Determination of the effective diffusion coefficient for 125I− in Beishan granite

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.

The diffusion of 75Se(IV) in Beishan granite – temperature, oxygen condition and ionic strength effects

To explore the diffusion behavior of 75Se(IV) in Beishan granite (BsG), the influences of temperature, oxygen condition and ionic strength were investigated using the through-diffusion experimental method. The effective diffusion coefficient D e of 75Se(IV) in BsG varied from 4.21×10−14 m2/s to 3.19×10−13 m2/s in our experimental conditions, increased with increasing temperature. The formation factor F f of BsG was calculated to be nearly constant in the range of temperatures investigated, suggesting that the inner structure of BsG had no significant change in the temperature range of 20–55°C. Meanwhile, the D e values of 75Se(IV) in BsG under anaerobic condition was significantly larger than that under aerobic condition, which may be attributed to the difference in the sorption characteristics and species distribution of Se and pH values. Moreover, the diffusion of 75Se(IV) was promoted with ionic strength increased from 0.01 M to 0.1 M, and then decreased at 0.5 M, mainly due to the combined effects of reduced double layers with increased ionic strength and increase of the solution viscosity at higher ionic strength.

The adsorption behavior of U(VI) on granite

The effects of pH, counter ions and temperature on the adsorption of U(VI) on Beishan granite (BsG) were investigated in the presence and absence of fulvic acid (FA) and humic acid (HA). The adsorption edge of U(VI) on BsG suggested that U(VI) adsorption was mainly controlled by ion exchange and outer-sphere complexation at low pH, whereas inner-sphere complex was the dominant adsorption species in the pH range of 4.0-9.0. Above pH 9.0, Na2U2O7 might play an important role in the rise of U(VI) adsorption again. Counter ions such as Cl(-), SO4(2-) and PO4(3-) can provoke U(VI) adsorption on BsG to some extent, which was directly correlated to the complexing ability of U(VI)-ligand. More noticeably, the large enhancement of U(VI) adsorption in the presence of phosphate can be attributed to the ternary complex formation (BsG-PO4-UO2), precipitation ((UO2)3(PO4)2(s)) and secondary phase (Na-autunite). Both FA and HA can slightly increase U(VI) adsorption at low pH, whereas they strongly inhibited U(VI) adsorption at high pH range. Artificial synthesized granite (AsG) prepared in the laboratory is impossible to use as an analogue of natural granite because of the large difference in the adsorption and surface properties.

The influence of pH on the sorption and diffusion of 99TcO4 − in Beishan granite

In order to deal with the potential highly increasing amount of high-level radioactive wastes and spent nuclear fuels along with the ambitious development of nuclear power plants, China needs to construct a geologic repository in the middle of the 21st century. After about 15 years of basic research, including site survey, radionuclide migration, Beishan area in Northwest China's Gansu Province has been considered as the most promising candidate place. The Beishan granite has been highly considered as one of the potential host rocks, and some geochemical and hydrogeologic work has been carried out with the granite.

In terms of pre-safety assessment of a potential site, the sorption, diffusion and migration of important radionuclides which tend to be released from the potential repository in the rock/water system play an important role and must be thoroughly investigated with the site specific rock/water characteristics. Due to the potential mobility of technetium under repository conditions, the investigation of the diffusion of technetium in different potential rocks has been studied, while few work focused on the rock surface characteristics. Since the diffusion/migration of radionuclides in the potential host rocks of geologic repositories is mainly through the pores/fractures of the host rocks, the surface characteristics of the rocks should play non-negligible role. In this paper, the influence of pH in the rock/water system on the diffusion of 99TcO4 − in Beishan granite is investigated with through-diffusion experiments and batch sorption experiments. Experimental results indicate that the effective diffusion coefficients (D e) of 99Tc in Beishan granite varied from 9.7 × 10−13 to 1.3 × 10−12 m2/s, and the distribution coefficients (K d) of 99Tc in Beishan granite decreased when the pH of the rock/water system increased which may due to the decreasing of the density of positively charged sites on the granite surface.

The influence of temperature on the diffusion of 125I− in Beishan Granite

China has planned to deal with the high-level radioactive wastes (HLW) in the middle of this century with deep geologic disposal method. The release of heat from HLW may cause the rise of temperatures in the surrounding backfilling materials or even the host rock of a repository. This brings about the concerns that the temperature elevation in the host rock may change the diffusion characteristics of key radionuclides and have some unpredictable effects in the performance assessment. In this paper, the influence of temperature on the diffusion of 125I− in Beishan Granite is studied by through diffusion method. The effective diffusion coefficients (D e) of 125I− in the granite from 27 to 50 °C are obtained and analyzed as a function of temperature. Our result indicated that the relationship between D e and temperature can be described by the modified Nernst equation, and the formation factors (F f) of the granite from 27 to 50 °C is constant with an average value of 1.03×10−4.