Fabrication And Stability Of Gd₂Zr₂O₇ Pyrochlore Waste Forms For Multi-nuclides Incorporation

To investigate the feasibility of Gd₂Zr₂O₇ pyrochlore used for disposal waste of multi-nuclides with multi-valence, simulated trialkyl phosphine oxides?TRPO developed by the Institute of Nuclear and New Energy Technology, Tsinghua University in China? waste was chosen as a simulated object. A series of?Gd,A?2?Zr,B?2O7 ceramics were successfully fabricated through a solid-state reaction. Phase composition, crystal structure, microstructure and chemical durability of the as-prepared simulating waste forms were characterized by means of X-ray diffraction?XRD?, Raman spectroscopy, electron backscattered diffraction?BSE?, scanning electron microscopy?SEM? and inductively coupled plasma mass spectrometer?ICP-Mass?. The phenomenon of alpha acceleration irradiation caused phase and microstructure were anazyed and elucidate their alpha irradiation damage mechanism. The main results of this work are as follows:?1? Gd₂Zr₂O₇ pyrochlore waste forms immobilized TRPO waste has been successfully prepared using Gd2O3, ZrO2 and TRPO waste as raw material, by the high temperature solid state reaction at 1500? for 72 h.?2? For Gd₂Zr₂O₇ solid solutions immobilized TRPO waste 1, the samples all exhibit a single pyrochlore structure. When the doping content of TRPO waste 2 is below than 35 wt.%, the samples show a single pyrochlore phase; The compositions corresponding to 35 – 65 wt.% adopt a single defect fluorite phase. With the enhancement of TRPO waste doping content, the lattice parameters display an approximately linear decrease. SEM results show that the samples we obtained are relatively dense and exhibit uniform contrast. In addition, the elements are almost distributed homogeneously in the solid solutions.?3? The density of simulated TRPO waste 1 form is about 4.303 5.630 g?cm-3, and its Vickers hardness is about 416.439 568.742 Kg?mm-2. The density of simulated TRPO waste 2 form is about 4.323 5.874 g?cm-3, and its Vickers hardness is about 416.428 552.642 Kg?mm-2.?4? The leaching concentration of all the elemnets from TRPO waste forms at 70°C is higher than that at 40°C, and is lower than 10-6 g/ml. The normalized release rate of each element in the solid solution has the highest values at the first day and decrease with leacing time. After 28 days, the normalized release rate of each element converges to constant value.?5? For TRPO waste forms with doping content 0, 25 wt.% and 40 wt.%, a crystal structure transformation from an ordered pyrochlore to a disordered fluorite structure occurred under 0.5 MeV He ions at the fluence of 1×1014- 1×1017 ions/cm2. No obvious amorphization was observed even to the highest fluence of 1×1017 ions/cm2. With an enhancement of ion fluence, the cell volume of the irradiated sample increase to some extent. At the same He ion fluence, the radiation damage tolerance of TRPO waste forms decreases with the enhancement of TRPO waste content. Raman results show that the main Raman peaks of each composition decrease in intensity with enhanced ion fluence, and the Raman modes shift towards higher frequencies. SEM observations show that there is no considerable variation in crystalline form of the sample after irradiation, and the grain size of the sample increases to some content. All the elments are almost distributed homogeneously in the irradiated sample.