05000498/LER-2012-001
| Telephone Number (Include Area Code) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| LER closed by | |
| IR 05000498/2012007 (20 November 2012) | |
| 4982012001R01 - NRC Website | |
No structures, systems, or components were inoperable at the start of the event that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
On March 27, 2012, Operations declared the Extended Range (E/R) Flux Lower Range Nuclear Instrument (NI-45A) inoperable based on the failure to meet Technical Specifications (TS) channel check acceptance criteria per the "Remote Shutdown Monitoring AND Accident Monitoring Instrumentation Channel Checks" Procedure . Subsequent troubleshooting discovered an issue with the channel's isolation circuit card, AT1. The AT1 card was found with a degraded condition which affected the indication of NI-45A Lower Range. With this degraded condition, Extended Range (E/R) Flux Lower Range Nuclear Instrument (NI-45A) would not have been able to perform its design function. After replacement of the AT1 card, the surveillance procedure was completed satisfactory on March 30, 2012 and the instrument was returned to an operable condition.
Each E/R channel has a Lower Range (which provides indication in shutdown conditions (counts per second)) and an Upper Range (which provides indication at power). For the time period under evaluation, Unit 1 was operating at power and the E/R Lower Range indication was saturated and considered unreliable. The E/R Lower Range indication is considered unreliable above approximately 1E-4% Power (100,000 CPS). At 100,000 CPS, saturation occurs in the Extended Range Lower Range Nuclear Instrumentation circuitry and a channel may indicate less than 100,000 CPS.
A review of channel data indicated that starting in December 2011; the NI-45 E/R Lower Range indication began a downward trend. Subsequent Channel Checks performed in January and February (monthly surveillance) were completed satisfactorily. These Channel Checks are performed with an acceptance criterion that notes the Lower Range indications may be unreliable, and that other indications such as E/R Upper Range output may be used to determine problems with the detector. During this time period, there were no issues found with the detectors E/R Upper Range output indication.
As stated previously, a degraded condition (downward trend) involving the E/R Lower Range indication had been evident since December 2011. Even though this indication is considered unreliable, there is reasonable confidence that in this instance, the indication showed the degrading performance of the AT1 card. Based on how the Surveillance procedure is performed, the Channel Check between NI-45A and NI-46A would have failed to meet the "factor of 10" acceptance criteria on February 29, 2012, (NI-045A = 10,527 cps, NI-046A = 106,057 cps). Therefore, it was concluded that the E/R Lower Range indication was inoperable from February 29, 2012 until the FORM 366 (10-2010) Nuclear or radiological safety was not affected. This event had no impact to the safety of the public or station personnel. This event did not impact the reliability of plant operation or production capacity. There was no impact to the Core Damage Frequency or Large Early Release Frequency associated with this event.
III. CAUSE OF THE EVENT
A. The root cause of this event was the lack of timely and dependable testing to identify a failure prior to exceeding the Limiting Condition for Operation (LCO).
This investigation identified that the monthly channel check test method provided in the Remote Shutdown Monitoring and Accident Monitoring Instrumentation Channel Checks procedure, OPSP03-SP-0001, is not a reliable method to determine operability of the low range channels due to their unreliable indication when at power. With the required monthly frequency, exceeding the 7 day LCO is possible. A more reliable method would be by using the presently designed built in test pushbuttons. Performing a periodic test to use pushbuttons S1 and S2 to test Integrated Computer System (ICS) point NI0045A would have identified the fault with the degrading AT1 much earlier and would have prevented this event from being reportable.
IV. CORRECTIVE ACTIONS
A. Initiate periodic preventive maintenance activities to press test switches 1 and 2 to test E/R Low Range indication twice weekly and verify startup rate indication on ICS.
FORM 366 (10-2010)