| High-level radioactive liquid waste（HLLW）is the waste liquid produced during the reprocessing of spent fuel.The treatment and disposal of high-level waste liquid has always been one of the difficulties affecting sustainable development of the nuclear industry.Vitrification is currently the only industrialized method to treat HLLW in the world.Borosilicate glass is widely used as confinement matrix for HLLW.Long-term storage of HLLW forms a layer of sludge at the bottom of the tank,that is,high-level radioactive sludge.The main components of existing high-level radioactive sludge in China are phosphomolybdic heteropolyacid salt,barium sulfate and nitrate.Due to the low solubility of molybdenum and sulfur in borosilicate glass(Mo O42-≤2 wt%,SO42-≤1 wt%),beyond the solubility limit,molybdates or sulphates can cause phase separation on the surface of the glass and affect its chemical stability of the waste glass.Currently,there is no effective method for treating and disposing of high-level radioactive sludge in China.In this thesis,simulated high-level radioactive sludge borosilicate waste glass was prepared by melting method.The effects of phosphomolybdic heteropolyacid salt（PM,the main component of high-level radioactive sludge）content and melting temperature on the structure and leaching resistance of borosilicate glass were systematically investigated.The dissolution of PM during melting was studied.The chemical stability of borosilicate waste glass was evaluated.The composition,structure and physical/chemical properties of simulated high-level radioactive sludge borosilicate waste glass were investigated.The leaching resistance of simulated high-level radioactive sludge borosilicate waste glass was evaluated under standard static conditions（PCT）and under"Thermodynamics-Hydrodynamics-Mechanics-Chemistry"（THMC）multi-field coupling conditions,respectively.The main results are as follows:（1）The effects of PM content and melting temperature on the structure of borosilicate waste glass was investigated.The results show that the waste glass with the addition of 0～4 wt%PM possesses the amorphous structure.White granular Ca Mo O4and Ba Mo O4 phases appear when PM content is higher than 5 wt%.The waste glass comprises two superimposed layers and is no longer homogeneous when the PM content is higher than 12 wt%.With the increase of PM content from 0 wt%to 12 wt%,the[Si O4]structure in the glass network structure gradually depolymerizes,the[Mo O4]2-structure gradually increases,and the contents of[BO4],Q3and Q4 units in the glass network decrease,leading to the depolymerization of the overall network structure of the waste glass.The main crystalline phases of the waste glass with 4 wt%PM content are Ca Mo O4 and Ba Mo O4 at 700～1050℃.The crystals gradually dissolve and the density of the waste glass gradually increases above1100°C.The Mo O3 content in the waste glass remains almost unchanged when the temperature increases from 800 to 1000℃,and decreases slightly from 1000 to 1150℃.（2）The effects of PM content on the leaching resistance of borosilicate waste glass were studied.The results of static leaching experiments（PCT）show that the normalized leaching rates of Si,B,and Na of the waste glass with 4 wt%PM content were lower than the original glass after 28d.With the increase of PM content,the normalized leaching rates of Si,B,and Na of the waste glass increase gradually.The normalized leaching rate of Mo of the waste glass with 8 wt%PM content was low because of forming a homogeneous glass-ceramic structure.In addition,the normalized leaching rates of Mo of the waste glass slightly increase gradually with the increase of PM content after 28d.（3）The effects of simulated high-level radioactive sludge content on the structure of borosilicate waste glass were investigated.The results show that the loading of simulated high-level radioactive sludge can reach 8 wt%.The waste glass with the addition of 0～8 wt%simulated sludge possesses the amorphous structure.Ca Mo O4and Ba Mo O4 phases appear when simulated sludge content is higher than 10 wt%.The waste glass comprises two superimposed layers when the simulated sludge content is higher than 12 wt%.The analyses of FTIR,Raman and MAS NMR show that with the increase of simulated sludge content from 0 wt%to 12 wt%,the[Si O4]structure in the glass network structure gradually depolymerizes,the[Mo O4]2-structure gradually increases,and the contents of[BO4]and Q4units in the glass network decrease,leading to the depolymerization of the overall network structure of the waste glass.（4）The leaching resistance of simulated high-level radioactive sludge borosilicate waste glass was evaluated under standard static conditions and under THMC multi-field coupling conditions,respectively.The results of static leaching experiments（PCT）show that with the increase of simulated sludge content,the normalized leaching rates of Si,B,Na and Mo of the waste glass decrease first and then increase after 28d.The normalized leaching rates of Si,B,Na and Mo of the waste glass with 8 wt%simulated sludge content are 5.23×10-3,5.35×10-3,1.56×10-2 and 1.94×10-3 g·m-2·d-1,decreasing to the minimum value.The results of the THMC multi-field coupling experiments show that the leaching rates of Si,B and Mo in the waste glass increased slightly for an early period and then remained stable.The leaching rate of each element in the waste glass with different simulated sludge content keep at 10-1order of magnitude after 28 d.The leaching rates of Si,B and Mo of the waste glass with 8wt%simulated sludge are 4.07×10-1,2.42×10-1 and 1.28×10-1 g·m-2·d-1. |
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