05000313/LER-2005-002
| Arkansas Nuclear One - | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) |
| 3132005002R00 - NRC Website | |
A. Plant Status At the time this condition was discovered, Arkansas Nuclear One, Unit 1 (ANO-1) was shutdown and in Mode 6 for refueling outage 1R19. Reactor core reload activities were in progress.
B. Event Description
ANO-1 Technical Specifications (TS) requires one source range nuclear instrumentation [IG] channel to be operable in Mode 6 and one additional channel to be operable during core alterations.
At 2134 on December 1, 2005, during the initial phase of reactor core reload activities, operators identified that source range nuclear instrumentation channel NI-0502 was inoperable.
During the initial phase of core reload, source range counts are typically less than 0.1 counts per second (CPS) until several fuel assemblies are placed near a detector. The reload sequence had begun with the loading of four twice-burned fuel assemblies, the first near NI 0502, the second near NI-0501, the third near NI-0502, and the fourth near NI-0501. As the fourth assembly was placed near N1-0501, operators noticed a disparity between the two source range channels. When the second fuel assembly was placed close to N1-0501, its indication increased to approximately 0.12 cps, whereas NI-0502 indicated approximately 0.04 cps with two assemblies close to it. Upon identification of the disparity, core alterations were immediately secured, as required by Technical Specifications.
The control room staff initiated an investigation to determine the cause of the disparity in readings between the two channels. Upon opening the signal processing drawer for NI-0502, operators discovered that the "non-operate" alarm light inside the drawer was illuminated.
This alarm indicates either a loss of minimum signal expected due to alpha decay within the detector, or power supply low voltage. It was subsequently determined that the high voltage power supply for NI-0502 had failed.
Due to the extremely low source range counts during the initial phase of core reload, the operator logs did not provide conclusive proof as to the time of failure of N1-0502; however, it was concluded that at least two fuel assemblies had been moved with only one source range channel operable.
C. Root Cause
The cause of the failure of the source range channel was a random failure of its high voltage power supply. The successful completion of surveillance testing prior to core reload did not identify any high voltage power supply degradation.
The root cause of this event involves the design of the source range instrumentation, which does not provide sufficient continuous functional monitoring capability for detection and annunciation of a channel failure during core alterations when indicated count rates are at the extreme low end of the scale. Although annunciation is available to indicate low count rates, there is no annunciation outside the signal processing drawer that would identify a power supply failure during low count rate conditions.
D. Corrective Actions
Prior to resumption of reactor core reload, the failed power supply was replaced and the instrument channel was tested and proven operable.
As stated previously, an alarm located inside each source range signal processing drawer indicates either a loss of minimum signal expected due to alpha decay within the detector, or power supply low voltage. The "Control of Unit 1 Refueling" procedure was revised to require inspection of these drawers prior to beginning refueling activities.
E. Safety Significance
The basis for the requirement for an additional channel of source range nuclear instrumentation 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, ANO-1 was in Mode 6 with core reload in progress. When NI-0502 was identified as being inoperable, only four fuel assemblies were loaded in the reactor vessel. Additionally, the redundant source range channel (NI-0501) was unaffected by this condition and remained operable and capable of alerting the operators of any unexpected reactivity changes. The unavailability of one channel of source range instrumentation did not adversely impact the operator's ability to perform required actions based on this parameter. Therefore, the safety significance of this event was determined to be minimal.
4 F. Basis for Reportability ANO-1 Technical Specifications requires that two channels of source range nuclear instrumentation be operable whenever core alterations are in progress. Since at least two fuel assemblies were moved with less than two channels operable, this condition represented operation prohibited by Technical Specifications and is reportable pursuant to 10CFR50.73(a)(2)(i)(B).
G. Additional Information
A previous similar event was reported in ANO Licensee Event Report 50-313/2004-001-01 dated August 16, 2004. Although the corrective actions associated with this report addressed identification of source range instrument signal failures, they were not adequate to identify a power supply failure during low count rate conditions.
Energy Industry Identification System (ENS) codes are identified in the text as [XX].