Pyrophoricity of uranium

Abstract

Uranium metal is pyrophoric and is capable of self-ignition in air provided conditions are favorable. Based on the data in this report, spontaneous ignition of spherical particles larger than 1/16 inch in diameter would not be expected to occur in air at room temperature (25{degree}C). The rate at which the uranium surface oxidizes in air, balanced against the rate at which the heat of reaction is lost to the surroundings, determines whether spontaneous ignition can occur. Heat loss to the surrounding environment depends on the thermal conductivity of the uranium including the oxide coating, and on the temperature gradient. The ignition temperature for uranium metal particles is a function of particle geometry, size or specific surface area, heating rate gas composition as well as the quantity and distribution of powder within a storage container. The most important variable; however, affecting the ignition temperature for single samples was found by Schnizlein and Bingle to be the specific surface area (surface area per gram) of the uranium particles. The ignition temperatures calculated from ANL data for 1/16, 1/4, and 1/2 inch diameter spherical particles are 333, 375, and 399{degree}C, respectively. The accuracy is believed to be about {plus_minus}l0%, which is based on theoretical and experimental results.