Mathematical and computational models for simulating transient nuclear criticality excursions within wetted fissile powder systems

This paper describes a novel methodology for the analysis of nuclear criticality excursions in fissile powder beds under wetting conditions. These potentially hazardous powder, slurry and sludge systems may be found in nuclear fuel manufacturing and fabrication facilities. A point neutron kinetics model was coupled with water infiltration, thermal–hydraulics and radiolysis models through the use of reactivity feedbacks. Good agreement in the water infiltration rate was found when comparing the water infiltration model used in this paper to experiments conducted by the French Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA). A case study was proposed whereby a sheet of fine water droplets from a sprinkler system came into contact with an open-topped bed of low enriched UO2 powder. Simulations indicate that the mean powder particle size had a strong effect on the time required for the water to percolate through the powder bed. Powder particle size was also predicted to have a moderate effect on the initial fission power spike. The fission energy released over the first 300 s of the nuclear criticality transient ranged from 65:28 MJ to 97:98 MJ depending on mean powder particle size. This is similar in magnitude to other simulated nuclear criticality excursions in powder beds. The model predicts that the initial fission power spike would be limited by the production of radiolytic gas and to a lesser extent the effects of Doppler broadening and thermal expansion. As expected, boiling and the associated steam production, was found to be an important phenomenon in the reduction of the fission rate through the negative void reactivity effect of the steam