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

A thermochemical analysis of the estimation of oxygen potential for mixed oxide fuel is presented. The MOX fuel is treated as a pseudo-quaternary solid solution of UO2–UcOd–PuO2–PuaOb with b = 1.5a and d = 2.25c. The hypo-stoichiometric fuel is viewed as UO2–(1/a)PuaOb–PuO2 pseudo-ternary system and the hyper-stoichiometric fuel is viewed as UO2–(1/c)UcOd–PuO2 pseudo-ternary system and the graphical representation is visualized as a ‘diamond plot’. The oxygen potential for MOX is determined either by PuO2–PuaOb equilibria or by the UO2–UcOd equilibria subject to the constraint that the oxygen potential is equal both in Pu–O subsystem and in U–O subsystem. The choice of stoichiometric parameters a and c affects the relative contribution of configurational entropy to the oxygen potential which determines the shape of the ‘S’-type curve near exact stoichiometry. From the analysis of literature data, a = 2 and c = 8 was derived at for the ‘diamond plot’ model. Oxygen potentials for MOX fuel with 21, 28% and 44% plutonia was estimated using the ‘diamond plot’ model. A simplified nomograph for the oxygen potential of MOX is presented.