OXYGEN POTENTIAL OF NEAR-STOICHIOMETRIC AND NONSTOICHIOMETRIC URANIA-25 MOL-PERCENT PLUTONIA SOLID-SOLUTIONS - COMPARISON OF THERMOGRAVIMETRIC AND GALVANIC CELL MEASUREMENTS

To resolve discrepancies between the existing low temperature Δ G ̄O data for solid solution mixed (U, Pu)-oxide nuclear fuel material, additional measurements have been performed on U0.75Pu0.25O2±x employing a combined thermogravimetric (TGA) and solid-electrolyte galvanic cell technique. These measurements, which were performed at temperatures between 800 and 1000°C, and for O : M ratios in the range 1.940 to 2.028, are reasonably self-consistent and show good agreement with the results of previous TGA measurements. However, they do not corroborate the earlier EMF cell measurements of Markin and McIver. Possible explanations for errors in these earlier EMF cell results are examined. The new results indicate that the Δ G ̄O of stoichiometric mixed oxide at typical outer surface fuel temperatures is close to -100 kcal/mol O2 (-419 kJ/mol O2). Attempts have been made to fit the new Δ G ̄O data to two equations derived from recent defect models, and it is shown that neither equation accurately represents the experimental po2 -x data over more than a short range of x. © 1979.