Experimental study on mass transfer behavior and mechanism in nuclide-containing sandy soil under temperature gradient

Abstract

The level of nuclear waste emergency disposal is an important factor restricting the rapid development of the nuclear energy industry under the trend of global energy revolution. To promote the effective application of artificial ground freezing (AGF) for the treatment of nuclear wastewater leakage, the mass transfer and migration mechanism of key nuclides (137Cs, 90Sr, and 129I) during unidirectional freezing of saturated silty sand were elucidated in this study. Temperature sensors and a nuclear magnetic resonance (NMR) were used to explore the internal temperature and moisture variation characteristics of nuclide-containing sandy soil during the freezing. Furthermore, using the designed device, a unidirectional freezing of different nuclide-containing sandy soils was performed to establish a relationship between the development characteristics of the freezing front and the mass transfer of nuclide. The results show that the T2 spectrum of nuclide-containing sandy soils has the most significant change from 0 °C to -1 °C, and the bound water content of I-containing sandy soil remains evident change when the temperature decreases below -1 °C. During unidirectional freezing, owing to the influence of the freezing front on nuclide migration, the resistivity curve exhibits a V-shaped jump, and the concentration peaks appear in turn along the freezing direction and show a stepwise increase. Nuclide mobility is closely related to the freezing rate of the corresponding location. With an increase in freezing thickness, the freezing rate decreases and the nuclide mobility increases. Owing to the combined influence of water-ice phase transition, concentration gradient and matrix suction in sandy soil, the concentration only increases within a small area at the face of the freezing front. The differences in migration behavior of nuclides in sandy soil driven by temperature gradient is mainly due to the different degrees of compression of nuclide ion on the diffusion double electric layer of soil particles, which results in different forms of unfrozen water within the sandy soils. The overall mobility of the three nuclides shows the following sequence: Cs>I>Sr. In the practical application of AGF, attention should be paid to the effects caused by the rapid migration of low-valence cationic nuclides and the retention of high valence cationic nuclides in the frozen wall.