The transition from natural uranium to 1.2% slightly enriched uranium (SEU) has been modelled in an 1100-day time-dependent fuel-management simulation for a CANDU 6 in which the reactivity devices have been relocated. The fuelling strategy was simply to replace natural uranium by SEU, two bundles at a time. Two main challenges were encountered during the transition: 1) maintaining critically when the transition first began, when the SEU was added to the low-worth locations at the channel ends, and 2) keeping maximum bundle powers to within acceptable limits when there were two or four SEU bundles in the channel. The ability to relocate the adjuster rods was found to have little impact on the ability to shape the axial power distribution during the transition. Alternate strategies are suggested for facilitating the transition.
