The reactions of sodium with urania, plutonia and their solid solutions

The thermodynamic conditions for the reactions of sodium with urania, plutonia and solid solutions of urania-plutonia have been determined using an electrochemical method. The thermodynamic oxygen potentials for the phase fields containing liquid sodium, the oxide and the compound Na3MO4 (M=U, Pu or Usub(1-x)Pusub(x)) have been measured in an EMF cell with a thoria-yttria electrolyte. Measurements were made in the temperature range 825-1000 K and for all these systems studied the values of the equilibrium oxygen potential were similar. Some measurements of oxygen potential were also made for the reaction of sodium with urania-plutonia containing cations which simulate the presence of dissolved fission products. The values for the oxygen potential were higher initially than those obtained for the systems without the addition of cations. However, there was a tendency for the EMF values to drift with time towards those for the pure mixed oxide. The derivation of the valency of the plutonium cations for plutonia and urania-plutonia solid solutions from the equilibrium oxygen potentials is also discussed in terms of the phase relationships and thermodynamic properties of the Pu-O and U-PuO systems. These threshold values of the oxygen potentials are also compared to those determined from a study of the products and the kinetics of the reactions of sodium with pellets of urania-plutonia. (orig.)