Isolating properties of a bentonite buffer in conditions of an underground repository for high-level radioactive wastes

The use of a bentonite buffer as an engineered barrier is specified in many contemporary designs of deep-seated underground repositories for high-level radioactive wastes (HLW). Briquettes of compacted bentonite are placed between a canister with HLW and rock. The thickness of the bentonite buffer is from 0.3 to 1.3 m. The isolating properties of bentonite are manifested in its low water permeability, swelling, plasticity, high sorption capacity, and high montmorillonite content (more than 60%). When saturated with water, bentonite swells and seals fissures in host rocks. In order to provide maximal durability of canisters and minimal solubility of HLW, the composition of bentonites is specifically selected to ensure reducing low-alkaline properties of pore waters. Due to low water permeability, radionuclides can migrate through the buffer only by diffusion. If a canister ensures the isolation of radionuclides from groundwater for 1000 years, a bentonite buffer increases this term by at least one order of magnitude. Only poorly sorbed long-lived radionuclides (C-14, I-129, Se-79, Cs-135, and Tc-99) can overcome a bentonite buffer, and only thousands of years after the failure of canisters. By the time of such failure, all short- and mid-lived radionuclides (Cs-137 and Sr-90) will decay, and 98% of HLW radioactivity will be caused by actinides. The bentonite buffer is capable of retaining these most environmentally hazardous radionuclides for a virtually unlimited period. The transfer of radioisotopes in colloidal form through the buffer is completely excluded due to the very fine size of pores in bentonite and the lack of open channels. In underground repositories of vitrified HLW, Se-79 and Cs-135 can overcome a bentonite buffer only after 10000 years, and Tc-99, only after 300000 years. The isotope Cs-135 is the main contributor to contamination of the biosphere. In this case, the radiation dose received by a human will not exceed 10(-3) of the dose obtained from back-round radiation. In repositories of spent nuclear fuel, C-14, Cl-36, and I-129, which are absent in vitrified HLW, will be the first to penetrate into groundwater through the bentonite buffer. However, their contribution to the radiation dose cannot exceed a few hundredth fractions of the background radiation. It would be reasonable to conclude that the use of this barrier allows us to ensure the reliable isolation of the most hazardous radionuclides (actinides) for the required period. The illitization of montmorillonite is the main process that can deteriorate the isolating properties of bentonite. The rate of this process depends on temperature and the potassium concentration in groundwater. The analysis of experimental and geological-geochemical data on the conditions of formation of clay minerals shows that, during the time when bentonite is undergoing the impact of a temperature up to 250degreesC, the amount of newly formed illite layers will be insignificant and will not influence the isolating properties of the buffer. This is all the more valid for the temperature of 100degreesC provided for in designs of underground HLW repositories in most countries. The isolating properties of bentonite could be considerably improved by introducing additives that decrease the solubility of HLW in pore water and improve bentonite sorption properties. The addition of iron filings or shavings increases the reducing capacity and heat conductivity of the buffer; the addition of vivianite decreases the solubility of HLW solidified into alumophosphate glass; and the addition of weathered basic rocks enriched with leucoxene and iron hydroxides improves sorption properties. The high technological and economic efficiency of the bentonite buffer allows us to consider this buffer as the main engineered barrier reliably ensuring the safety of underground HLW repositories.