Wetting-Induced Volumetric Collapse of UO2 Powder Beds and the Consequence on Transient Nuclear Criticality Excursions

<p>Mathematical and computational models are proposed to simulate wetting-induced volumetric collapse of fissile powder beds. Slumping, nuclear thermal hydraulics, radiolytic gas and steam production models are coupled with point neutron kinetics to investigate transient nuclear criticality excursions in two 5-wt%&nbsp;enriched UO₂&nbsp;fissile powder beds with varying levels of wetting-induced volumetric collapse. The two beds are distinguished by their mean powder particle size of 30&nbsp;&micro;m and 100&nbsp;&micro;m. For the UO₂&nbsp;powder beds modelled, the re-distribution of UO₂ powder and moderator due to slumping introduced a negative reactivity into the system. This increased the amount of time taken for a delayed critical state to be reached once infiltration began, and also reduced the total fission energy generated over the course of the simulated transient. The total fission energy generated ranged from 42&nbsp;MJ&nbsp;to 48&nbsp;MJ&nbsp;100&nbsp;seconds after the initial nuclear criticality excursion was observed for the 30&nbsp;&micro;m sized UO₂ powder bed. The fission energy of the larger sized powder bed (100&nbsp;&micro;m), varied from 42&nbsp;MJ&nbsp;to 57&nbsp;MJ. Larger discrepancies between the slumped and un-slumped initial peak power are predicted. Peak powers varied from 29.2&nbsp;MW to 106&nbsp;MW for the smaller-sized powder particles, whereas for larger particles, the peak powers varied from 255&nbsp;MW to 501&nbsp;MW.</p>