Hot-press Sintering And Chemical Durability Of Fluorapatite Ceramic Waste Form

Due to the high radiotoxicity and long half-life of minor actinides,their safety immobilization treatment becomes a major concern and technological difficulty in the management of high-level radioactive waste（HLW）.Fluorapatite ceramic was used to immobilize the simulated minor actinides in this paper,the chemical durability of the waste form was studied systematically as well.The pure fluorapatite was synthesized by the solid state reaction and the fluorapatite ceramic waste form was prepared by hot-press sintering using Ca F₂,Ca₂P₂O₇,Ca CO₃,Na₂CO₃,Sm₂O₃,Eu₂O₃and Gd₂O₃ as the raw materials.The phase composition,phosphate group,bulk density,Vickers hardness,microstructure and hot-press sintering process of fluorapatite ceramic waste form were analyzed through XRD,Raman,FTIR,density tester,Vickers hardness tester,SEM and EDS.The better hot-press sintering processes of fluorapatite ceramic waste form were optimized by relative density and Vickers hardness.The chemical durability and the nuclide leaching behavior of fluorapatite ceramic waste form in the simulated deep geological environment have been investigated.This thesis includes the following three parts:（1）In this study,the low-temperature and controllable synthesis of pure fluorapatite in the atmosphere was carried out.The study found that the pure fluorapatite can be successfully synthesized at 900℃for 4 h,when the extra 46 wt%Ca F₂ was added to the reaction raw materials and the reaction raw materials were pressed before the chemical reaction.The pure fluorapatite is well crystallized and has a uniform grain size.The grain size is between 2 and 4μm.The elements such as calcium,phosphorus,oxygen and fluorine are uniformly distributed in pure fluorapatite.Pure fluorapatite only contains phosphate groups and the adsorbed water,without any hydroxyl and carbonate groups in the samples.It indicates that the formation of carbonate and hydroxyl groups could be suppressed by the presence of fluoride ion.The possible mechanism of the synthesis of pure fluorapatite is that the pressing of raw materials and the extra addition of46 wt%Ca F₂ make the raw materials to form a"volatilization-compensation"equilibrium at the calcination temperature of 900℃,which ensures the synthesis of pure fluorapatite.（2）In this study,the simulated nuclides（samarium,europium,gadolinium）doped fluorapatite ceramic waste forms were successfully fabricated by hot-press sintering process.The samarium/europium/gadolinium doped fluorapatite ceramic waste form with the highest relative density（99.46%,99.23%and 99.58%）and maximum Vickers hardness（3.704,3.92 and 3.7913 GPa）were successfully fabricated by hot-press sintering at1100℃for 2 h,1150℃for 3 h and 1200℃for 2 h,respectively.（3）In addition,the chemical durability of samarium/europium/gadolinium doped fluorapatite ceramic waste forms were systematically evaluated under the THMC coupling effects.No obvious changes were found in the crystal phase composition and phosphate group of the samarium doped fluorapatite ceramics before and after the leaching experiments.After being soaked in p H=3 solution,the monazite crystals containing calcium(Ca_0.16Eu_0.96PO_4.45,Ca_0.34Gd_1.19PO_4.37)were detected on the surface of the europium/gadolinium doped fluorapatite ceramic waste form,respectively.The Sm/Eu/Gd doped fluorapatite ceramic waste forms possess high chemical durability in the neutral and alkaline leachates at 90℃.The lower normalized leaching rates of Sm,Eu,and Gd in the fluorapatite ceramic waste forms were¹0^-6（Sm）,¹0^-7（Eu）and¹0^-7（Gd）g·m^-2·d^-1,respectively.The above normalized leaching rates of fluorapatite ceramic waste forms were 3-4 orders of magnitude lower than that of the nuclide normalized leaching rate(¹0^-3 g·m^-2·d^-1)in the borosilicate glass waste form.The preparation technologies used in the project were simple and practical.The sintering process was carried out at a lower temperature.The fluorapatite ceramic waste forms with large density,high purity and superior chemical durability were obtained.The preparation technologies would help to realize the resource utilization of phosphorus containing HLW,and the engineering application of HLW immobilization treatment.The related research results provided the theoretical basis and technical support for the optimization and preparation of high-performance phosphorus and minor actinides containing HLW ceramic waste forms,and laid a solid foundation for promoting the applied basic research and safe disposal of HLW immobilization using the ceramic.