Transport properties of iodine and tellurium in a thoria-2 mol% urania matrix

Abstract

Diffusion properties of the volatile fission products iodine and tellurium inside the thoria based fuel matrix (ThO 2-2 mol% UO 2) were obtained by studying the release kinetics of the species from trace-irradiated fuel samples at different temperatures (1290–1790 K) in post-irradiation annealing experiments. Analysis of the kinetic data shows that the release through bulk diffusion is too low to be measurable below 1273 K. The apparent bulk diffusion coefficients for the two volatile species could be evaluated at higher temperatures in a well-defined powder sample (37–45 μm particle size, 24 m 2 kg −1 BET surface area) of the high-density fuel (>95% T.D.). Temperature dependences of the diffusion coefficients of iodine and tellurium derived from this study could be expressed in the form of Arrhenius equations as logD ′ I ( s −1)=−(15 000±1000)/T−1.19±0.63 , and, logD ′ Te ( s −1)=−(25 600±1200)/T+6.14±0.77 . Activation energies and frequency factors for diffusion of the two species as obtained from the Arrhenius equations are 286 kJ mol −1 and 6.4 × 10 −2 s −1 for I and 491 kJ mol −1 and 1.38 × 10 6 s −1 for Te. The two sets of kinetic data are quite different and they suggest that the two species have different mechanisms of their transport in the fuel matrix. The results were compared with reported data in pure urania and thoria matrices.