Sensitivity Studies to Assess the Impact of Geometry and Operating/Boundary Condition Perturbations on Thermal-Hydraulic Behavior of Advanced Fuel Channels in Pressure Tube Heavy Water Reactors with Uranium and Thorium-Based Fuels

This paper reports the results of subchannel thermal-hydraulic studies (using the ASSERT-PV code) of the effects of variations and uncertainties in operating/boundary conditions and geometry on the predictions of pressure drop, dryout power, and dryout location for two types of advanced, nonconventional fuels in a pressure tube heavy water reactor (PT-HWR) fuel channel with 12 fuel bundles. The fuel bundles tested include a 37-element fuel bundle made with SEUO₂ (1.2 wt% ²³⁵U/U), with a central fuel element made of ThO₂, and 35-element fuel bundle made with (LEU,Th)O₂, using 5 wt% ²³⁵U/U low-enriched uranium (LEU), 50 wt% LEUO₂, and 50 wt% ThO₂. Results indicate that for a range of flow conditions, the dryout power for the thorium-based 35-element fuel bundle is 10% to 26% higher than that for the uranium-based 37-element fuel bundle. Variation/uncertainty in the pressure tube diameter has the most significant impact on the pressure drop, dryout power, and dryout location. Results from these studies may have implications for the operations of PT-HWRs with advanced fuels, and further modifications may be desirable to further enhance thermal-hydraulic margins.