Simulation of Hypothetical Criticality Accidents Involving Homogeneous Damp Low-Enriched UO₂ Powder Systems

This paper describes the development of a computer model for predicting the excursion characteristics of a postulated, hypothetical, criticality accident involving a homogeneous mixture of low-enriched UO₂ powder and water contained in a cylindrical blender. The model uses point neutronics coupled with simple lumped-parameter thermal-hydraulic feedback. The temperature of the system is calculated using a simple time-dependent energy balance where two extreme conditions for the thermal behavior of the system are considered, which bound the real life situation. Using these extremes, three different models are developed. To evaluate the models, we compared our results with the results of the POWDER code, which was developed by the Commissariat à l’Energie Atomique/United Kingdom Atomic Energy Authority (CEA/UKAEA) for damp powder systems. The agreement in these comparisons is satisfactory. Results of the excursion studies in this work show that approximately 10¹⁹ fissions occur as a result of accidental water ingress into powder blenders containing 5000 kg of low-enriched (5%) UO₂ powder.