Fuels comprised of the element thorium have become increasingly popular with researchers and the public as the next generation fuel due to its ability to produce its own fissile element (U-233) and generate lower concentrations of heavy actinides. The use of thorium can possibly lead to a self-sustaining cycle whereby the addition of fissile material is not required and that the fuel can breed sufficient amounts of U-233 for a continuous supply. Research into thorium use in CANDU reactors has mainly been focused on using driver elements such as U-235 or Pu-239 to initiate the nuclear reaction by taking advantage of bundle design or by mixing the thorium and driver fuel together; however, these methods have added complexities and may not lead to a pure thorium fuel cycle, but extend the life of current nuclear fuels used. This thesis will investigate a simpler means of utilizing thorium for the intent of breeding U-233 through the use of pure thorium bundles in a once-through cycle by the ways of a heterogeneous core loading in a CANDU-6 reactor model. A 3x3 multi-cell model using DRAGON 3.06K will simulate the dual fuel model by having the centre lattice enclosing the thorium bundle and the outer eight lattices enclosing the enriched uranium bundles as the driver fuel. Next, the diffusion code DONJON 3.02E is used to produce time-average, instantaneous, and initial startup full-core simulations. As well, a brief look at the refuelling operations on the thorium channels will be done. The presence of a thorium bundle places a negative reactivity load on the multi-cell, but causes a positive insertion of reactivity for a coolant void and shutdown scenario. In the full-core modelling, the final core configuration chosen shows that thorium channels should be located in the inner core rather than in the most outer channels to produce a flattening effect on the radial profile. Thorium channels will require a combination of SEU and thorium bundles in an attempt to maintain channel power levels. Specifically, the use of 4, 6, or 8 Th bundles were investigated. The most optimal core performance shown has a radial form factor of 0.816, a total average core burnup of 18.32 GWd/t, and operates within designed power limits. It is possible to implement pure thorium bundles into a reactor set in a dual fuel mode. A careful consideration of where thorium bundles should be located in the core can help flatten the radial power distribution and help the reactor operate within the operating licensing parameters without the use of adjuster rods while breeding U-233 for a future thorium fuel cycle. / Thesis / Master of Applied Science (MASc)