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Event Report Forms (ERFs) are filed to the IAEA. Sometimes there are multiple event reports, updating the rating, the event description. Here you only see the latest event report

ERF send date: Fri, 07-06-1991
ERF description: During routine calibration of the Regional Overpower (ROP) detectors for Shutdown System One and Shutdown System Two on 91/05/30, an incorrect calibration factor was applied. The calibration factor used was applicable for operation at 96% full power, however, the reactor was at 100% full power. This error in calibration was discovered approximately six hours later at which time all detectors were recalibrated to the correct value for operation at full power. The calibration error was in the unsafe direction, therefore, the rip effectiveness on this parameter for both shutdown systems was reduced for the period which the incorrect calibration existed.
BASIS FOR RATING: This event has been rated as Level 2, an "Incident", on the INES scaleunder the category of "Defence-in-depth Degradation". The effectiveness ofthe Regional Overpower trip parameter was reduced on both shutdown systems, however, alternate trip parameters provide back-up to ROP for most events. On the basis of partial effectiveness of the ROP parameter and theavailability of alternate trip parameters, the safety function availabilitywas deemed "adequate", with respect to the INES definition, to limit theconsequences of any initiator (Table A1, expected initiator, reactor trip).
DIFFICULTIES IN RATING: Table A1 yields a choice of level 1 or 2. Level 2 was chosen because both shutdown systems were affected by this common mode failure.
ERF rating date: 01-01-1991
ERF contact person: Mr. W. Andersen
ERF address: 700 University Avenue, Canada
ERF affiliation: Ontario Hydro H11-B1, Nuclear Technology Services Division

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