Influence of superficial oxidation on the pyrophoric behaviour of uranium hydride and uranium powders in air

Pyrophoric behaviours in air of uranium hydride and uranium powders have been studied on samples of small mass – several tens of milligrams – in a thermogravimetric device and on samples of several grams in an instrumented furnace. Results show that ignition can occur at room temperature for both materials but only in the second device and provided that powders are not superficially oxidized. Chemisorption of oxygen on particles is suspected to be responsible for ignition at room temperature. This phenomenon is not taken into account in classical theories of ignition.When ignition does not occur at room temperature, it occurs above at least 90 °C when heating at a rate of 5 °C min^?1. Ignition seems then to result principally from the large increase of oxidation rates with temperature.