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Clay-based matrices for chloride- and fluoride-containing radioactive wastesдипломная работа (Магистр)
- Научные руководители: Матвеенко Анна Вячеславовна, Петров Владимир Геннадиевич
- Автор: Yin Yihang
- Тип: Магистр
- Организация, в которой проходила защита: МГУ имени М.В. Ломоносова
- Год защиты: 2025
- Аннотация: The construction of a closed nuclear fuel cycle system in the field of nuclear energy has become a major contemporary research issue. Uranium-plutonium recycling and minor actinide transmutation through spent nuclear fuel (SNF) reprocessing, combined with the development of new reactor technologies, can significantly enhance the sustainability of nuclear energy systems. In the treatment of SNF containing uranium-plutonium mixed nitrides produced by fast neutron reactors (FNR), pyrochemical or combined technologies are required to realize the recovery of fissile materials such as uranium and plutonium. This process inevitably includes the usage of the chlorine salt system and is accompanied by the segregation and accumulation of the fission products, which ultimately results in the formation of chlorine-containing high-level radioactive wastes (HLW)[1]. As well molten salt reactors (MSR) suggested to use fluorine-containing salt mixture as the heat-transfer medium for core operation[2]. This poses a serious challenge to conventional technologies of immobilization of radioactive waste (RW). Although the current mainstream borosilicate glass curing system is widely used, there are significant limitations in its inclusive performance for chlorine and fluoride-based special RW[3]. Although phosphate glass shows better chemical stability in fluoride solidification, but for the curing of chlorides, traditional glass and cement substrates still generally have low sealing efficiency, long-term stability is insufficient and other problems, resulting in increased risk of radionuclide leaching[3,4]. In contrast, ceramic materials, with their unique crystalline structure, show significant advantages in chemical inertness, thermo-mechanical stability and resistance to radiation damage, providing an important direction for the development of new curing matrix. The construction of a closed nuclear fuel cycle system in the field of nuclear energy has become a major contemporary research issue. Uranium-plutonium recycling and minor actinide transmutation through spent nuclear fuel (SNF) reprocessing, combined with the development of new reactor technologies, can significantly enhance the sustainability of nuclear energy systems. In the treatment of SNF containing uranium-plutonium mixed nitrides produced by fast neutron reactors (FNR), pyrochemical or combined technologies are required to realize the recovery of fissile materials such as uranium and plutonium. This process inevitably includes the usage of the chlorine salt system and is accompanied by the segregation and accumulation of the fission products, which ultimately results in the formation of chlorine-containing high-level radioactive wastes (HLW)[1]. As well molten salt reactors (MSR) suggested to use fluorine-containing salt mixture as the heat-transfer medium for core operation[2]. This poses a serious challenge to conventional technologies of immobilization of radioactive waste (RW). Although the current mainstream borosilicate glass curing system is widely used, there are significant limitations in its inclusive performance for chlorine and fluoride-based special RW[3]. Although phosphate glass shows better chemical stability in fluoride solidification, but for the curing of chlorides, traditional glass and cement substrates still generally have low sealing efficiency, long-term stability is insufficient and other problems, resulting in increased risk of radionuclide leaching[3,4]. In contrast, ceramic materials, with their unique crystalline structure, show significant advantages in chemical inertness, thermo-mechanical stability and resistance to radiation damage, providing an important direction for the development of new curing matrix. In this work, we used three different clays as substrates for the synthesis of ceramic samples contained at least 20 wt. % of chloride or fluoride electrolyte as a simulant HLW: Cl-containing RW generated during pyrochemical treatment of SNF from FNR and F-containing RW generated during MSR operation. After a systematic evaluation of the phase composition, mechanical strength and hydrolysis resistance of the samples, the results show that three investigated clays cloud be used for the synthesis of matrix materials, and the characteristics of the compounds obtained successfully meet the requirements for the immobilization of RW. The study also reveals the effects of different clay types and chlorine and fluorine additions on the microstructure and elemental distribution of the ceramic samples through SEM and EDS analyses, and finds that the clay compositions significantly modulate the material properties, and the chlorine and fluorine systems show significant differences in elemental distribution and microformations. So, the main results are: 1. Mechanical properties: compressive strength ≥ 10 MPa; 2. Leaching rate of the Cs ≤ 10-5 g/(cm2·day); 3. The phase composition analyses demonstrated that all three clays formed a dense aluminum-silicate skeleton structure after sintering, incorporating the most significant elements from the waste simulators. 4. I4 clay has the best overall performance in the chlorine/fluorine system, with a homogeneous microstructure and no elemental aggregation. 5. In the Cl system, the element distribution is uniform and the skeleton is stable, while in the F system, the localized F enrichment problem needs to be optimized. 6. We need to focus on the mechanism of Zr enrichment in H3Cl and the regulation of fluorine localization in I4F. Comparison of all the clay samples was provided during every result discussion and in sum: • all three clays exhibit roughly equal levels of leaching resistance in chlorine-containing samples; • based on SEM as well as EDS plot analysis, the chlorine- and fluorine-containing samples of the I4 clay exhibit whatever is best in terms of morphology and elemental distribution combined; • I4 proved to be much stronger than the other two in terms of mechanical strength. Overall, I4 clay performs best in the immobilization of chlorine and fluorine-containing RW. Three clay samples with different mineralogical and chemical characteristics were analyzed and compared to determine their ability to immobilize chlorinated and fluorinated waste from FNR and MSR, thus in general providing new options for the use of natural clays as a matrix for immobilizing HLW. Literature 1. Shadrin A.Yu. et al. РH process as a technology for reprocessing mixed uranium–plutonium fuel from BREST-OD-300 reactor // Radiochemistry. 2016. Vol. 58, № 3. P. 271–279. 2. Gregg D.J. et al. Hot Isostatically Pressed (HIPed) fluorite glass‐ceramic wasteforms for fluoride molten salt wastes // Journal of the American Ceramic Society. 2020. Vol. 103, № 10. P. 5454–5469. 3. Vaishnav S. Structural characterization of sulphate and chloride doped glasses for radioactive waste immobilisation. Sheffield Hallam University, 2018. 4. Sun Y.-P. et al. Properties of phosphate glass waste forms containing fluorides from a molten salt reactor // Nuclear Science and Techniques. 2016. Vol. 27, № 3. P. 63.
- Добавил в систему: Матвеенко Анна Вячеславовна