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

Transient Nuclear Criticality Excursion Analysis of Highly Dispersed Particulate Three-Phase Fluidised Systems

“…The proposed study examines how the transition between the first two of these three hydrodynamic states may affect a transient nuclear criticality excursion in a partially saturated UO 2 powder bed. The proposed model extends the work of Jones et al (2022), to include a phenomenological model, describing gas-bubble induced fluidisation of the UO 2 powder bed, leading to the re-distribution of fissile mass into a highly dispersed suspension. Additionally, a sedimentationdispersion model, similar to the approach proposed by Cova (1966) has been implemented to represent the gravity-driven advective flow of agglomerated, suspended UO 2 powder particles, as well as gasbubble induced turbulent mixing in the suspension.…”

“…The generalised form of the equations governing component and region mass transfer in a three-phase UO 2 powder bed, contained within a cylindrical vessel, can be derived from specific governing equations outlined by Jones et al (2022) for the conservation of water, steam, radiolytic gas and UO 2 powder in the dry powder, settled wet powder and pooled water/suspension regions. The proposed model extends the work of Jones et al (2022) by outlining a governing equation for powder present in the suspension, located axially above the settled wet powder bed. The mass transfer of UO 2 powder across the interface between the suspension and settled wet powder regions is governed by fluidisation and sedimentation phenomena.…”

“…At low gas velocities, the bed remains in a stationary or fixed bed state. This type of bed has been modelled previously by a number of authors (Jones et al, 2022;Basoglu, 1992;Basoglu et al, 1994;Yamane et al, 2003). As the gas flow rate exceeds the minimum fluidisation velocity, the powder particles become fluidised.…”

“…Previous mathematical and numerical models of this type of postulated transient nuclear criticality excursion by Jones et al (2022) considered the effect that a range of phenomena would have on the nuclear criticality excursion dynamics. Models of water infiltration into an initially unsaturated UO 2 powder bed were successfully compared to experimentally derived data by Rozain et al (1991).…”

“…Models of water infiltration into an initially unsaturated UO 2 powder bed were successfully compared to experimentally derived data by Rozain et al (1991). Jones et al (2022) predicted that the resulting nuclear criticality excursion led to significant thermal energy deposition in the UO 2 powder and water, causing significant steam production through the boiling of water on the surface of the UO 2 powder particles. Additionally, large volumes of radiolytic gas, produced through the radiation induced dissociation of water molecules, were predicted.…”