Chemical reactions between sodium and (U,Pu)02 mixed oxides |
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Authors: | R Lorenzelli T Athanassiadis |
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Affiliation: | C.E.A. — Département d''Etude des Combustibles á base de Plutonium — C.E.N. Fontenay-aus-Roses, B.P.6, 92260 Fontenay-aux-Roses, France |
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Abstract: | 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 . 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 , 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 , 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-MO2-x-Na phase field with M = U0.8Pu0,2 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: 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. |
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