Chemical reactions between sodium and (U,Pu)02 mixed oxides

The three phase field MO₂-Na₃MO₄-Na has been studied in detail. It was shown that Na₃MO₄ exists at least in two crystallographic forms and may be above 1200°C, a third one fcc, with a small cell $\text{a = 4.77}\text{A}\text{?}$. This latter form which is considered in the literature as the “normal” uranate cannot be stable in the given conditions of preparation. The low temperature form, simple cubic $\text{a = 9.54}\text{A}\text{?}$, is the one to which the wrong Na₁₁U₅O₁₆ formula has been attributed up to now. It transforms at ca 975°C into a face-centered cubic form with $\text{a = 9.56}\text{A}\text{?}$, stable at least up to 1200°C. Isomorphism between Na-uranate and Na-uranoplutonate has been established for Pu content up to 30%, but there is still some doubt about the nature of the plutonate which is formed by direct reaction between PuO₂ and Na.For the compositions studied (Pu ≤ 30%) the monovariant domain adjacent to the “MO₂”-Na₃MO₄-Na domain, contains the three phases Na₃MO₄, Na₆MO₆ and Na.Oxygen potentials corresponding to the Na₃MO₄-MO_2-x-Na phase field with M = U_0.8Pu_0,2 have been deduced from the measurement of x₀ in equilibrium conditions. It has been foun'd that X_₀ was dependent on T, varying from 0.007 at 550°C to 0.040 at 1200°C. Corresponding values of the oxygen potentials can be represented by the expression: $\text{?}\text{sm = ? 907, 394 + 224.9 T J/mol 0}{}_2$ A thermodynamic treatment shows that this expression is quasi-independent of Pu content (Pu≤30%) and must be very close to the one valid for the UO₂-Na₃UO₄-Na domain.In conclusion, we examine very schematically how these results can offer guidelines to appreciate the evolution of failed breeder pins in reactor conditions.