Preliminary assessment of the impact of extrapolation distance on the results of Time-Average calculations in CANDU analyses

Abstract

The time-average calculations are typically used in CANDU full-core fuel management to capture the average behaviour of the core over a long period of time. This approach is necessary since CANDU reactors, which utilize natural uranium fuels, need to be refueled on a daily basis (or a few times a week) in order to keep the reactor critical. Solving the neutron diffusion equations for a time-average core is no different than for any snapshot of core configurations. The only difference is related to the burnup definition of the fuel bundles where instead of having a single value for each bundle, various other parameters are taken into account such as the fueling scheme (i.e., the number of bundles fueled during each refueling operation), the target exit irradiation or burnup, and the dwelling time (i.e., the duration spent by a bundle in one position in the core). Details regarding time-average calculations will not be discussed here. Instead, one of the components for a typical full-core physics calculation will be evaluated in more detail, namely the extrapolation distance which represents the distance from the edge of the core where the neutron flux goes to “zero”. The impacts of varying the extrapolation distance on the fueling rates and on the worth of the liquid zone controller are examined. The results of the study indicate that the total number of fuel bundles needed to support an operation of a CANDU 600 MW reactor for a year could vary by approximately ± 70 bundles and the worth of the liquid zone controller can vary by approximately 0.18 mk. Finally, it should be noted that the results presented herein are part of the initial assessments for developing a fuel-channel-dependent extrapolation distance for CANDU application.