(247b) Analysis of Air Oxidation of Plutonium Metal

Rates of oxidation of plutonium metal alloys exposed to high-temperature air are analyzed. Department of Energy facilities, including the Savannah River Site, utilize direct metal oxidation in air (DMO) to convert surplus plutonium metal alloys to a stable oxide form. The plutonium oxide can be either stored or used as feed for other processes. Typically, DMO is performed in an electrically heated furnace; the furnace supplies the heat needed to raise the metal temperature to its ignition point, which is a requirement for oxidation in dry air.

Oxidation rates are calculated for oxidation of a 3.5-in. diameter by 1.25-in. thick cylinder and for a 4-in. by 4-in. by 0.125-in. piece of foil. The calculations demonstrate that the oxidation rates are not limited by the build-up of the oxide powder layer on the surface of the metal but instead are kinetically controlled. The calculations also show that the air supply to the furnace must be throttled to limit the furnace temperature after ignition.

Separate analyses are included to estimate the ignition temperature, the minimum oxygen concentration in the air, and the porosity of the oxide powder generated by the oxidation. The ignition temperature is calculated based on the Frank-Kamenetskii theory for thermal reaction excursions. The minimum oxygen concentration is estimated from the likelihood that an oxygen molecule will diffuse to an oxidation reaction site on the metal surface. The porosity is calculated from the assumption that the oxide layer expands to a minimal extent in the direction normal to the metal surface.