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The fluid transfer of radionuclides in the geologic medium is considered under conditions when radionuclides are contained in fluids not only as solutes but also as colloids. The effect of colloidal transport of radionuclides on the rate of spreading of radioactive contamination in an underground medium is estimated, with assessment of this effect in mathematical models describing the transport of radionuclides by subsurface water. For this purpose, the exchange of radionuclides between subsurface water, colloid, and an immobile solid phase is considered, taking into account the precipitation of colloidal particles on both the immobile solid phase and other colloidal particles and their recurrent mobilization into the liquid phase. It is noted that, in real colloidal transfer, the heterogeneity of the geologic medium and colloidal particles in subsurface water is of great importance. The known models of colloidal transfer of radionuclides are evaluated on the basis of the analysis performed.

World‐class unconformity‐related U deposits in the Athabasca Basin (Saskatchewan, Canada) are generally located within or near fault zones that intersect the unconformity between the Athabasca Group sedimentary basin rocks and underlying metamorphic basement rocks. Two distinct subtypes of unconformity‐related uranium deposits have been identified: those hosted primarily in the Athabasca Group sandstones (sediment‐hosted) and those hosted primarily in the underlying basement rocks (basement‐hosted). Although significant research on these deposits has been carried out, certain aspects of their formation are still under discussion, one of the main issues being the fluid flow mechanisms responsible for uranium mineralization. The intriguing feature of this problem is that sediment‐hosted and basement‐hosted deposits are characterized by oppositely directed vectors of fluid flow via associated fault zones. Sediment‐hosted deposits formed via upward flow of basement fluids, basement‐hosted deposits via downward flow of basinal fluids. We have hypothesized that such flow patterns are indicative of the fluid flow self‐organization in fault‐bounded thermal convection (Transport in Porous Media, 110, 2015, 25). To explore this hypothesis, we constructed a simplified hydrogeologic model with fault‐bounded thermal convection of fluids in the faulted basement linked with fluid circulation in the overlying fault‐free sandstone horizon. Based on this model, a series of numerical experiments was carried out to simulate the hypothesized fluid flow patterns. The results obtained are in reasonable agreement with the concept of fault‐bounded convection cells as an explanation of focused upflow and downflow across the basement/sandstone unconformity. We then discuss application of the model to another debated problem, the uranium source for the ore‐forming basinal brines.

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