05000325/LER-2009-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 08-14-2009 |
|---|---|
| Report date: | 10-13-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3252009003R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
Introduction Initial Conditions At the time of the event, Unit 1 was in Mode 1, operating at approximately 100 percent of Rated Thermal Power (RTP). The "A" Loop of the Residual Heat Removal (RHR) system [BO] was inoperable and under clearance to support planned maintenance.
Reportability Criteria This event resulted in the High Pressure Coolant Injection (HPCI) system [BJ] being declared inoperable.
As such, this event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D), as an event or condition that could have prevented the fulfillment of the safety function of a system that is needed to mitigate the consequences of an accident. The NRC was initially notified of this event on August 15, 2009. (i.e., Event Number 45264).
Event Description
On August 14, 2009, at approximately 19:58 EDT, the Unit 1 HPCI steam supply line inboard isolation valve (i.e., 1-E41-F002) isolated due to a spurious ambient temperature high signal from the Nuclear Measurement Analysis and Control (NUMAC) Steam Leak Detection Monitor (SLDM) [IJ]. Multiple annunciators [IB] for the SLDM system were received in the Control Room. The Reactor Operator verified that the NUMAC steam leak detection module located on panel P614 indicated fault and trip conditions, which would have caused the inboard isolation signal. The HPCI inboard isolation valve went closed as expected, due to the isolation signal. The HPCI suppression pool suction inboard isolation valve (i.e., 1-E41-F042) was already closed prior to the isolation signal and remained closed. Due to the isolation, HPCI was declared inoperable in accordance with Technical Specifications (TS) 3.5.1, "ECCS-Operating.
The SLDM system is comprised of instrument channels which monitor parameters indicative of failure of high pressure piping or steam-driven components outside primary containment on the Main Steam [SB], HPCI, Reactor Core Isolation Cooling (RCIC) [BN], and Reactor Water Cleanup (RWCU) [CE] systems.
The SLDM system also provides the Control Room operators with temperature indications of the areas being monitored. The steam leak detection system for HPCI, RCIC, and RWCU consists of four NUMAC microprocessor units located in the Control Room back panel. The NUMAC modules provide indication and trip channels for HPCI, RCIC, and RWCU leak detection. In addition, the NUMAC output signals interface with the Emergency Response Facility Information System (ERFIS) and Process Computer [ID] for various calculations and monitoring of plant parameters (i.e., Heat Balance).
Investigation of this event found that a NUMAC steam leak detection module (i.e., 1-B21-XY-5948B) failed in a manner that caused two temperature inputs (i.e., one HPCI leak detection input and one Reactor Event Description (continued) Building 20-foot south temperature indication) to be well above the setpoint, resulting in an inadvertent HPCI isolation when no valid signal was present. The SLDM instrument monitors the ambient and differential thermocouple temperatures, provides display of local temperatures and trip conditions, and provides trip outputs for system isolation and alarm functions. Temperature compensation, calibration, and mathematical operations are performed by the instrument computer. The failure occurred on the 1-B21- XY-5948B module Al thermocouple input card. The input card is one of six input cards on the module, with six isolated thermocouple signal conditioners in each input card. The purpose of the thermocouple input card is to provide valid temperature information to the leak detection monitor and to cause system isolation when a valid temperature input exceeds pre-established values. The investigation determined that a ceramic capacitor on the isolated thermocouple signal conditioner (i.e., 1B51AN), for Reactor Building 20-foot south temperature indication, on the Al thermocouple input card had failed. This created a short between the -15 volt power supply (i.e., Pin 17) and the ground (i.e, Pin 23) which caused the Reactor Building 20-foot south temperature indication to fail high and caused a spurious HPCI ambient temperature high signal that resulted in the HPCI isolation. Operability of HPCI was restored at 20:00 hours on August 15, 2009, after completing repairs and post-maintenance testing.
Event Cause The select cause of this failure is an early random failure of a ceramic capacitor on the isolated thermocouple signal conditioner on the Al input card of the NUMAC module. The failed ceramic capacitor had experienced less than half of its design life and one-sixth of the mean time between failures expectation.
Based on the physical evidence, it is believed that the ceramic capacitor developed a short over time and drew enough current to produce severe thermal degradation of the input board around this ceramic capacitor resulting in its failure. The failure of the ceramic capacitor on the input board created a spurious ambient temperature high signal that caused a HPCI isolation when no valid signal existed.
Safety Assessment The safety significance of this event is considered minimal. The Primary Containment Isolation steam leak detection system functioned as designed. With the HPCI system isolated and inoperable, the remaining Emergency Core Cooling Systems (ECCS), with the exception of "A" loop of RHR (i.e., Low Pressure Coolant Injection), along with the RCIC system and the Automatic Depressurization (ADS) system were operable and available to provide adequate core cooling if needed. No other actuations occurred, and plant staff took immediate and proper actions to return the HPCI system to operable.
Corrective Actions
The following corrective action was completed.
- Replacement of the failed thermocouple input module.
Previous Similar Events
A review of LERs and corrective action program condition reports for the past three years was completed, and the following similar occurrences were identified.
Nuclear Condition Report (NCR) 312335 documented a December 23, 2008, failure of a NUMAC thermocouple input card which resulted in an isolation of the RWCU system. The cause of the failure was contributed to a random failure of a circuit board component (i.e., AS card), however, erratic readings observed by a Senior Reactor, Operator at the time of the failure, indicate that this failure mechanism was not the same as that being reported in LER 1-2009-003. Additionally, preliminary repair summary report data confirm that the December 23, 2008, event was due to failure of different component. A second new input card installed from stores also failed. The corrective actions for this failure were to replace the cards, and send the failed cards to General Electric (GE) for a complete failure analysis. The preliminary repair summary report indicates that these failures were not repeatable. The corrective actions resulting from the report had not been finalized or implemented at the time of this failure. Thus, the corrective actions could not have reasonably been expected to prevent this event.
NCR 316200 documented a January 22, 2009, failure of a NUMAC thermocouple input card which resulted in the module experiencing a self-diagnostic test fault. The vendor repair of this component in 2001 apparently did not detect an intermittent problem that caused the subsequent faults. This was the same card that had previously been replaced on December 23, 2008 (i.e., AS card). The corrective actions were to replace the card, and send the failed card to GE for a complete failure analysis. The preliminary repair summary report indicates that these failures were not repeatable. The corrective actions resulting from the report had not been finalized or implemented at the time of this failure. Thus, the corrective actions could not have reasonably been expected to prevent this event.
Commitments No regulatory commitments are contained in this report.