05000263/LER-2002-007
| Monticello Nuclear Generating Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2632002007R00 - NRC Website | |
Description On October 21, 2002, while at 100% power, during the performance of quarterly surveillance 0003, Drywell 'High Pressure Scram and Group 2, 3, and Secondary Containment Isolation Test and Calibration Procedure, it was noted that two of the four drywell pressure instrument channel 2(B and C) high drywell pressure as-found trip setpoint values were greater than Technical Specification (TS) limits, but within the As-Found Limit (Allowable Value) of 81 inches Water Column (WC) specified in the surveillance (see area of interest below). The High Drywell Pressure TS Trip setting is required to be less than or equal to 2 psig (55.4 inches WC) for reactor protection, primary and secondary containment isolation functions. The B and C channels (PS-5-12B and C) were determined to have as- found trip setpoints of 56.0 inches WC (2.024 psig). The A and D channel as-found values were within TS limits.
The As-Found Acceptance Band criteria is based on an allowable deviation from the TS trip setting which takes into account drift and uncertainty to assure that analytical limits of the safety analyses are not exceeded. However, the allowable deviations are contained in tables in the Bases of the TS and should not be construed as an allowance to deviate from the TS. The application of the Allowable Deviations (from the Bases) to the Trip Settings (in the TS) is illustrated below.
Analytical Limit Uncertainty (excluding drift) As-Found Limit (Allowable Value) � r{ Area of interest } Allowable Deviation (Uncertainty with Drift).
Technical Specification Trip Setting Nominal Trip Set Point As-Found Acceptance Band As-Left Acceptance Band For all four channels evaluated during the surveillance, the as-found value was within the Allowable Value, and as-left settings were adjusted if necessary and verified to be within the TS trip setting. Since the as-found condition was within the Allowable Value, the safety function of each channel was not affected since the analytical limit could not have been exceeded. Therefore, while the as-found condition did not meet TS requirements, the safety function of the instruments was not affected.
1 ENS System Code-VB 2 EDS Component Code-PS
Event Analysis
Analysis of Reportability This report is being submitted in accordance with 10 CFR 50.73(a)(2)(vii) since a single cause or condition caused multiple channels of a system designed to shutdown the reactor, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident became inoperable.
The "cause or condition" is that the calibration procedure used the allowable deviation in the TS Bases to allow the as-found condition to exceed a TS trip setting.
Each channel is calibrated separately and is returned to the nominal trip setpoint prior to calibrating the next channel. Thus, no two channels are allowed to remain outside the TS trip setting at the same time.
However, for the purposes of 50.73(a)(2)(vii), both channels could probably be assumed to be outside the TS trip setting (i.e., inoperable) at the same time, since the channels are calibrated back to back.
This event does not constitute a safety system functional failure because all channels would have been able to perform their safety function.
Safety Significance
This condition is not considered safety significant. While the as-found condition was outside the TS setting value, the instruments would have functioned within analytical limits in order to perform their safety function.
It was recognized that instrument setpoint drift, inherent instrument error, operator setting error, etc.
cause deviations that could move instrument settings beyond TS setpoint. These deviations were accounted for in transient analyses. Instrument setpoint calculations and surveillance procedures were written based upon preventing instrument settings from exceeding analytical limits. Acceptance criteria ensure that an analytical limit is not exceeded. The deviation tables were provided and described in the Bases section to clearly show that analytical limits would not be exceeded due to these effects.
All instrument as-left values were within the TS value specified in the main body of the TS. Although the instrument as-found values could exceed the TS values, they were not caused by a "knowingly set" condition. When accounting for the various uncertainties, the as-found criteria assures that the TS setting, as modified by deviation, is not exceeded. Therefore, the analytical limit is preserved and all potentially affected systems and components are operable and can fulfill their safety function.
This condition has been evaluated by the Monticello Plant Probability Risk Assessment (PRA) Group.
The evaluation determined there was no significant impact on the values for core damage frequency or large early release frequency.
Per revised surveillance procedure, an as-found value that exceeds the TS limit setting requires a condition report be generated in the Corrective Action Program (CAP). The process includes an operability evaluation of the involved systems, structures and components, cause determination and assessment.
Cause
While the deviation tables were included in the originally issued TS Bases, the TS Bases are not to be used to further qualify a TS requirement. The cause of the condition is the failure to consider incorporating the deviation table information into the TS tables.
Corrective Action As discussed in LER 2002-002 (Previous Similar Event below), a review is being performed to determine whether changes to the TS instrument tables are required to incorporate Bases deviation table information into the TS.
The Site recognizes that due to the long-term nature of the corrective actions and the continuous and frequent performance of surveillances on equipment associated with the involved TS Instrumentation Tables, the potential exists for further reportable events. The Site also recognizes that future occurrences do not adversely affect the safety function of plant equipment. Interim actions are being taken to minimize the opportunity for future occurrences prior to the completion of long-term corrective actions. In cases where TS limits are exceeded, the CAP is utilized to evaluate each occurrence.
Interim corrective actions presently include:
- Revising Procedure 0003 to include a note to adjust as-left trip settings before approaching as- left setpoint limits
- Recalibration of drywell high pressure switches to tighter as-left trip settings
- Identify and revise other surveillance procedures regarding as-left adjustment Failed Component Identification None Previous Similar Event be Outside Technical Specification Value, was submitted for events where as-found setpoints exceeded the TS limit during a given calibration period. The cause of the event was calibration procedures used the allowable deviation in the TS Bases to allow the as-found condition to exceed a TS trip setting. The condenser low vacuum scram instruments were found to have exceeded TS trip settings. The instrumentation was able to fulfill its safety function.