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 UO 2 ―U c O d ―PUO 2 ―PU a O b with b = 1.5a and d = 2.25c. The hypo-stoichiometric fuel is viewed as UO 2 -(1/a)Pu a O b -PuO 2 pseudo-ternary system and the hyper-stoichiometric fuel is viewed as UO 2 ―(1/c)U c O d ―PuO 2 pseudo-ternary system and the graphical representation is visualized as a 'diamond plot'. The oxygen potential for MOX is determined either by PUO 2 ―PU a O b equilibria or by the UO 2 ―U c O d 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.