Shutdown mechanisms for a hypothetical criticality accident involving highly enriched uranium powder

This work is a study of the physical mechanisms which would cause an accidental criticality excursion involving highly-enriched uranium (HEU) powder to shutdown (i.e., become subcritical) naturally. The study analyzes an excursion resulting from the continuous pouring of slightly damp HEU powder (either UO₃;, or UF₄;, containing 93.2 wt% U²³⁵; and 1.5 wt% H₂;O) onto a concrete floor resulting in a rectangular pile. For simplicity, the width and depth are chosen to be the same, and the height of the pile determines the system reactivity. The external reactivity insertion is caused by mass addition to the system.

The reactor point kinetics equations with simple thermal-hydraulic feedback are used to model the transient behavior of the fissile powder pile. An energy balance is performed to determine the bulk temperature of the system.

Dispersion of the damp HEU powder is the most important shutdown mechanism in the system. Initially, air is trapped inside the void spaces of the powder. As the system temperature rises above saturation conditions for water, the water in the powder is vaporized, thus producing a mixture of vapor and air in the interstitial volume. As the trapped air and vapor expand, the powder particles are dispersed and the excursion is terminated.