Simulations of a Pressure-Tube Heavy Water Reactor Operating on Near-Breeding Thorium Cycles

The pressure-tube heavy water reactor (PT-HWR) has excellent potential as an operational technology to exploit the use of thorium. Reactor core configurations of an existing PT-HWR design with thorium-based fuels were simulated using the DRAGON/DONJON reactor physics code suite. The ultimate goal of this work was to achieve a self-sufficient equilibrium thorium cycle with a fissile inventory ratio (FIR) greater than unity (FIR >= 1.0) by altering the fueling configuration and leaving the reactor model relatively unchanged from the existing 700-MW(electric)-class PT-HWR design. A further constraint was the license requirements limiting the maximum channel and bundle powers of existing PT-HWRs. To improve the breeding potential in the PT-HWR, heterogeneous seed and blanket core configurations were selected for assessment as opposed to using a homogenous core configuration with one single type of fuel. A number of bundle design concepts were modeled with DRAGON: A 24-element variant of the internally cooled annular fuel bundle was chosen for the seed fuel, and a conventional 28-element bundle was used for the blanket fuel. Two annular heterogeneous core configurations were considered: inner seed outer blanket (ISOB) and inner blanket outer seed (IBOS). Time-average and instantaneous power calculations were performed using DONJON It was found that while the ISOB configuration could attain net breeding (FIR >= 1.0), the maximum channel and bundle powers exceeded the defined limits. When the reactor was derated to reduce these powers, the fuel cycle fell just below net breeding, although it did have a very high FIR. The IBOS configuration could meet the power limits without derating but was not self-sufficient. Despite not being net breeders, the FIR in both cases was very close to unity (0.986 to 0.995). Work is continuing to further optimize the fuel bundle concepts and core configurations and to achieve net breeding. Overall, the PT-HWR shows great promise for the current-generation implementation of the thorium fuel cycle.