Transport properties of I, Te and Xe in thoria–urania SIMFUEL

Diffusion properties for volatile fission products iodine and tellurium, and gaseous product xenon in a solid solution of thoria–2 mol% urania doped with fission products for simulating 2 at.% burnup were obtained by studying the release kinetics of the species from trace-irradiated fuel samples at different temperatures using post-irradiation annealing technique. Bulk diffusion coefficients for Xe, I and Te were evaluated in a well-defined powder sample of particle size in the range of 37–45 μm (25 m2 kg−1 BET surface area) with 97% of theoretical density. Temperature dependences of the apparent diffusion coefficients of Xe, I and Te derived from this study could be expressed in the form of Arrhenius equations for the respective cases as ln DI/Simfuel′ (s−1) = −(19,480 ± 3300)/T − 9.3 ± 2.2 and ln DTe/Simfuel′ (s−1) = −(31,234 ± 3000)/T − 3.7 ± 2.0, (1273 ⩽ T/K ⩽ 1773) and ln DXe/Simfuel′ (s−1) = −(22,755 ± 1364)/T − 0.003 ± 0.775, 1700 ⩽ T/K ⩽ 1800. For Xe diffusion the activation energy and frequency factor are 189 kJ mol−1 and 0.997 s−1, respectively. For I and Te the activation energy values are 162 and 260 kJ mol−1, respectively. The respective frequency factors for I and Te are 9.1 × 10−5 and 2.5 × 10−2 s−1. On comparison with the reported data in pure urania and thoria matrices a lowering of activation energy for all three species was observed in case of the fission product doped matrix. On the other hand the frequency factor has increased only in the case for diffusion of Xe. This suggests different mechanisms of transport for Xe and volatile fission products I and Te in the fuel matrix.