Matrices for the immobilization of waste from pyrochemical processing of spent nuclear fuel

Nowadays countries operating nuclear power plants have an ambitious goal - closing the nuclear fuel cycle. One of the major tasks is the management of spent nuclear fuel (SNF). Processing of mixed nitride uranium- plutonium SNF suggests the use of LiCl-KCl electrolyte in the pyrochemical or combined technology. However, the industrial application of pyrochemical processing cannot be implemented until methods for the immobilization of the resulting radioactive waste (RW) are developed. Studies with glass composite materials show that for reliable fixation of a significant amount of alkali chlorides they must be preliminary converted into the form of phosphates or oxides, i.e. conventional technological operation forms. Ceramic or glass-ceramic matrices are more reliable alternatives to vitrified matrices. To reduce the release of radionuclides from the matrix into the biosphere, the matrix material should be as close as possible to the surrounding host rocks in its chemical and phase composition. Therefore, in this work we propose to use the bentonite clays as a possible material for matrices for a specific radioactive waste arose from pyrochemical processing of SNF. These finely dispersed materials, consisting of more than 70% of the layered mineral montmorillonite have a large specific surface, become very durable after annealing like all clays and are able to retain alkali metal ions due to their layered structure. Specific compositions and temperature-time regimes were chosen empirically for immobilization of the spent LiCl-KCl electrolyte with addition of fission products. As a result, strong ceramic samples with an electrolyte content of up to 30 mass. % were synthesized. The addition of silicon-containing compounds to the matrix base material were investigated to increase mechanical strength characteristics and to reduce the temperature and an overall cost of the technology. The formation of new mineral phases during annealing was revealed, and the absence of the effect of radiation exposure up to 100 MGy on the resulted matrix was observed. The studied characteristics included mechanical strength, frost resistance and water resistance, phase composition, morphological structure, and hydrolytic stability. A set of modern analytical and physicochemical methods were used to support the results: X-ray diffraction with in-situ heating in situ, ICP- AES and ICP-MS, mercury porosimetry, X-ray fluorescence analysis, etc. Thus, the fundamental possibility of using bentonite clays as a material for immobilizing the spent chloride electrolyte from pyro-processing of SNF has been demonstrated. (authors)