05000313/LER-2001-004
| Dccket Number | |
| Event date: | 07-24-2001 |
|---|---|
| Report date: | 09-20-2001 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3132001004R00 - NRC Website | |
A. Plant Status At the time of this event, Arkansas Nuclear One Unit 1 (ANO-1) was operating in steady-state conditions at 100 percent power.
B. Event Description
An automatic reactor trip occurred due to a malfunction of the Electro-Hydraulic Control (EHC) [Ja] System of the Turbine Generator [TA].
At approximately 0600 on July 24, 2001, the plant experienced a small increase in generated megawatts (MW) and a decrease in steam header pressure. Control Room Operators noticed that the setter display for the Turbine Generator EHC controls appeared to be locked up but the reference display appeared to be responsive.
After the transient, the plant stabilized. System Engineering was notified and was requested to provide troubleshooting assistance. At 0639, a second transient began with indications similar to the earlier occurrence. As the plant started to stabilize, generated MW decreased rapidly due to rapid closure of the turbine governor valves. The Reactor Protection System (RPS) [JC] initiated an automatic reactor trip on high Reactor Coolant System (RCS) (AB] pressure. The trip was not complicated. No actuation of Engineered Safety Features (ESF) [JE] systems occurred, and all control rods [AA] fully inserted. Once Through Steam Generator (OTSG) [AB] safety valves opened, as expected, for a short period. The unit was promptly stabilized in hot shutdown conditions with temperature controlled by turbine bypass valves [JI] and OTSG water level controlled by the Main Feedwater System [SJ]. Following an investigation into potential causes of the event, the reactor was critical at 0322 on July 25, 2001.
Reactor power was maintained at approximately five percent while troubleshooting continued. At 0403 on July 26, 2001, the Turbine Generator was tied to the grid, and the unit reached full power at 2300 that same day.
C. Root Cause
The root cause for the abnormal governor valve movement and subsequent plant trip was a failure of the reference up-down counter card in the EHC System. This counter is responsible for the governor valve demand signal. A failure of the setter up- down counter card also occurred. The timing of the setter counter card failure is uncertain and may have happened earlier than the reference counter card failure. However, the failure of the setter counter had no direct impact on the plant trip since PAC FORA! 316A Hilt FORM 385* UAL NUCLEAR REGULATORY COMMISSION 04m) DOCKET (2) 2001 LER NUMBERL II FACILTrf NAME (1) i unuerami_ REVISION
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004 00 it only tracks the reference counter during load control. The counter chip for the hundreds digit failed on both cards, and each card exhibited erratic behavior. The failures were confirmed by bench testing. No circuit related link to the two failures could be identified. The only interaction between the two cards besides the power supply is the digital comparator card on the counter card outputs. A review of the comparator circuit revealed no failure mode that could have resulted in the counter card failures. Both counter cards showed signs of excessive heat that may have accelerated the probability of failures.
Deficiencies involving loose connections and high resistance on signal and power grounds were found during troubleshooting on the EHC cabinet; however, these conditions are not believed to be related to the card failures.
D. Corrective Actions
The failed up-down counter cards were part number 2822A8G01 style TTL supplied by Westinghouse (Manufacturer W120). The original cards had been part number 398522 style HTL. The type card that failed had a dropping resistor and voltage regulator that added additional heat. The older style HTL cards were used as replacements.
Other immediate actions included checking of other EHC circuits for failures, checking EHC cabinets for noise or grounds, tightening loose connections, and testing power supplies.
An EHC equipment reliability study is being conducted. Results of this study will be utilized to determine the need for system upgrades and improved maintenance strategies.
E. Safety Significance
Safety systems operated as designed following the trip and the plant was safely placed in stable hot shutdown conditions. The RPS functioned properly and there were no actuations or conditions warranting actuation of any ESF system. Therefore, this event had minimal safety significance.
F. Basis for Reportability The automatic RPS actuation is being reported pursuant to 10CFR50.73(a)(2)(iv)(A). This condition was reported to the NRC Operations Center pursuant to 10CFR50.72(b)(2)(iv)(B) at 0900 CDT on July 24, 2001.
G. Additional Information
There have been no previous similar events reported by ANO as Licensee Event Reports (LERs).
Energy Industry Identification System (EIIS) codes are identified in the text as (XX).