Diffusion of99Tc in granite: A small scale laboratory simulation experiment

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.

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.

Nuclear and radiochemistry in China: present status and future perspectives

Nuclear and radiochemistry is one of the frontier areas of chemistry with high impact on national security, energy supply, scientific advances, social and economic development. Nuclear and radiochemistry in China is now experiencing a renaissance, which is being strongly motivated by China’s huge demand for nuclear energy. With this in review, the progress in nuclear and radiochemistry of China is selectively addressed. Some hot topics have been summarized and the main research results achieved by Chinese scientists in this field are highlighted, with emphasis on the basic nuclear chemistry, actinide and trans-actinide chemistry, chemistry of spent nuclear fuel reprocessing, radioanalytical chemistry, environmental radiochemistry and radiopharmaceutical chemistry, etc. Some measures about how to promote the radiochemical education and research in China are suggested, and future perspectives are briefly outlined as well.

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.