Coupled point neutron kinetics and thermal-hydraulics models of transient nuclear criticality excursions in wetted fissile uranium dioxide (UO2) powders

Abstract

This thesis describes a phenomenologically based mathematical and computational methodology for the simulation of a postulated transient nuclear criticality excursion initiated by the incursion of water, from a fire-sprinkler system, into a bed of dry UO2 powder. These potentially hazardous multi-phase dispersed particulate systems may form as a result of a fire or explosion in a nuclear fuel fabrication facility. The models proposed in this thesis aim to support nuclear criticality safety analysis and assessment. In addition, the development of these models aims to support emergency planning and preparedness.

The point neutron kinetics equations are coupled to phenomenological models of water infiltration, sedimentation, fluidisation, nuclear thermal hydraulics, radiolysis and boiling, through the use of multivariate reactivity feedback components. The spatial and temporal solution of this set of equations enables the modelling of postulated transient nuclear criticality excursions in highly dispersed three-phase particulate systems of UO2 powder. The results indicate that there is the potential for large positive reactivities to be added to a UO2 powder system as pores become filled with water. Generally, thermal expansion and Doppler broadening are insufficient to control the transient, leading to significant radiolysis and boiling on the surface of the UO2 powder particles. Radiolytic gas and steam bubble induced fluidisation and sedimentation significantly alters the characteristics of a transient nuclear criticality excursion and should be carefully considered.

Research has also been undertaken examining transient nuclear criticality excursions in weak intrinsic neutron source UO2 powder systems by solving the forward probability balance equation and using a Gamma probability distribution function to estimate mean wait-time probability distributions. Significant variations in the potential initial peak power are predicted for highly enriched, wetted, UO2 powders as a function of the stochastic behaviour associated with criticality excursions in low neutron population systems.