Thermal diffusion of chlorine in uranium dioxide

Abstract

In a nuclear reactor, isotopes such as 35Cl present as impurities in the nuclear fuel are activated by thermal neutron capture. During interim storage or geological disposal of nuclear fuel, the activation products such as 36Cl may be released from the fuel to the geo/biosphere and contribute to the “instant release fraction” as they are likely to migrate in defects and grain boundaries. In order to differentiate diffusion mechanisms due to “athermal” processes during irradiation from thermally activated diffusion, both irradiation and thermal effects must be assessed. This work concerns the measurement of the thermal diffusion coefficient of chlorine in UO2.37Cl was implanted at a 1013 at/cm2 fluence in depleted UO2 samples which were then annealed in the 900−1200 °C temperature range and finally analyzed by Secondary Ion Mass Spectrometry (SIMS) to obtain 37Cl depth profiles. The migration process appears to be rather complex, involving mechanisms such as atomic, grain boundary, directed diffusion along preferential patterns as well as trapping into sinks before successive effusion. However, using a diffusion model based on general equation of transport, apparent diffusion coefficients could be calculated for 1000 and 1100 °C and a mean activation energy of 4.3 eV is proposed. This value is one of the lowest values compared to those found in literature for other radionuclides pointing out a great ability of chlorine to migrate in UO2 at relatively low temperatures. In order to unequivocally determine the diffusion behaviour of both implanted and pristine chlorine before and after thermal annealing, the structural environment of chlorine in UO2 was examined using micro X-ray fluorescence (micro-XRF) and micro X-ray absorption spectroscopy (micro-XAS).