related inverters convert the DC power to 120 VAC to supply critical instrumentation and control system loads.
The ECCS Inverter input fuse is a high speed fuse used to protect the silicone-controlled rectifier in the event of a fault or overload. Just prior to the loss of the ECCS inverter, the inverter was in service and, 250 VDC RMOV Board 3A was powered from its alternate source. WA postulated that during the transfer of 250 VDC RMOV Board back to the normal source the inverter experienced a voltage transient. This transient induced a current overload on the input fuse which caused it to clear. Following the fuse replacement, the ECCS inverter output voltage and frequency was monitored for approximately one hour with the replacement fuse in service. Since the fuse was replaced, the inverter has operated continuously with no abnormalities. The cleared fuse was sent to TVA Central Labs for analysis. No irregularities could were identified.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of this event were not significant. BFN TS allow continued power operations for up to 14 days with HPCI inoperable as long as RCIC is operable. As soon as it was determined that HPCI was inoperable, operations verified RCIC was operable and then entered the appropriate LCO. During the timeframe that HPCI was inoperable; the other required Emergency Core Cooling systems remained operable. Therefore, TVA concludes that the health and safety of the public was not affected by this event.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions
The cleared fuse was replaced. The ECCS inverter output voltage and frequency was monitored for approximately one hour with the replacement fuse in service.
B. Corrective Actions to Prevent Recurrence (1) TVA will revise the operating instructions to require the affected ECCS ATU Inverters de-energized prior to a scheduled transfer of the input voltage source.
VII. ADDITIONAL INFORMATION
A. Failed Components
BUSS Semiconductor fuse, part No. FWH-125A, 550V AC/DC manufactured by Bussmann, Inc.
B. Previous LERs on Similar Events None.
C. Additional Information
Corrective action document for this event is PER 127921.
D. Safety System Functional Failure Consideration:
This event involved a safety system functional failure as referenced in 10 CFR 50.73(a)(2)(v), and will be included in Performance Indicator Reporting in accordance with NEI 99-02.
E. Scram With Complications Consideration:
This event did not result in a complicated scram as defined in NEI 99-02.
VIII. COMMITMENTS
None.
(1) TVA does not consider these corrective actions as regulatory requirements. The completion of these actions will be tracked in TVA's Corrective Action Program.