05000368/LER-2002-002

LER-2002-002,

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3682002002R00 - NRC Website

A. Plant Status At the time this event occurred, Arkansas Nuclear One Unit 2 (ANO-2) was operating in Mode 1 (Power Operation) steady-state conditions at approximately 100 percent power.

B. Event Description

At 0400 CST on December 19, 2002, a Main Turbine [TA] trip occurred. This resulted in automatic actuation of the Reactor Protection System (RPS) [JC] and a reactor trip. All Control Element Assemblies (CEAs) [AA] fully inserted. No actuation of an Emergency Core Cooling System (ECCS) [JE] occurred. Plant response to the trip was uncomplicated and the plant was promptly stabilized in Mode 3 (Hot Standby) conditions.

Troubleshooting revealed that the Main Turbine trip resulted from failure of the reverse power (anti-motoring) relay for the Main Generator [TB] . After the failed relay was replaced, the reactor was critical at 2146 on December 19, 2002. The unit returned to full power operation at 0251 on December 21, 2002.

C. Root Cause

The failed relay was a Model 12GGP53C1A manufactured by General Electric Company (manufacturer code G080). The relay is a sensitive three phase time delay power directional relay designed to provide anti-motoring protection for steam turbine generators upon loss of its prime mover.

The failed relay received a thorough inspection. There was internal damage in the form of metal particles, a broken pivot bearing on the lower rotating element, and a coating of black powder throughout the surfaces inside the relay. A failure mode analysis could not be completed without potentially destructive testing and disassembly. Inspections and tests verified that the failure had not resulted from conditions external to the relay. A review of maintenance history showed that this relay had been in service since initial plant construction in the late 1970s. Industry experience with this model relay indicates that the frequency of significant problems is low. One relay failure in the mid-1990s at another facility was determined to have resulted from a loose clutch mechanism (essentially a set screw). Periodic testing and calibration of this relay was last performed in October 2000.

The root cause of the relay failure is indeterminate pending completion of inspection and testing by the manufacturer.

D. Corrective Actions

The relay was replaced. The replacement relay was monitored during the subsequent plant startup and performed satisfactorily.

The failed relay is being evaluated by the manufacturer to determine if additional information regarding the root cause of the failure can be obtained.

Based on the industry experience review, applicable procedures will be enhanced by a revision to add specific steps to check for clutch tightness during routine relay tests and calibrations.

E. Safety Significance

The RPS actuated as expected to cause a reactor trip when the Main Turbine tripped. All CEAs fully inserted. There was no ECCS actuation or radioactive release during this event. The electrical grid remained stable and all Emergency Diesel Generators [EK] were operable. The plant was stabilized in hot standby conditions without complications. Therefore, this condition had minimal actual safety significance.

F. Basis for Reportability Automatic actuation of the RPS is being reported in accordance with 10CFR50.73(a)(2)(iv)(A). A notification of this event was made to the NRC Operations Center in accordance with 10CFR50.72(b)(2)(iv)(B) at 0443 CST on December 19, 2002.

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] .