An evaluation of the net breeding capability of heterogeneously-fuelled pressure-tube heavy water reactors with the thorium fuel cycle
MetadataShow full item record
The thorium fuel cycle is a promising future option for an alternative to uranium. The pressure tube heavy water reactor (PT-HWR) has for decades been considered capable of achieving net breeding on the thorium fuel cycle, but this capability has not been conclusively demonstrated. The goal of this work was to perform reactor physics modelling of the 380-channel, 700 MWe PT-HWR, attempting to achieve a self-sustaining equilibrium thorium cycle with specific focus on operational viability. The DRAGON neutron transport code was used to model ThO₂/²³³UO₂ fuel lattice cells. Sensitivity analyses were performed for the fissile nuclide content, specific power, and average fuel temperature. Thorium-based fuels are found to have a strongly negative power coefficient of reactivity, leading to criticality concerns in the event of a prolonged shutdown. Several fuel bundle concepts were modelled in order to determine the most favourable assembly for breeding. The results showed no significant benefit to other concepts, therefore the standard 37-element bundle was selected due to its wealth of operational experience and well-known operating margins. Homogeneous fuelling was found to be impractical for breeding, and heterogeneous fuel bundles were found to not offer significant improvement while increasing complexity. As a result, heterogeneous core configurations using homogeneous bundles were investigated in full core calculations. The DONJON core physics code was used in conjunction with homogenized cross-sections calculated by DRAGON to model the complete reactor, including reactivity devices and supporting structures. Seven fuelling configurations were simulated and iteratively improved through an empirical process. Ultimately, none of the studied configurations could achieve net breeding. The most favourable configuration was found to be a heterogeneous seed and blanket core where the blanket (composed of 1.4 at% ²³³U) was placed in the central region and the seed (composed of 1.6 at% ²³³U) was placed in the periphery. Two variants of this configuration (differing on the refuelling scheme used in the blanket) were further investigated with instantaneous power simulations with 10 full power days of refuelling. Both variants were found to abide by the existing license limits on maximum bundle and channel power. The variant using 4-bundle shift in both blanket and seed could tolerate an increase in reactor power to 110% FP while maintaining a comfortable margin to safety. A number of postulated misfuelling events were simulated for both variants, and the responses were found to be controllable by the existing reactivity systems. It is noted that there are significant sources of error in the results of this work. Advances in computational methods as well as better nuclear data for the thorium fuel cycle are required for more accurate predictions. Overall, while the PT-HWR has great potential as a very high converting reactor, based on the results of this work, the existing design cannot operate as a net breeder. From an economic perspective, a gain in power output may be more beneficial than an entirely closed fuel cycle.