Evaluating the Behaviour of the Infinite Multiplication Factor of Depleted Uranium Fuel Bundles in CANDU Analyses

Abstract

The CANDU reactor design utilizes natural uranium fuel bundles throughout its lifetime except during the initial core configuration. In order to suppress the excess amount of positive reactivity, the depleted uranium fuel bundles are utilized in the initial core along with the usage of strong neutron absorbers (i.e., boron or gadolinium) in the moderator. The term depleted is used to indicate that the contents of ²³⁵U in the fuel are less than the ones corresponding to the natural uranium bundle. Typically, one particular “enrichment” is utilized in the initial core of CANDU reactors. A study will be performed to optimize the distribution of the depleted uranium bundles in the initial core as well as trying to utilize various level of “enrichments”. As a precursor to that optimization study, some understanding of the behaviours of depleted fuel bundles is required. The objective of this paper is to evaluate the neutronics behaviour of depleted uranium bundle with different levels of enrichment, in particular the behaviour of the infinite multiplication factor. The variations in the behaviour of this quantity as a function of the “enrichment” level, the level of poison in the moderator, and the amount of pressure tube diametral creep are evaluated.