05000263/LER-2002-006
| Monticello Nuclear Generating Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| 2632002006R00 - NRC Website | |
Description On October 8, 2002, station personnel were performing a review of Nuclear. Instrumentation surveillances. While performing this review, it was determined that the APRM1 Recirculation Flow Instrumentation Calibration Surveillance procedure allowed a condition that could momentarily prevented a flow bias scram signal. The surveillance required station personnel to close the valves2 to isolate the transmitter3 and then open the equalizing valve resulting in a half scram. During the time frame required to open the equalizing valve and obtain the half scram signal, there were no operable APRM instrument channels available to provide a flow bias scram signal in the trip system being tested due to the transmitter being isolated. Engineering placed a hold on the surveillance procedure and initiated a procedure revision to correct the condition.
Event Analysis
Analysis of Reportability The event is rep-ortable under 10 CFR 50.73(a)(2)(v)(A), "Any event or condition that could have prevented the fulfillment of the'safety function of structures, or systems that are needed to: (A) Shutdown the reactor and maintain it in a safe shutdown condition." Since
- the flow bias neutron scram provides protection for the fuel safety limit in the unlikely event of a thermo-hydraulic instability, the fulfillment of a needed safety function was not met for the momentary period during surveillance performance.
This event constitutes a safety system functional failure.
Safety Significance
The safety significance of the event is assessed as minimal. The transient and overpressure safety analyses take credit for the clamped Hi-Hi Scram setpoint at and this setpoint was unaffected by a loss of flow bias instrumentation. The thermo- hydraulic stability analysis considers the effects of flow-biased setpoint; but this is a concern only at reduced recirculation flows. Flows were normal so the conditions, which would warrant this trip, were not present.
The PRA Group has concluded that although the condition of operating without the intended number of APRM instrument channels in the operable condition is highly undesirable, the 1 EllS System Code IG 2 Component Code RN 3 Component Code FFT additional risk that can be attributed to inadvertent past operations in this condition at Monticello would be extremely small for the following reasons:
- The flow biased APRM scram setting is clamped at a value of clamp is not affected by the flow input from the recirculation system.
- There are operating restrictions, which prevent the reactor from being operated in high- power low-flow regions where thermal-hydraulic instabilities are considered to be a potential threat. There are also abnormal operating procedures which address the monitoring and control of neutron flux oscillations, should such oscillations resulting from thermal-hydraulic instability occur for any reason. Thus, barriers in addition to the flow- biased scram exist to protect the fuel safetylimit from challenges arising from thermal- hydraulic instability concerns.
- The fraction of operating time since 1992 when the flow-biased scram was more limiting than the 120% clamp value was small and limited to plant startups, shutdowns, and significant power reductions. Furthermore, within this subset of operating time, the time when core operation was near the regions of increased thermal-hydraulic instability susceptibility (high power/low flow) was even smaller. Finally, the momentary time periods within the procedure when the surveillance was performed while operating at � , high power/low flow core conditions would be a very small subset of the prior two conditions.
Cause
The cause_ of the event was an incorrect procedure revision perfo-rmed in 1992, which allowed the transmitter valves to be isolated prior to equalizing transmitter-pressure with the
- - resulting half scram. The procedure writer appears to have not recognized the significance, of the two-step process of valve isolation. During an early 1990's effort to reduce inadvertent scrams, the manual half scram insertion during this procedure was eliminated- with the expectation that the half scram would occur as the transmitter was isolated.
Corrective Actions
The surveillance procedure has been revised to include a step to insert a half-scram signal prior to performing the isolation of the transmitter valves (Rev 29 of Surveillance 0026).
Failed Component Identification No component failure occurred in this event.
Previous Similar Events
reactor scram that occurred during an instrument surveillance due to a valve operation error. The LER describes a physical plant event rather than failure to comply with the plant's licensing bases with respect to equipment operability determination.
Return to Service of Individual Channels during Instrument Surveillance," describes a technical specification violation due to having more than the allowed number of instrument channels and trip systems to be considered inoperable. The event was the result of a procedural deficiency where independent verifications of instrument channels were done after all the channels were calibrated and tested rather than after each individual channel was calibrated and tested.
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