05000263/LER-2002-002
| Monticello Nuclear Generating Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2632002002R01 - NRC Website | |
Description During an observation of a Rod Block Monitor' Functional Test and Calibration, it was noted that the instrument calibration As-Found Acceptance Band criteria allowed exceeding the Technical Specification (TS) value (see "Area of Interest" below). The actual As-Found calibration data was within TS values for the observed procedure. 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. It should be noted that the following statement was included in the original TS Bases and was never removed: "A violation of this specification is assumed to occur only when a device is knowingly set outside the limiting trip settings, or, when a sufficient number of devices have been affected by any means such that the automatic function is incapable of operating within the allowable deviation while in a reactor mode in which the specified function must be operable or when actions specified are not initiated as specified.
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) � (P.
Area of interest A } Allowable Deviation (Uncertainty with Drift) Technical Specification Trip Setting Nominal Trip Set Point As-Found Acceptance Band As-Left Acceptance Band I EMS System Name: � JC During review of past instrument calibration data on February13, 2002, the following occurrences were discovered in which two channels in the same division of an instrument function were found to be outside the TS trip setting but within the drift included in the allowable value:
As-Found Date Instrument Function TS Trip Setting As-Found Data Accept. Band 11/27/00 PS-5-11A2 Low Vacuum Scram3 .22 in. Hg 21.95 21.65 to 22.85 11/27/00 PS-5-11B2 Low Vacuum Scram3 ?.22 in. Hg 21.80 21.65 to 22.85 11/27/00 PS-5-11C2 Low Vacuum Scram3 ?.22 in. Hg 21.90 21.65 to 22.85 03/05/01 PS-5-11C2 Low Vacuum Scram3 22 in. Hg 21.90 21.65 to 22.85 03/05/01 PS-5-11D2 Low Vacuum Scram3 .?.22 in. Hg 21.90 21.65 to 22.85 In all cases, the as-found value was within the allowed drift, and as-left settings were within the TS trip setting. Since the as-found condition was within the allowed drift, the safety function of each channel was not affected since an 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.
Event Analysis
Analysis of Reportability This report is being submitted in accordance Wth 10 CFR 50.73(a)(2)(vii): "Any event where a single cause or condition caused ...".
The "cause or condition" is that the calibration procedure used the allmnable 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.
Plant technical staff procedure review found that there were two occasions where multiple channels of the Turbine Condenser Low Vacuum trip were found to have exceeded TS settings during a single calibration period. In both occasions, the as-found data was within the As-Found Acceptance Band criteria. Channels were found to have exceeded their TS settings during instrument surveillance testing. The condition was not in compliance with the plant's TS because the setpoint methodology is based on information that is not located in the main bodyof the TS.
2 Component Function Identifier: PS 3 EllS System Name: JE Future instances of common mode failures of instrument channels due to the use of deviation table information (e.g., two channels found between the TS setting and the as-found limit) will be reported as a supplement to this report. This does not relieve the responsibility to report failures associated with other causes as required by the reporting criteria.
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 precisely upon preventing instrument settings from exceeding TS 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 and found to have no effect on Core Damage Frequency because the instruments and the PRA model assumptions were not affected.
Cause
While the deviation tables were included in the originally issued TS Bases, including the statement quoted above, the TS Bases are not to be used to revise a TS requirement. The cause of the condition is failure to consider incorporating the deviation table information into the TS tables.
Corrective Action As discussed above, plant staff determined that there was no effect on the safety function of plant instrumentation. The affected systems and components would have performed their safety functions. The TS Bases allowance to deviate from the TS is no longer considered to be valid.
I The plant technical staff performed reviews and noted that several procedures contained instrumentation as-found acceptance criteria that permitted exceeding the TS value without a means of requiring the condition to be entered in the plant's Corrective Action Program. The plant technical staff will review all potentially affected procedures. If required, procedures will be revised to add a step requiring the initiation of a condition report to acknowledge and disposition conditions in which the as-found value exceeds the TS value. If it becomes apparent during the course of the review that the as-left condition should be revised to minimize the possibility of a future as-found value not meeting the TS, an appropriate procedure change will be made.
A review will be performed to determine whether changes to the TS instrument tables are required to incorporate the Bases deviation table information into the TS.
Failed Component Identification No components failed.
Previous Similar Event None.