Removal And Immobilization Of Radionuclides From Spent Molten Salts From The Electrometallurgical Treatment For Spent Fuel

Rapid development of nuclear industries will bring about large amount of spent fuels containing much radionuclide, which threatens ecological environment seriously. Therefore, spent fuel reprocessing is an important guarantee for the sustainable development of nuclear industries.Among many spent fuel reprocessing method, electrometallurgical dry reprocessing based on high temperature molten salt medium has fully been developed and studied most extensively. However, small amount of toxic and radioactive elements, such as La?Ce?Fr?Nd?Sm?Gd?Cr?Ni?Sr?U? Pu, still remains in the used molten salt (hereinafter called as spent molten salt). In this thesis, hydrothermal and high temperature solid-state methods were used to remove and immobilize the nuclides in spent molten salts aiming at the recycling use of molten salts and reducing of high level radioactive waste. XRD, ICP, and XRF were used to analyze the phase and composition of sediment products, solidfied forms and the molten salts after purification.Hydrothermal method:La3+?Ce3+?Nd3+?r3+ and Ni2+ were used to simulate the fission products, corrosion products and actinides. A certain amount of KOH was added to the aqueous solution of spent molten salts, and the mixture was reacted in hydrothermal condition to precipitate the above ions. It has been shown that majority of these ions could be removed from the spent molten salts when the pH value of the mixture solution rise to 9.High temperature solid-state reaction:(1) zirconia was used as a matrix to remove and immobilize rare earths, such as La3+?Ce3+?Pr3+?Nd3+?Sm3+? Gd3+ through the reaction between Zr and rare earth salts to produce rare earth zirconates (Ln2Zr2O7). The results show that the products crystallize in pyrochlore structure for La3+?Pr3+?Nd3+and Sm3+, and defect fluorite structure for Gd3+, whereas only partial Ce4+ transform to Ce2Zr2O7.(2) Gd2Zr207 was used as a matrix to remove and immobilize La3+?Ce3+? Nd3+?Cr3+ and Ni2+. Partial Gd3+ was replaced by one or more the above ions to synthesize Gd2Zr2O7 in molten salts to immobilize these ions in Gd2Zr207 lattice. The results show that immobilized ratio of Cr3+ is almost zero, whereas those of other ions are higher than 99.9% when 10% of singular element was immobilized at 800?. When 10% mixed ions are co-doped in Gd2Zr2O7, the immobilized ratio of Cr3+ increased obviously to 80%.(3) Gd2Zr2O7 was used as a matrix to remove and immobilize Sr2+. It is shown that the immobilized ratio of Sr2+ in Gd2Zr2O7 could reach 99.7% when 10% of Sr2+ replacing Gd3+This research work provides some experimental and theoretical data for the cleaning and recycling of the spent molten salts from the electrometallurgical treatment for spent fuel.