05000313/LER-2004-001

LER-2004-001,

Docket Number
Event date:	09-29-2004
Report date:	08-16-2009
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3132004001R01 - NRC Website

YES NO SUBMISSION

(If yes, complete EXPECTED SUBMISSION DATE) X DATE (15) ABSTRACT (16) U (ANO-1 )U in Mode 6 for refueling outage 1R18, operators discovered that one of two redundant trains of source range neutron flux monitoring instrumentation was inoperable.

A ANO-1 technical specifications (TS) require one source range monitor to be operable in Mode 6 and one additional monitor to be operable during core alterations.A A review of plant computer historical data determined that the indication for the green train source range nuclear instrument failed at 1052 on April 29, 2004, but was not immediately detected because the count rates indicated on both channels were at the extreme low end of the scale.A At the time of the failure, core off-load activities were in progress and continued until all fuel assemblies were removed from the reactor core at 2159.

A Therefore, for approximately 11 hours1.273148e-4 days <br />0.00306 hours <br />1.818783e-5 weeks <br />4.1855e-6 months <br /> the unit was in a condition prohibited by TS due to the undetected inoperability of one of the two required source range channels. The redundant source range channel remained operable and unaffected by this condition.

A The cause of this event was determined to be an undetected failure of the signal processor drawer circuit breaker.

Following replacement of the failed components and post maintenance testing, the source range channel was declared operable.

A. Plant Status At the time this condition was discovered, Arkansas Nuclear One, Unit 1 was shutdown and in Mode 6 for refueling outage 1R18.

B. Event Description

At 2251 on April 29, 2004, during the removal of inverter Y-28 [EF] from service for planned maintenance, the ANO-1 shift manager observed that the plant computer [ID] indication for the green train source range nuclear instrumentation [IG] did not exhibit the expected response when its power supply was de-energized. Specifically, operators anticipated that the indicated count rate would decrease when the inverter was shutdown; however, the indication did not change.

The control room staff began an investigation by reviewing historical trend data on the plant computer, the Safety Parameter Display System (SPDS) [ID], and operator log readings taken at 0835 and 1944 CDT. Due to the extremely low readings, the operator logs did not provide conclusive evidence as to the time of failure of the affected instrument channel. However, it was determined from the evaluation of the plant computer and SPDS data that the green train source range nuclear instrumentation channel had failed at 1052 CDT. Additionally, the review determined that the redundant red train source range instrumentation was functioning normally.

During a portion of the time that one source range channel was inoperable, the failure had not yet been detected and transfer of fuel assemblies from the reactor vessel to the spent fuel pool continued. While such core alterations are in progress, ANO-1 technical specifications require that two source range neutron flux monitors shall be operable. With one of the required channels inoperable, core alterations and any operations that would dilute RCS boron concentration to less than the technical specification limit must be immediately suspended. Contrary to this requirement, while one of the required monitors was inoperable, the transfer of fuel assemblies continued until the core off-load was complete. This condition prohibited by technical specifications was present from 1052 until fuel off-loading activities were completed at 2159 on April 29, 2004.

Once core alterations were terminated, no fuel remained in the core and the technical specification requirement was no longer applicable.

C. Root Cause

An analysis has been performed to determine the specific cause of the failure of the source range instrumentation. The results of the evaluation have determined that the probable event initiator was a random failure of the signal processor drawer circuit breaker (circuit breaker found open). Technicians also observed that the breaker felt mechanically weak (i.e., moved easily from ' trip to close positions). Although additional failed components were identified (i.e., power supply and pre-amplifiers), they were not event initiators, but were a consequence of weaker electronic components that were stressed when the system was re-energized to support troubleshooting and repair activities.

It has also been concluded that the configuration of the Gamma Metrics source range instrumentation does not provide sufficient continuous functional monitoring capability for detection and annunciation of a channel failure during core alterations; particularly when indicated count rates are at the extreme low end of the scale.

D. Corrective Actions

Prior to resumption of core alterations for the core reload, the failed circuit breaker, the power supply, and two pre-amplifiers were replaced. The instrumentation was then tested and operability was demonstrated by performing a Channel Operability verification in accordance with the Source Range Channel Calibration procedure. This procedure is used to support compliance with technical specification surveillance requirements. Test results were satisfactory and the green train source range monitoring channel was returned to operable status.

Planned corrective action includes development of a method for continuous functional monitoring of Gamma Metrics source range instrumentation to enable the timely detection of a failure of a required channel.

E. Safety Significance

The basis for the requirement for an additional channel of source range neutron flux monitoring during core alterations is to ensure redundant monitoring capability when positive reactivity changes are being made to the core. At the time of this event,

G. Additional Information

There have been no previous similar events reported by ANO as Licensee Event Reports.

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