Evaluation of uranium-based fuels augmented by low levels of thorium for near-term implementation in pressure tube heavy water reactors

Thorium, a fertile nuclear fuel which is nearly three times as abundant as uranium, represents a long-term energy source that could complement uranium and eventually replace it. To facilitate the gradual transition from uranium-based fuels to thorium-based fuels, it may be advantageous in the near-term to introduce small amounts of thorium (less than 7% of the total fuel mass) into uranium-based fuels in pressure-tube heavy water reactors (PT-HWR). Down-blending natural or slightly enriched uranium dioxide with thorium dioxide for fuel pellets placed at the ends of the fuel stack of a conventional 37-element fuel bundle could help reduce axial power peaking, while incorporating thorium dioxide into the central element could reduce coolant void reactivity (CVR). A series of 2-D lattice physics simulations were carried out to evaluate the performance and safety characteristics of uranium-based fuel bundles with small amounts of thorium fuel added. The simulation results were complemented by an approximate model for evaluating the potential economic characteristics. The cases studied involve modifications to fuel composition, central element materials and the addition of thorium dioxide to the fuel stack. Results suggest it should be possible to incorporate thorium into the fuel cycle using existing 37-element fuel bundle geometry. Advantages to incorporating thorium include: a reduction in the CVR through a thorium central element, breeding of ²³³U, maintaining front-end fuel costs at or below the price of natural uranium fuel and maintaining maximum linear element ratings within 6% of those achieved using natural uranium 37-element fuel. (author)