Influence of γ-irradiation and oxygen conditions on the diffusion of I-125 in crushed Beishan granite

The diffusion of iodine (labeled with ¹²⁵I^-) in compacted Beishan granite (BsG) was investigated using the in-diffusion capillary method at pH ~2.0 to ~11.0 under oxygen and irradiation conditions. With the advantages of simple and easy operation of the capillary diffusion devices, this work makes a preliminary investigation on the irradiation condition that cannot be performed by the traditional diffusion experiment. In this study, D_a values of ¹²⁵I as a tracer in the form of iodide were determined to investigate and assess the influence of irradiation and oxygen conditions, which must be considered in-situ environment of the geologic repository. The results indicate that capillary method is a simple and efficient method to study the diffusion behavior of radionuclides, which is a relatively feasible to quickly obtain the diffusion coefficient, especially for some special conditions, e.g., oxygen and irradiation conditions. The diffusion results showed that D_a values of ¹²⁵I^- range from 1.4 × 10^-10 to 1.5 × 10^-9 m²∙s^-1, which was much faster than other nuclides, such as ⁷⁹Se (10^-11 m²∙s^-1). The batch adsorption experiments in this paper and related studies showed that the sorption could be neglected, i.e., ¹²⁵I^- is a weak adsorbent nuclide. Its diffusion and sorption process were hardly affected by pH、oxygen and irradiation conditions, which is very different from ⁷⁹Se. Besides, ionic strength has a significant impact on the diffusion rate of ¹²⁵I^-, which is closely related to the double electric layer. Overall, this study indicates that natural BsG couldn't greatly attenuate the mobility of ¹²⁵I^- in the deep geologic repository and new retardation method or materials need to be explored.