Thorium, a fertile nuclear fuel that is nearly three times as abundant as uranium, represents a long-term energy source that could complement uranium and eventually replace it. With the expected refurbishment and new construction of pressure tube heavy water reactors (PT-HWRs) within the international community, there is an opportunity to gain experience with thorium-based fuels and to start the transition toward the use of thorium as part of the nuclear fuel cycle.

This paper presents an evaluation of fuel types that could be implemented in the near-term to transition into thorium-based fuels in current PT-HWRs. The near-term fuel consists of small amounts of thorium (in a traditional 37-element fuel bundle that is mostly filled with natural uranium or slightly enriched uranium). In addition, a modified 37-element fuel bundle type comprised of slightly enriched uranium fuel (1.2 wt% 235U/U or less), a thorium central element, and the mass equivalent of 1-cm thorium end pellets was studied. Both lattice physics depletion simulations and full-core time-averaged neutron diffusion simulations were carried out to evaluate the performance and safety characteristics of the different studied full-core configurations.

The results demonstrate that adding small amounts of thorium into the fuel of a 37-element bundle is feasible, through enrichment, without reducing power in the reactor or incurring a severe burnup penalty. The most viable core configuration is a core filled with modified 37-element fuel containing slightly enriched uranium dioxide with 1.2 wt% 235U/U. Even with the addition of 1.2 kg of thorium metal to the bundle, significant gains are achieved, including an increased margin to maximum bundle power limit of 40 kW and a 50% increase in fissile utilization.