Oxygen potential measurements of pyrochlore-type Zr2M2O7+x (M= Pu, Ce) by EMF method

Abstract

Pyrochlore-type (P-type) zirconia has been widely studied as a disposable form of the high-level radioactive nuclear waste. We have developed the EMF (electromotive force) measurement system using the cubic zirconia oxygen sensor, and investigated the relation between oxygen potential (g(O2)), oxygen composition (x) and temperature (T) for oxygen-excess pyrochlore Zr2Pu2O7+x between 798 K and 1078 K over 0.42<x<0.95. Similar experiments were also carried out for P-type Zr2Ce2O7+x. It was found that g(O2) of P-type Zr2Pu2O7+x. are significantly higher than those of oxygen deficient fluorite PuO2-x at the same values of the oxygen to metal molar ratio (O/M) or the molar ratio of Pu(Ce)3+ ions to the total Pu(Ce)3++Pu(Ce)4+ions (y).g(O2) of P-type Zr2Pu2O7+x. are about -150 kJ/mol lower than those of P-type Zr2Ce2O7+x at the same values of O/M or y, though the shape of g(O2) versus x curves resembles each other over the range of x investigated in this study. These results are consistent with the fact that g(O2) of PuO2-x are about -150 kJ/mol lower than those of CeO2-x at the same values of O/M or y, and their g(O2) curves resemble each other.