05000423/LER-2001-004
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4232001004R00 - NRC Website | |
I. Event Description
On August 23, 2001, with reactor power at 100 percent, Technical Specification 3.3.3.6 Action Statement a. was entered due to the Channel 1 Gamma Metrics Neutron Flux Monitor [IP] exhibiting erratic indication. The Channel 1 Gamma Metrics Neutron Flux Monitor was restored to operable status on August 24, 2001. Subsequent investigation identified that the Channel 1 Gamma Metrics Neutron Flux Monitor began exhibiting the erratic behavior as early as August 12, 2001. As a result, the Channel 1 Gamma Metrics Neutron Flux Monitor was inoperable for more than 12 days. This exceeded the 7 day allowed outage time.
The Gamma Metrics Neutron Flux Monitor Channels 1 and 2 are required to be operable in Modes 1, 2, and 3 by Technical Specification 3.3.3.6, Accident Monitoring Instrumentation. Restoration of an inoperable Gamma Metrics Neutron Flux Monitor Channel is required within 7 days or a plant shutdown to Mode 3 is required within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and Mode 4 the following 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
Since the Channel 1 Gamma Metrics Neutron Flux Monitor was inoperable for more than 12 days, which is greater than the allowed outage time (7 days) plus shut down time to Mode 3 (6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />), this is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by Technical Specifications.
II. Cause of Event
The cause of the erratic indication by the Channel 1 Gamma Metrics Neutron Flux Monitor was a loose wiring lug fastener on the output of the isolator in the signal processor. The Channel 2 Gamma Metrics Neutron Flux Monitor was inspected and no loose wiring lug fasteners were discovered.
The Gamma Metrics Neutron Flux Monitor displays are located in the Main Control Room, but they are not on the Main Control Board. The human factor design of the Gamma Metrics Neutron Flux Monitor displays is adequate since the indications are not used during normal plant operation. Surveillance procedures were reviewed and determined to adequately address Technical Specification requirements.
The investigation of this event identified a Man-Machine Interface issue as the root cause. No alarm or warning function was provided for this instrumentation to aid the operators in identifying potential equipment problems.
III. Analysis of Event
The Gamma Metrics Neutron Flux Monitor Channels may be used post-accident to verify that the reactor is subcritical and remains subcritical. The ability to shut down the reactor and maintain it in a safe shutdown condition is one of the four safety functions of structures and systems listed by 10 CFR 50.72(b)(3)(v) and 10 CFR 50.73(a)(2)(v), Event or Condition That Could Have Prevented Fulfillment of a Safety Function. During the time period that the Channel 1 Gamma Metrics Neutron Flux Monitor was inoperable, Channel 2 Gamma Metrics Neutron Flux Monitor and additional backup indications were operable providing sufficient monitoring capability to verify the shutdown condition of the reactor, if required. This event did not result in a loss of a safety function, and is of low safety significance.
IV. Corrective Action As a result of this event, daily monitoring of the Gamma Metrics Neutron Flux Monitor Channels was established to ensure equipment operability. Daily monitoring will continue until a deviation alarm between the Gamma Metrics Neutron Flux Monitor Channels is implemented.
V. Previous Occurrences
No previous similar events/conditions were identified.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].