Removal Of Fission Elements (Re³⁺, Cs⁺, Sr²⁺, Ba²⁺) And Glass-Ceramic Solidification From Spent Salt Of Electrolytic Refining

Nuclear energy has become an alternative for traditional fossil fuels because of its high efficiency and clean characteristics,and has been widely used in military and civilian fields.The radioactive spent fuel generated from nuclear power plants needs to be effectively processed to realize fuel recycling.At present,molten salt electrolysis is considered as one of the most effective means to treat spent fuel.During electrolysis,spent fuel is dissolved in molten salt,and U,Pu,and minor actinide can be electrolytically recovered,but the fission products will be concentrated in the salt,which will inevitably affect the separation efficiency of actinides.Therefore,it is necessary to replace the electrolyte salt.In order to reduce the volume of waste and improve the utilization rate of salt,it is necessary to purify the electrolyte salt.The salt-containing waste obtained after purification treatment need to be further vitrified before geological disposal to prevent nuclear diffusion in nature.However,the treatment of saline waste is a challenge in the academic circles.In this paper,the simulated fission elements RE³⁺（La,Sm,Nd,Dy）,Cs⁺,Sr²⁺and Ba²⁺enriched in molten salt were removed by two-step precipitation method,achieving the separation of simulated fission elements from molten salt.The simulated nuclide was further solidified in the ceramic lattice by high temperature pressureless sintering method,and the ceramic was embedded in the glass,and finally the composite of glass and ceramic（glass-ceramics in this paper）was obtained.The properties of solidified glass-ceramic were measured to determine its element leaching rate and glass corrosion resistance.The specific research contents of this paper are as follows:The removal of RE³⁺,Cs⁺,Sr²⁺and Ba²⁺from LiCl-KCl molten salt was studied using Na₃PO₄ and K₂CO₃ as precipitants.The thermodynamic feasibility of the removal reaction was determined by thermodynamic calculation.The results of analysis showed that the reaction Gibbs free energy of the of REPO₄,Sr CO₃ and Ba CO₃ in molten salt became less than zero.The results of single precipitation experiment in molten salt showed that adding Na₃PO₄ could convert RE³⁺and Cs⁺into insoluble phosphate and adding K₂CO₃ could convert Sr²⁺and Ba²⁺into insoluble carbonate.On-line monitoring of phosphate precipitation process was carried out by square wave voltammetry（SWV）.The results showed that the peak current density corresponding to the reduction of RE³⁺decreased gradually with the addition of Na₃PO₄.When the current peak disappeared,the precipitation reaction between RE³⁺andPO₄^3-were completed,reaching the monitoring endpoint.Molten salt containing RE³⁺,Cs⁺,Sr²⁺and Ba²⁺was used to simulate waste electrolyte,and two-step precipitation was carried out.The removal rate of Cs was 79.17%,the removal rate of RE was above95.00%,and that of Sr and Ba were 99.20%and 99.64%respectively.Borosilicate and iron phosphate glass substrates were used to embed the ceramics containing salt REPO₄,Cs₃PO₄,sodalite（Na₈（Al Si O₄）₆Cl₂）and Ba_0.6Sr_0.4Ti O₃,and the waste salts containing RE³⁺,Cs⁺,Sr²⁺,Ba²⁺and Cl^-were immobilized together.Sr CO₃,Ba CO₃ and Ti O₂ were mixed for pressureless sintering to further obtain Ba_0.6Sr_0.4Ti O₃ ceramic.Cs,Sr and Ba were pre-fixed in the crystal lattice of Cs₃PO₄ and Ba_0.6Sr_0.4Ti O₃ceramics.Because Cs₃PO₄ was easily soluble in water,REPO₄ and Cs₃PO₄ were not recrystallized,and Cl^-existed in the salt containing salt precipitates.The salt contents of La PO₄,Sm PO₄,Nd PO₄,Dy PO₄and Cs₃PO₄ were 6.10,10.20,6.70,9.41 and 9.20 wt.%respectively.The Cl^-in the salt phase was treated with zeolite 4A(Na₁₂Al1₂Si₁₂O₄₈)to generate sodalite,and Cl was pre-fixed in the ceramic lattice of sodalite.Thermogravimetric analysis showed that REPO₄ and Cs₃PO₄with salt had good heat resistance.The maximum embedding rate of the two kinds of glass to the mixed waste reached 30 wt.%.On the 28th day,the normalized leaching rates of RE³⁺,Cs⁺,Sr²⁺,Ba²⁺and Cl^-in the two kinds of glass-ceramics were in the range of 10^-7～10^-2 g·m^-2·d^-1,which meet the requirements of Chinese nuclear industry standards.To immobilize all kinds of ceramics containing fission elements,REPO₄,Sr₅（PO₄）₃Cl and Ba₅（PO₄）₃Cl ceramics were embedded in iron phosphate glass matrix,and the waste salts containing RE³⁺,Sr²⁺,Ba²⁺and Cl^-were immobilized together.RE could be pre-fixed in the crystal lattice of REPO₄ ceramics,but Cl^-existed in the sediments containing salt Sr CO₃ and Ba CO₃.Sr CO₃ and Ba CO₃ containing waste salt were mixed with NH₄H₂PO₄ for pressureless sintering,and further Sr₅（PO₄）₃Cl and Ba₅（PO₄）₃Cl ceramics were obtained.Cl in waste salt was pre-fixed in the crystal lattice of chlorophytate type Sr₅（PO₄）₃Cl and Ba₅（PO₄）₃Cl ceramics.Both ceramics had high crystallinity.The maximum embedding rate of ceramics mixture in iron phosphate glass could reach 30 wt.%.It was measured that the normalized leaching rate of RE³⁺,Sr²⁺,Ba²⁺,Cl^-were within the range of 10^-7～10^-3 g·m^-2·d^-1 on the 28th day,which meet the requirements of Chinese nuclear industry standards(<1 g·m^-2·d^-1).The corrosion mechanism of solidified glass-ceramics in simulated groundwater was explored.The results showed that the glass would undergo hydration reaction,and the glass surface would corrode and form new crystals.By studying the corrosion mechanism of iron phosphate glass-ceramics,it was found that Fe²⁺exchanged with H⁺in the solution under acidic conditions,which showed that the glass was dissolved layer by layer.When glass was immersed in alkali solution,the cation at the end of phosphate chain was hydrated,and the glass structure was destroyed by OH^-corrosion in alkali solution.The loose glass network structure lead to the glass surface falling off in fragments or blocks.Although the corrosiveness of alkaline environment to the cured glass-ceramics was weaker than that of acidic environment,the enhancement of alkalinity would still promote the corrosion of the cured glass-ceramics.This study has laid a solid foundation for building underground laboratories and mastering the geological disposal technology of high-level radioactive waste in the future.