Combined matrixes for the immobilisation of organic liquid radioactive waste containing Cs-137 and Sr-90
It is proposed to use combined matrixes for the immobilization of the organic liquid radioactive waste (spent tributylphosphate in hydrocarbon diluent and technological oils) containing radionuclides Cs-137 and Sr-90. Higher charging of the matrixes with waste is achieved by preliminary mixing of the latter with thermally expanded graphite. Bentonite clays containing 65% montmorillonite should be added to the compounds to ensure tight trapping of cesium. It is shown that 25 vol.% is the optimal content of extractant in the compound and 20 vol.% is the optimum for the oil. Minimum curing time between the cement gauging and long-term storage or disposal is to be 42 days. Coating the compound surface with waterproof enamel creates no significant barrier for the leaching of cesium but, however, slows down the water uptake by the compound and therefore postpones the start of emersion of the radionuclides into liquid phase. Mechanical strength of obtained compounds fits the set requirements (GOST R 51883-2002). Average leaching rate of 137Cs was found to be no greater than 1 mg/(cm2·day); the same index for <sup>90</sup>Sr + <sup>90</sup>Y - no greater than 0,1 mg/(cm<sup>2</sup>·day). The kinetics of cesium leaching is limited by diffusion; the same of strontium and yttrium is limited by sorpiondesorption processes. Leaching rates of the radionuclides decrease as follows R(<sup>137</sup>Cs) > R(<sup>90</sup>Sr) >> R(<sup>90</sup>Y).