Investigation of Se(IV) diffusion in compacted Tamusu clay by capillary method

Sorption behavior and mechanism of U(VI) on Tamusu clay in the presence of U(VI)-CO3 complexes

The sorption behavior of U(VI) on Tamusu clay sampled from a pre-selected high-level radioactive waste (HLW) disposal site in Inner Mongolia (China) was studied systematically in the U(VI)-CO3 solution at pH 7.8 by batch experiments. The results demonstrated that the distribution coefficients (Kd) decreased with the increasing values of pHinitial, [U(VI)]initial, and ionic strength, but increased with the extended time and the rising temperature. The sorption was a pH-dependent, heterogeneous, spontaneous, and endothermic chemical process, which could be better described by Freundlich isothermal model and pseudo-second-order kinetic model. The presence of humic acid (HA) or fulvic acid (FA) significantly inhibited the U(VI) sorption, due to the enhanced electrostatic repulsion between the negatively charged HA/FA adsorbed on the clay surface and the negative U(VI) species, as well as the well dispersed HA/FA aggregates in solution wrapping the U(VI) species. The FTIR and XPS spectra indicated that the HCO3- groups on the surface of Tamusu clay after hydroxylation and the ‒OH groups in HA/FA were involved in the U(VI) sorption. The results reported here provide valuable insights into the further understanding of U(VI) migration in geological media.

A sensitive improved method for analyzing diffusion coefficients of Cs in compacted bentonite with different lengths

Based on the one-dimensional diffusion theory, the diffusion parameters were obtained from numerical fitting by the Cyclic Initial Value (CIV) program written by MATLAB. Taking the through-diffusion experimental of cesium (stable isotope 133Cs) as an example, on the premise of ensuring accuracy, fitting calculation was used to obtain the diffusion equilibrium time of Cs in different lengths bentonite column. The fitting results of diffusion test for tritium water (HTO) and Cs are both very well. The calculation results of the equilibrium time for Cs diffusion show that the equilibrium time obtained by CIV is less than the experimental period in both groundwater (GW) and seawater (SW). In GW environment, when the sensitivity is at the maximum setting value, the diffusion coefficient of Cs in 1.5 cm bentonite column could be calculated in a shorter period of time. Compared with the experimental period, the time was shortened by 110 days. The main purpose is to verify the feasibility of CIV through the experimental data of Cs in different column lengths. The CIV program can also be used to fit and calculate the experimental data of other ongoing diffusion experiments and get the equilibrium time required for diffusion experiments. It shows that the proposed models offer the advantages of saving experimental time and reducing experimental waste.

Investigation of Re(VII) diffusion in Tamusu clayrock core by through-diffusion method

Tamusu area is the primary pre-selection site of clayrock disposal repository for high-level radioactive waste (HLW) in China. However, the research on the migration behavior of nuclides in Tamusu clayrock is still in its infancy. For the first time in laboratory, the diffusion behavior of Re(VII) in Tamusu clayrock core was studied by means of through-diffusion method. The effects of pH, ionic strength and humic acid on the diffusion behavior of Re(VII) in clayrock were studied. The effective diffusion coefficient, apparent diffusion coefficient and rock capacity factor value were obtained. All the experimental conditions of Re(VII) diffusion in Tamusu clayrock are compared with other geological samples under the same conditions in literature data. The diffusion mechanism of radionuclide in Tamusu clay is discussed, which can provide experimental data for site selection and safety assessment of high-level radioactive waste repository in China. The experimental results showed an effective application and reference for the countries disposed HLW in mudrocks or clayrocks, such as France, Belgium etc. in Europe. Moreover, this research can provide the original data support for the metallogenic regularity and prospecting prognosis of rare element rhenium in different geological environments.