The Effect of the Hydrogen to Heavy Metal Ratio (H/HM) on Reactivity and Discharge Isotopics of Homogeneous Thoria-Urania Fuel

Calculations were performed using MOCUP, which includes the use of MCNP for neutron transport and ORIGEN for depletion. The MOCUP calculations were done using a unit cell (pin cell) model, where the ThO₂ varied from 65–75wt% and the UO₂ varied from 25–35wt%. The fission products and actinides being tracked in the calculations account for >97% of the parasitic captures in the fuel. The fuel pin was surrounded by four reflecting planes, where typical parameters were used for a 17×17 PWR assembly. The hydrogen to heavy metal ratio (H/HM) was varied by increasing or decreasing the water density in the cell. The results show that the drier lattices have insufficient reactivity due to the limited enrichment of the uranium. However, a slightly wetter lattice will increase the reactivity-limited burnup by 26% for the 25% UO₂ – 75% ThO₂, and 19% for the 35% UO₂ – 65% ThO₂ as compared to the standard coolant density. This is appears to be consistent with similar studies done with all-uranium lattices, where advantages are gained by hardening or further softening the neutron spectrum. Although some advantage is gained by softening the spectrum, the same can be said of all-uranium fueled cores. The spectral changes and, to a lesser extent, competing resonances between the ²³⁸U and bred-in ²³³U appear to hamper advantages in the conversion of thorium in homogeneous fuel that might otherwise be gained by shifting the neutron spectrum. Physically separating the uranium and thorium (e.g., in micro-heterogeneous and seed-and-blanket arrangements) have been shown alleviate this problem. A change in moderation may further enhance the reactivity-limited burnup of these lattices, and will be the focus of future work.