Study On The Mechanism Of Microwave Coupled Coal Fly Ash Immobilization Of Multivalent Radioactive Waste

The treatment and disposal of high-level waste have always been a worldwide problem,and the safe treatment and disposal of multivalent long-lived radionuclides have become a bottleneck that restricts the sustainable development of nuclear energy.At the same time,the annual discharge of coal fly ash（CFA）in China amounts to about 600million tons and its utilization rate is only 70%,resulting in the accumulation of more than 3 billion tons of CFA,which has brought enormous environmental pressure to China.Inspired by the above,the method,microwave coupled CFA for immobilization of multivalent radioactive waste,is proposed in this thesis.The CFA is employed as the curing substrate and reducing agent.It is expected that the research results can provide a theoretical basis and technical support for the optimization and preparation of multivalent radionuclides bearing waste forms.The main findings are as follows:（1）The chemical composition,phase composition,microstructure, and dielectric properties of CFA were explored to reveal the basic characteristics and microwave sintering properties of CFA.It was determined that crude CFA is mainly composed of Al₂O₃,SiO₂,and alkali or alkaline earth metal oxides,and the main phases are SiO₂,mullite,and carbon.Moreover,the microscopic particles of CFA consisted mainly of spherical particles of different diameter sizes with a wide range of particle size distribution and a double normal peak shape with a D（50）value of 15μm.By testing the dielectric parameters of CFA and its main components as well as the effect of temperature on the dielectric properties（dielectric constant,dielectric loss factor,dielectric loss angle tangent,and microwave penetration depth）of CFA,it was demonstrated that CFA is suitable for microwave heating.（2）The immobilization of low-valent radionuclides in microwaved sintered fly ash was successfully achieved.The immobilization mechanism was revealed by evaluating the phase composition,microstructure,and chemical stability of the sintered body.（I）For monovalent（Cs⁺）and divalent(Sr²⁺)radionuclides,as the ionic radius of Al³⁺is similar to that of Si⁴⁺,Al³⁺can substitute Si⁴⁺in an ordered or disordered manner during the sintering process,and the charge difference caused by the substitution of Al³⁺for Si⁴⁺was compensated by the positively charged Cs⁺or Sr²⁺cations,which would react with Al³⁺and Si⁴⁺to produce CsAlSi₂O₆and SrAl₂Si₂O₈.Notably,the Cs and Sr elements are uniformly distributed in the sintered matrix without obvious enrichment.（II）For trivalent(Nd³⁺)radionuclides,during the sintering process,Nd³⁺reacts with Si⁴⁺and Ca²⁺to produce Ca₂Nd₈（SiO₄）₆O₂.Moreover,the Nd element is uniformly distributed in the cured body without obvious enrichment,and the maximum solid solution amount also exceeds 30 wt.%at the optimum sintering temperature.（Ⅲ）For the co-doping of divalent and trivalent radionuclides,the Nd element is fixed in the glass network structure,while the Sr element is incorporated in the form of SrAl₂Si₂O₈.（Ⅳ）The chemical stability of the sintered sample was tested using the PCT method,and the results demonstrated that the samples bearing 30 wt.%nuclides prepared at the optimum sintered temperature were all of the excellent chemical stability with leaching rates of about 0.08×10^-5g·m^-2·d^-1for Cs and3.64×10^-5g·m^-2·d^-1for Sr at 42 days.（3）The microwave-enhanced CFA carbothermal reduction technique was successfully developed for the immobilization of high-valent radionuclides.The immobilization mechanism of microwave-enhanced fly ash carbon-thermal reduction immobilized high-valent radionuclides was revealed by evaluating the phase composition,microstructure,and chemical stability of the sintered body.Part of Ce⁴⁺was carbon-thermal reduced to Ce³⁺at 1200℃under nitrogen atmosphere,and Ce element was incorporated in the glass network structure,while the solid solution amount of CeO₂in the cured body under air atmosphere was less than 5 wt.%.It indicated that the solid solution amount in the cured body can be significantly enhanced by regulating the valence state of the nucleophile(Ce⁴⁺→Ce³⁺).（4）A new method for the preparation of high-performance microwave-sintered bodies derived from alkali-activated CFA was established.During the alkali activation process,the amorphous glass phase and the quartz component of CFA undergo desilication.The silicate network structure gradually depolymerized,and about 28%of Al（Ⅵ）was transformed to Al（Ⅳ）,which replaced Si⁴⁺in[SiO₄].Due to the difference in bond length,bond angle,and binding energy between[AlO₄]and[SiO₄],the silicate structure of alkali-activated CFA was more unstable than that of the original CFA,and the silicate structure was excited and the reactivity was enhanced.The chemical stability of the sintered forms was tested using the PCT method and the results indicated that the sintered bearing 30 wt.%Nd₂O₃were of excellent chemical stability at the optimum sintering temperature,and that the leaching rate of Nd at a concentration of 100 g/L of the alkaline solution became lower than that of the original CFA.The lowest leaching rate of Nd elements was around 0.74×10^-6g·m^-2·d^-1.