| The separation process of actinides and fission product from high level liquid waste (HLLW) have some drawbacks such as the low separation efficiency and high equipment requirement. In the study, in order to immobilize actinides and fission product Sr of radioactive liquid wasete simultaneously and stably,the author has designed and synthesized a new simulated waste form,Ln2Zr2O7-SrZrO3,in which Ln and Sr are used to simulate An and 90Sr respectively, and carried out the following research work for the phase structure, chemistry and radiation stability.The Phase composition, microstructure and elemental distribution of the waste form were investigated by XRD, Rietveld structure refinement, SEM/EDS, TEM and so on. The results show that multiphase waste form is composed only by cubic phase Ln2Zr2O7 (Ln= La, Nd, Eu, Gd; LnZO) and quadrature perovskite phase SrZrO3(SZO). no other phase is produced, and Ln and Sr can be fixed their most stable lattice sites; Both of LnZO and SZO are completely compatible, the content of element Ln, Zr, Sr changed regularly with the design of the formula, the experimental results are consistent with the new and designed waste form; The crystal structure of waste form consists of two phases, the bright phase is the LnZO with pyrochlore structure, the dark phase is the SZO with perovskite structure. The crystal growth is mainly polygonal morphology, the structure is dense and has low porosity; TEM results for 0.5NSZO waste form show that material has a clear boundary and the interface does not produce a typically interphase. Selected area electron diffraction pattern of NZO phase described that the cation is ordering arrangement highly, EDS confirmed that the doping content each other for Sr and Nd elements is very little.The above experimental results show that the new waste form can immobilize actinides and fission product Sr simultaneously and stably, and have a strong ability to adapt to the waste ingredients, which consistent with author’s research expectation.The physical properties of the sample test results show that the body density of pyrochlore and perovskite phase is about 93～95% of the theoretical density,which indicates that the single-phase sample sinterd at 1673K reaction sufficiently and the structure is dense, which Consistents with the experimental results of SEM and TEM; the vickers hardness values of pyrochlore and perovskite samples is high relatively, which indicates the structure of waste form is dense and the binding force of ion is strong; the multiphase form’s density value and vickers hardness value are just between LnZO and SZO, and decrease progressively and regularly with the increase of Sr content.The preparation method and crystal structures for CSO and CSZO,who contain variable valence of lanthanide elements Ce, was explored and researched. The results show that:In this experiment, the single-phase k-CeZrO4 and multiphase waste form CeZrO4-SrZrO3 containing k phase can be prepared by graphite powers at various reducing temperatures (1223≤Tred.≤1573K) for 24h in a self-made reducing device. The method adopted in our experiment is simpler, safer and more economic than the reported one of using a reduction-reoxiation two-step procedure by Omata et al. It can be considered as a better substitute method to prepare. The reduced phase is identified as the k-CeZrO4 phase with the space group of P213, in which Ce and Zr ions are in an ordered arrangement similar to that of the pyrochlore Nd2Zr2O7 and the ordering level between Ce and Zr ions in the pyrochlore-type k-CeZrO4 phase decreased with decreasing Tred.The graphite reduction mechanism is considered as follows:the heating and heat preservation procedures are considered as equivalent to CeZrO3.5+δ phase formation process.the cooling process is equivalent to carry out the reoxidation for CeZrO3.5+δ compound accordingly.The characteristics of easy decomposition for k-CeZrO4 phase shows that the structural stability is poor. Therefore, CSZO waste form with k phase is not in the column of performance study.The chemical stability of NSZO and 0.5LnSZO series were estimated by the MCC-1 method.The normalized leaching rates for target elements of Nd、Sr、Zr were lower than or equal to the leaching rates of a single-phase pyrochlore or Synroc. The leaching resistance of each element is reducing generally according to the order of Zr>Nd> Sr, the low leaching rate of Zr is attributed to the Zr-O covalent bond; the leaching depths of Ln and Zr were only 0.5nm and 0.01nm respectively, but the Sr element is about ～um magnitude, the etching depths above eletments are quite with pyrochlore-rich synroc.The leaching resistance of 0.5LnSZO waste form is similar to 0.5NSZO, which indicates that the Multiphase materials are suitable for most Ln series.The above analysis shows that Multiphase waste form have excellent chemical stability.The XRD、SEM/EDS were used to study waste form’s leaching surface.Results indicate that single-phase NZO has good chemical stability, but the leaching surface of single-phase SZO seems to be "molten" after leaching 72d and has white precipitates, which is corresponding to the diffraction peaks of amorphous phase and a new phase from XRD figure, the result means SZO phase’s surface structural haa occurred rearrangements, and the structure stability is less than NZO, but the generated amorphous phase effectively prevented further leaching of elements; the structure of multiphase waste form containing NZO and SZO is still stable in a leaching condition of high temperature and long time, which is in accordance with the low leaching rate; in addition, after recombination with NZO,the structure stability of SZO has been greatly improved; Leaching mechanism for various elements is different, and the leaching mechanism Zr is more complex, simply exchange mechanism or dissolution mechanism can not be used to explain.In Lanzhou Modern Physics, we test the waste form’s irradiation resistance on 320Kv highly charged ions integrated research platform. The XRD, glancing-incidence X-ray diffraction and Raman were used to analyse the irradiated sample. The results show that the diffraction peaks of NZO,0.5NSZO and SZO samples reduced regularly and width, but there was no completely metamictization; For irradiated samples at 5dpa, the result of glancing-incidence XRD shows that Kr ion energies are deposited at 650nm,600nm and lum respectively in the sample’s subsurface, and has partial amorphization, but above or below the damage layer structure is similar to the body; When the radiation dose is equivalent (1dpa), the basic structure of NSZO and 0.5LnSZO has unchanged. The above results show that the designed multiphase waste form has excellent irradiation resistance. |
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