Computational Model for the Oxygen Potentials of Mixed Uranium‐Plutonium Oxide

Abstract

A model based on simple thermodynamic considerations for calculating oxygen potentials of mixed uranium-plutonium oxide fuel was developed. Oxygen potential values obtained using this model are presented as a function of temperature and O/M ratio for mixed oxide fuel containing 15 to 30 mol% plutonium oxides. Comparison with experimental data from the literature shows reasonably good agreement.

Estimation of oxygen potential for U–Pu–O mixed oxide fuel

“…The thermal contributions are independent of the values of the stoichiometric coefficients a and c. For example, invoking the thermochemical data employed by Besmann and Lindemer [7], the thermal term can be shown to be The choice of stoichiometric parameters a and c affects the relative contribution of the configurational entropy to the oxygen potential. Krishnaiah and Sriramamurti [6] have used a = 2 and c = 4 and Besmann and Lindemer [7] have used a = (4/3) and c = 2. Effect of these parameters on the predicted oxygen potentials is shown in Fig.…”

“…The present work attempts to generalize the thermochemical approaches [6,7]. The MOX fuel is treated as a pseudo-quaternary solid solution of UO 2 subsystems, namely UO 2+x and PuO 2+y solid solutions.…”

“…Some of them are based on defects [2,3] and some on average valency for plutonium or uranium in the solid solution [4,5]. Krishnaiah and Sriramamurti [6] as well as Besmann and Lindemer [7] used the thermochemical approach for the estimation without invoking either the presence of defects or average valency. Krishnaiah and Sriramamurti [6] employed equilibrium constant approach, in which expressions for two independent and simultaneous equilibria among plutonium oxides and among uranium oxides were setup.…”

“…Krishnaiah and Sriramamurti [6] as well as Besmann and Lindemer [7] used the thermochemical approach for the estimation without invoking either the presence of defects or average valency. Krishnaiah and Sriramamurti [6] employed equilibrium constant approach, in which expressions for two independent and simultaneous equilibria among plutonium oxides and among uranium oxides were setup. By invoking the condition that the oxygen potential in the Pu-O pseudo-binary system is equal to that in the U-O pseudo-binary, the non-linear expressions are solved for the respective unknowns using iterative procedures.…”

“…(1)-(6) are solved iteratively to arrive at the equilibrium composition and the oxygen potential. In this scheme there is a finite possibility for the presence of U c O d -type oxide in the hypo-stoichiometric fuel and for the presence of Pu a O b -type oxide in the hyper-stoichiometric fuel[6].…”