Syntheses Of Uranium-Rich Pyrochlore Synroc

High-level radioactive waste(HLW) contains the toxic, long half-life and strong radioactive nuclides. As a result, how to safely and effectively handle and disposal the HLW becomes the great challenge for protecting environment and for supporting the sustainable development of nuclear energy. HLW geological disposal is the most suitable disposal method so far. One of the key techniques is the preparation of waste form that can adapt to disposal conditions for long time. Synroc waste form with good leaching resistance, low inflation, stable chemical properties and good thermal stability was treated as the favorable waste forms for disposal. Pyrochlore(CaUTiO7), as one kind of synrocs,normally has large capacity of uranium bearing, high density and high leaching resistance.It is considered to be the favorable waste form for HLW geological disposal. However, it is necessary to conduct further studies for future possible utilization.By using sol-gel method, and using ethylene glycol solution with citric acid as sol agent, dry gel was prepared. And then the dry gel was treated under the condition of 650℃and 2 h preservation by TG and DTA, which yielded the fine powders of precursor. EDS analyses indicates that the chemical proportion of precursor is of Ca:U:Ti:O≈1:1:1.5:7,which can be used for the sintering synthesis experiment.Systematic orthogonal experiments revealed the best conditions for sintering experiment: 1200 ℃ sintering temperature, 9 MPa cold pressing pressure, 5% binder and3 h heat preservation time. XRD results of Ti doping pyrochlore indicate that diuranate calcium is relatively lower when the proportion of U:Ti=2, while the pyrochlore presents higher crystallization degree with higher compacted density under the sintering temperature of 1200 ℃.This study also inspects the influence of Al3+charge to condensate, as compensation charge. XRD diagrams indicated that the condensate is still dominated by pyrochlore witha small amount of brannerite(UTi2O6) which yielded the different diffraction peaks. The chemical compositions analysis by EDS. The results show that the solid solution quantity of uranium increased with the chemical formula of Ca0.823U1.134Ti1.827Al0.138O7 when adding Al3+ charge to condensate. SEM microstructure images illustrated that the crystal of the grain boundary precipitates was rectangular in shape, while the crystal with Al3+compensation charge is regular octahedron in shape.The chemical stability of syntheses condensate under the best sintering conditions was conducted by static leaching experiment of MCC-1. It can be found that elements leaching rate in simulated groundwater is slightly higher than that in deionized water. Both syntheses condensate and condensate mixed with Al3+ as compensation charge have good chemical stability and leaching resistance. However, the normalized leaching rate of uranium is slightly higher in condensate mixed with Al3+ as compensation charge.