Oxygen potential measurement of (Pu_0.928Am_0.072)O_2–xat high temperatures

Abstract

The oxygen potentials of (Pu 0.928 Am 0.072 )O 2-x were measured as a function of oxygen to metal ratio (O/M) in the temperature range from 1473 to 1873 K by the gas equilibrium method using thermogravimetry.The measured data were about 200 kJ/mol higher in the O/M ratio range from 2.00 to 1.96 and about 50 kJ/mol higher in the reducing region (below O/M ratio = 1.94) compared with those of PuO 2-x . The measured oxygen potentials were analyzed by the point defect chemistry method. From this analysis, the oxygen potential of (Pu 0.928 Am 0.072 )O 2-x was represented as a function of O/M ratio and temperature and calculated results were consistent with the measured data.

Evolving Defect Chemistry of (Pu,Am)O2±x

“…Values of x in (Pu 1– y Am y )­O 2– x as a function of oxygen partial pressure at y values of (a) 0.09 and (b) 0.072, with comparison to the experimental results of Osaka et al and Matsumoto et al …”

“…At near-stoichiometry, our model shows that the presence of Am results in [e – ] ≪ [h + ], in contrast to Am-free PuO 2 , where [e – ] = [h + ] at near-stoichiometry. Therefore, to construct equations describing the defect chemistry of (Pu,Am)­O 2– x , we cannot say that [e – ] = [h + ] as suggested by Matsumoto et al Instead, we propose that at low concentrations of V O 2+ and near-stoichiometry, the formation of V O 2+ is charge-compensated by the removal of holes, which exist at concentrations many orders of magnitude greater than V O 2+ at near-stoichiometry. The defect reaction and the corresponding equilibrium constant ( k 1 ) are written as We see in Figure that at near-stoichiometry, the concentration of holes can be considered fixed.…”

“…This explains the curve we see in Figures and : as Am (+IV) is reduced, the rate of V O 2+ formation decreases and when reduction is complete, [V O 2+ ] remains constant. As oxygen partial pressure is reduced further, the value of x in (Pu,Am)­O 2– x will continue to evolve. The start of reduction in Pu is predicted to result in the formation of defect clusters; however, both cluster formation and the higher defect concentrations at low O/M ratios are beyond the capabilities of the point defect model.…”

“…Osaka et al experimentally determined the oxygen potential of (Pu 0.91 Am 0.09 )­O 2– x as a function of the O/M ratio, as well as measuring the deviation x from stoichiometry as a function of oxygen partial pressure. Matsumoto et al experimentally studied the oxygen potential of (Pu 0.928 Am 0.072 )­O 2– x at high temperatures as a function of the O/M ratio and constructed point defect equations to describe the deviation x from stoichiometry. A doubly charged vacancy is predicted as the source of all hypostoichiometry .…”

“…Matsumoto et al experimentally studied the oxygen potential of (Pu 0.928 Am 0.072 )­O 2– x at high temperatures as a function of the O/M ratio and constructed point defect equations to describe the deviation x from stoichiometry. A doubly charged vacancy is predicted as the source of all hypostoichiometry . Using X-ray absorption spectroscopy, Belin et al were able to quantitatively determine Pu and Am valences in the reduction process of (Pu,Am)­O 2– x , validating an earlier prediction made by Osaka et al that all Am (+IV) will reduce to Am (+III) prior to any reduction in Pu (+IV) occurring.…”