Phase Structure Evolution And Chemical Stability Studies Of Hollandite For Cesium And Its Daughter Barium Immobilization

The reprocessing of nuclear spent fuel generates a lot of high-level radioactive waste with high toxicity,strong radioactivity and half-life.It must be isolated from the biosphere in very durable solid matrices.Synroc solidification is called as“the second-generation solidified high-level radioactive waste”.A lot of research work on the preparation,incorporated capacity and stability of synroc solidification has been carried out at home and abroad.Hollandite is one of the main mineral phase of Synroc-C,and it is a stable mineral phase with self-regulating ability.In Hollandite,large-size ions(Cs⁺,Ba²⁺,Sr²⁺)can be incorporated into its lattice sites or gap by isomorphic substitution or low temperature co-melting to realize the lattice solidification of radionuclides.However,radionuclides are unstable,and daughter products are quite different from their parents due to a decay.For example,when ¹³⁷Cs releases?-particle decay to ¹³⁷Ba,the chemical valence changes from+1 to+2,and the ion radius decreases by²0%.Therefore,it is necessary to pay attention to the influence of decay daughters when evaluating the properties of solidification.In this study,hollandite was used as Cs host,the non-radioactive ¹³³Cs and ¹³³Ba were chosen as alternatives for simulating the parent ¹³⁷Cs and daughter ¹³⁷Ba,respectively.The solid solutions of?Ba,Cs??M,Ti?₈O₁₆(M=Al³⁺,Ti³⁺,Fe³⁺)was prepared by high temperature solid state method and spray pyrolysis method,and the effect of decay daughter on the phase composition,microstructure and stability of the solidified body was investigated.The prepared samples were analyzed by means of TG-DSC,XRD,SEM-EDS,TEM-EDX and other tests.The XRD patterns were refined by Rietveld,and the effects of different concentrations of Cs⁺and Ba²⁺on the lattice parameters and microstructure of the solidified body were analyzed.Furthermore,the leaching stability of the samples are evaluated by static leaching test.?1?Using carbonate and oxide as raw materials,[Cs_xBa_y][(Al³⁺,Ti³⁺)_2y+xTi⁴⁺_8-2y-x]O₁₆?0.4?x,y?0.8?and[Cs_xBa_y][Fe³⁺_2y+xTi⁴⁺_8-2y-x]O₁₆?0.3?x,y?0.75?ceramics were prepared by high temperature solid state method at 1250°C for 4 h.The solidified body has complete crystallization,uniform distribution of elements and dense micro-morphology.?2?Base on the rietveld results,it is found that when the amount of Cs doping decreases from 0.8?0.75?to 0.4?0.3?,the lattice volume of the solidified body only shrinks by less than 1.5%,while the Cs/Ba-oxygen bond does not appear serious distortion.The standard leaching experiment of MCC-1 shows that the Cs release rate is about two orders of magnitude lower than that in the glass waste forms,and the normalized leaching rate of Cs decreases with the increase of Cs concentration.?3?The(Cs_0.8-xBa_0.4+x)(Al_1.6+xTi_6.4-x)O₁₆?0?x?0.4?ceramics were synthesized at1050°C from raw materials that were pretreated by spray pyrolysis.The grains are uniform rod-shaped,integrity crystal,the elements are uniformly distributed on the grains,and more than 85%of the Cs is retained in the solid solution.