HIGH-TEMPERATURE ELECTRICAL-CONDUCTIVITY AND CONDUCTION MECHANISM OF (U, PU)O2+/-X AT LOW-OXYGEN PARTIAL PRESSURES

The electrical conductivities (sigma) of U1-yPuyO2+/-x were measured in the oxygen partial pressure range from P(O2) = 10(-15) to 10(-1.5) Pa at 1273 K. Below about 10(-5) Pa, the electrical conductivity was independent of P(O2) and the sigma versus y curve showed a maximum at y = approximately 0.5. The conduction mechanism in this region was analyzed on the basis of the hopping or small polaron conduction formalism. The maximum was found to be reasonably explained by the disproportionation reaction Pu4+ + U4+ = Pu3+ + U5+. The activation energy for electric conduction was measured by quenching the U1-yPuyO2+/-x specimens from 1273 K. The observed (apparent) activation energy steadily increased with increasing y from 0.52 eV at y = 0.05 to 0.75 eV at y = 0.9. This change was well followed by assuming that the activation energy for electric conduction between uranium ions is different from that between plutonium ions. Discussion was made on the rate of the disproportionation reactions, which is low enough to yield the temperature independent concentrations of U5+ and/or Pu3+ when quenching.