Investigation On SAC Wasteform Of Performance And Leaching/γ Irradiation Resistance Mechanisms For Intermediate And Low-level Radioactive Waste

Nuclear energy,as a kind of clean energy,is widely used in the field of power generation.The large amount of intermediate-and low-level nuclear waste produced by nuclear power plants is one of the important factors restricting the development of nuclear power.It is very important to dispose of intermediate-and low-level nuclear waste efficiently and safely.The commonly used ordinary Portland cement immobilization technology has problems such as low mechanical strength of the wasteform,high nuclides leaching rate and insufficient resistance to gamma irradiation.This leads to the low package capacity of the wasteform,making its higher storage and transportation costs.Studies have shown that sulfoaluminate cement（SAC）wasteform has high mechanical strength and excellent leaching resistance performance,making it a potential cementitious material that can immobilize low-and medium-level nuclear waste.This subject uses SAC as the main cementitious material to study the basic properties of SAC wasteform that immobilized radioactive spent resin,boron-containing liquid waste and incineration ash,and then systematically optimize its formulation.Also,its mechanisms of leaching andγ-irradiation resistance have been studied in detail.A material with good mechanical properties and excellent durability that solidified intermediate-and low-level radioactive waste was obtained;The mechanism of SAC immobilized radionuclides is explained,mainly through the lattice immobilization and surface adsorption of ettringite,the surface adsorption of AH₃ and the physical barrier of SAC dense structure;In addition,aiming at the phenomena of poor leaching resistance of Cs⁺,the targeted modification of nano materials（KNFC and LDHs）was carried out.First,the compressive strength,impact resistance,immersion resistance,leaching resistance,freeze-thaw resistance and gamma radiation resistance of the above-mentioned multiple wasteform were studied.The results showed that the various properties of the SAC nuclear waste wasteform have good performance under the premise of higher package capacity.Among them,the compressive strength of the radioactive waste resin wasteform was still above7 MPa when the package capacity is 60%,which met the requirements of GB 14569.1-2011;the performance of the wasteform of the boron-containing waste liquid with a concentration of2.1%is excellent.For a concentration of 11%borate liquid waste,the mechanical properties of the boron-containing waste liquid wasteform basically met the requirements of the national standard,and the durability needs to be further improved.The wasteform of incineration ash with a content of 50%still has good mechanical properties and durability.When the package capacity is high,the other durability properties of all kinds of wasteform also meet the requirements of the national standard.Aiming at the shortcomings of poor impact resistance of the wasteform at high package capacity,the experiment optimized its impact resistance by adding copper-coated steel fiber,which improved the fracture toughness of the wasteform at high package capacity and reduced it during transportation.The possibility of breakage was reduced,when subjected to an impact.Moreover,the thermal conductivity and thermal diffusivity of the wasteform added with steel fibers were significantly improved,which helps to quickly conduct the heat generated in the process of cement hydration and reduces the damage of the matrix caused by thermal stress.In addition,the response surface method was used to further optimize the wasteform.The optimized formula is:14-20%fly ash,5-12%zeolite and 3-8%silica fume.Verification of the optimized formula found that the mechanical properties,water resistance,and freeze-thaw resistance of the wasteform all met the requirements of the national standard,and the hydration heat release is reduced by 38.4%,which effectively reduced the heat release concentration.The immobilization mechanism of SAC for nuclear waste has been systematically studied,mainly including the study of the interaction between nuclide ions and SAC,the immobilization and leaching mechanism of nuclides,the interaction with the SAC wasteform,and the immobilization mechanism.Studies have found that nuclides such as Cs⁺and Sr²⁺have an inhibitory effect on the early hydration of cement,but have little effect on the later hydration.The study on the leaching model of nuclide found that the leaching of Cs⁺conforms to the FRDM and DDIM models;the leaching of Sr²⁺conforms to the DDIM model.The immobilization performance and immobilization mechanism of the two main hydration products of SAC（ettringite and aluminum gel）on the nuclide were studied separately,and it was found that both have excellent immobilization effects on the nuclide ion.The immobilization of Cs⁺by ettringite is mainly through its surface adsorption and inter-column adsorption,while the immobilization of Sr²⁺has both adsorption and solid solution displacement;the immobilization of nuclides by aluminum gel is mainly through surface adsorption and bonding.A preliminary study on theγ-irradiation resistance mechanism of the SAC nuclear waste wasteform was carried out,including the effects ofγ-irradiation on the mechanical strength,hydration behavior and microstructure stability of the wasteform.The results show thatγ-irradiation has no obvious deteriorating effect on the mechanical strength of the cement matrix,and even contributes to the development of its later strength.However,the radioactive waste resin has obvious deterioration,which leads to a decrease in the performance of the spent resin wasteform after irradiation.γ-irradiation promotes the hydration of ye’elimite and C₂S in SAC,and optimizes the pore structure of the matrix.The stability of the microstructure of the wasteform was studied by XRD,XPS,FT-IR and Solid-State MAS-NMR,and it was found that the structure of ettringite had a certain lattice distortion or lattice relaxation,which was due to thatγ-irradiation caused the decomposition of interlayer water;the AH₃ afterγ-irradiation changes to a more disordered amorphous state.In order to further improve the leaching resistance performance of Cs⁺for the SAC wasteform,Li-Al-LDHs and KNFC,which can immobilize Cs⁺,were initially synthesized and modified,and their immobilization effect in the cement matrix was studied.XRD and XPS analysis have found that the distance between the modified Li-Al-LDHs layers is significantly increased,and it has a strong ability to absorb Cs⁺.Also,modified Li-Al-LDHs and KNFC both promoted the early hydration of SAC and improved the leaching resistance performance of Cs⁺.