Determination of Realistic Margins to Fuel Heatup for Slow Loss of Regulation Accidents in a CANDU Reactor

Abstract

The primary safety objectives related to Loss of Regulation (LOR) accidents are meeting acceptable dose rates and heat transport system overpressure protection limits. Within this context the licensing requirements for operating CANDUs (E.g., R-8) require the prevention of fuel and pressure tube failures. To further simplify these acceptance criteria, the CANDU industry has adopted the prevention of dryout as a sufficient but not necessary criterion for ensuring fuel and channel integrity for slow LOR events. However, CANDU reactor designs operate at much lower heat fluxes than Light Water Reactor (LWR) designs and hence the use of dryout (or in the LWR case, Departure from Nucleate Boiling) as an acceptance criteria is excessively conservative since the sheath and fuel temperature excursions in the post-dryout regime are much more benign. It has also been shown experimentally that operation in the post dryout regime for long durations is acceptable provided that the integrated effects of time and temperature are accounted for. This important design feature of a CANDU has thus far not been credited in LOR safety analysis. In order to provide a realistic assessment of the margin during postulated LOR accidents the margin to more realistic fuel failure limits must be included. Furthermore, the use of Extreme Value Statistics provides a more accurate framework for establishing the uncertainty in the estimated outcomes by examining not just the uncertainty in individual fuel channels or trip instrumentation responses, but rather the uncertainty in computing maxima and minima of the quantity in question. This paper presents the results of best estimate analysis of the margin to fuel failure during a slow Loss of Regulation accident accounting for the extremal nature of the accident analyses and the actual margins to fuel safety.