Monte Carlo criticality analysis of simple geometries containing tungsten–rhenium alloys engrained with uranium dioxide and uranium mononitride

The critical mass and dimensions of simple geometries containing highly enriched uranium dioxide (UO2) and uranium mononitride (UN) encapsulated in tungsten–rhenium alloys are determined using MCNP5 criticality calculations. Spheres as well as cylinders with length to radius ratios of 1.82 are computationally built to consist of 60 vol.% fuel and 40 vol.% metal matrix. Within the geometries, the uranium is enriched to 93 wt.% uranium-235 and the rhenium content within the metal alloy was modeled over the range of 0–30 at.%. The spheres containing UO2 were determined to have a critical radius of 18.29–19.11 cm and a critical mass ranging from 366 kg to 424 kg. The cylinders containing UO2 were found to have a critical radius ranging from 17.07 cm to 17.84 cm with a corresponding critical mass of 406–471 kg. Spheres engrained with UN were determined to have a critical radius ranging from 14.82 cm to 15.19 cm and a critical mass between 222 kg and 242 kg. Cylinders which were engrained with UN were determined to have a critical radius ranging from 13.81 cm to 14.15 cm and a corresponding critical mass of 245–267 kg. The critical geometries were also computationally submerged in a neutronically infinite medium of fresh water to determine the effects of rhenium addition on criticality accidents due to water submersion. The Monte Carlo analysis demonstrated that rhenium addition of up to 30 at.% can reduce the excess reactivity due to water submersion by up to $5.07 for UO2 fueled cylinders, $3.87 for UO2 fueled spheres and approximately $3.00 for UN fueled spheres and cylinders.