A thermodynamic study of the Pu-Am-O system

Vapour pressure measurements were performed on a (Pu0.756Am0.244)O2−x sample using Knudsen cell mass spectrometry. The total and partial vapour pressures of the gaseous species have been measured in the temperature range from 2000 to 2300 K. The evolution of the plutonium and americium bearing species was also determined as a function of time, in order to evaluate the congruent vapour composition. At constant temperature, the energy of ionising electrons was stepwise increased and the ionisation efficiency curves were recorded. The results were combined with the assessment of the Pu-Am-O system using the CALPHAD method. To obtain the model of this ternary system, the data on the Pu-Am and Am-O binaries have been evaluated and the optimised phase diagrams are presented. A consistent thermodynamic description of the ternary was obtained which allows the calculation of the ternary phase diagram, the oxygen potential for (Pu, Am)O2±x and the equilibrium partial vapour pressures.