Mechanical retardation of actinide-bearing colloid migration from an underground repository: Theory and experiment

A necessary condition for the sustainable development of nuclear power is the safe disposal of vitrified high-level radioactive waste from the processing of spent nuclear fuel in underground repositories. Their safety depends on the rate of radionuclide transport by groundwater from the repository to the biosphere. A significant fraction of the radionuclides is carried by groundwater in colloidal form. The radionuclide-bearing colloid (radiocolloid) is the most mobile and therefore the most ecologically hazardous form of radionuclide transport. The radiocolloid can be retained mechanically in the rock if the colloid particle size is larger than diameter of rock pore channels. Transport of radionuclide-bearing colloid by groundwater is considered. (1) A technique for determining the size distribution of pore channels in the rock is developed. It is based on measurements of the gas permeability of rock samples at different pressures. (2) The technique is applied to rock samples from the Nizhnekanskiy Massif (Russia), which has been selected for the development of a high-level radioactive waste repository. (3) Simulated radionuclide-bearing colloids were obtained by water leaching of aged aluminum phosphate glass with radionuclide simulants. The glass composition is similar to that used for vitrification of liquid HLW at the Russian radiochemical plant. (4) The size distribution of the colloid was determined by filtering the leachates through membranes of different pore sizes. (5) Mechanical retention of the colloid by the rock is estimated theoretically. The estimation is based on size distributions of colloid particles and diameters of pore channels in the rock. It is shown that 99% of the colloidal form of the actinide simulants (Ce, Nd, U) can be mechanically retained in the rock of the Nizhnekanskiy Massif.