A SEMI-EMPIRICAL ANALYSIS OF IGNITION BEHAVIOUR OF PLUTONIUM

Experiments are described in which small cylinders of plutonium were heated slowly to ignition in air and subsequently quenched with nitrogen. The behaviour of both the pure metal and the alloy stabilised in the delta phase was studied. The temperature in air at which the reaction became self-sustaining was found to be about 500 °C for the delta alloy and about 400 °C for the pure metal. Measurements were also made of the oxygen concentrations which would support a self-sustaining reaction. A simple theory has been developed, assuming as a first approximation that the oxidation rate of plutonium is directly proportional to the partial pressure of oxygen. This assumption was found to be satisfactory for the delta alloy, the reaction having an activation energy of about 25 kcal/g · atom. The reaction of pure plutonium was more complex and appeared to depend on the previous history of the specimen. The theory also predicts zones outside which plutonium will not undergo a self-sustaining reaction with oxygen. It is shown that massive specimens of delta alloy should not ignite if the oxygen concentration is less than 4 × 10-2 atm. In oxygen concentrations less than this a steady reaction occurs, but in order to maintain the temperature additional heat is required. The ignition behaviour of the pure metal varied in an irregular manner from specimen to specimen. However, for any particular specimen predictions based on the theory are in agreement with the observed behaviour. The presence of water vapour in amounts up to 16 mg/litre had no observed effect on the ignition behaviour. © 1969.