Conference Proceedings Paper
Instrument Calibration Optimization at Bruce Power: ECI Loops
9th International Conference on CANDU® Maintenance - 2011 December 04-06
Vinod Chugh (AMEC NSS)
Mariela Angelova (AMEC NSS)
Ranbir Parmar (AMEC NSS)
Vivian Wang (AMEC NSS)
Hong Xie (AMEC NSS)
Joan Higgs (AMEC NSS)
Jim Schut (Bruce Power)
Ian Cruchley (Bruce Power)
Most instruments in a nuclear power plant are calibrated at regular intervals to ensure consistency with the assumptions in the plant Technical Specifications and/or Safe Operating Envelope (SOE) compliance limits (e.g., As-Found Tolerance). In the Instrument Uncertainty Calculations (IUC), As-Found Tolerance for instrument drift is estimated based on statistical analysis of As-Found and As-Left calibration data such as that carried out for Bruce NGS by EPRI in 1998. Bruce specific drift values were found to compare favorably with industry benchmarks.
Recently a significant amount of work has been done by EPRI and IAEA on extending calibration intervals of safety related instrument. Reduction in calibration frequency reduces time commitments on the part of Authorized Nuclear Operators and safety system qualified Control Maintenance Technicians, and allows more schedule flexibility.
To establish the proof of concept, As-Left/As-Found tolerances and available margins have been evaluated for the Bruce B Emergency Coolant Injection (ECI) system instrument loops to determine whether an extension of the calibration period from one year to two or three years is justifiable on the basis that the risk of non-compliance with SOE is small and is acceptable. Out of a total of 104 loops with 301 instruments that were analyzed, 225 instruments (75%) are qualified for calibration interval extension to three years.
Sensitivity assessment of the effect of proposed changes in calibration intervals on the ECI system unavailability has also been performed based on the current Bruce Power ECI unavailability model. The results show that, assuming that failure rates are independent of the calibration intervals, the largest increase in ECI Predicted Future Unavailability is approximately 17% for a very small LOCA accident if a three-year calibration interval is implemented.
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