Research Of Containing Characterisics With Calciumiron Phosphate Glass Nuclear Waste Forms

Glass in the CaO-Fe2O3-P2O5 system has received the considerable attention for high-level radioactive wastes immobilization because of their greater tolerance for radionuclides, low melting temperature and viscosity in the high temperature, good chemical durability. Between rare earth elements and actinides elements in the valence, extranuclear electron arrangement and ionic radius depend upon the same coordination number are substantially similar, which leaded to separate difficultly from the actinide. In addition, some rare earth elements were the main products of nuclear fission waste. Therefore, in the paper the calcium iron phosphate base glasses and use the matrix glass solidified five kinds of rare earth elements yttrium (Y), lanthanum (La), neodymium (Nd), samarium (Sm), gadolinium (Gd) in their structure, thermal stability and crystallization properties were studied by means of X-ray diffraction (XRD), Scanning electron microscopy (SEM), Fourier transform infrared spectra (FT-IR), Raman spectroscopy, UV-vis spectroscopy, Differential scanning calorimetry (DSC). The studies have shown that:1. The calcium iron phosphate glass matrix xCaO-(32-x)Fe2O3-68P2O5 has a wide range of glass formation. Spectroscopic analysis clarified that in the glass network (P-O- Ca2+-O-P) chain was formed by replacing P-O- when CaO content was less than 16 mol%, so that the glass structure become more compact, which resulted in the increasing of hardness and glass transition temperature; when CaO content was more than 16 mol%, calcium ions addition formed (P-O-Ca2+-O-P) and P-O-Ca by replacing P-O-P and P-O-Fe, resulting in decreasing of chemical stability of glass. When Fe2O3 was replaced by CaO, the structure of the matrix glass converted from pyrophosphate to metaphosphate species, so the average phosphate chains increased and the content of the non-bridging oxygen in the glass system reduced. After the two steps heat treatment with glass samples, the major crystalline phases were changed with the increasing of CaO content. By sour leaching the heat-treated samples, only FeP04 was selectively dissolved out. CaO addition in the glass system caused the increasing of crystallization activation energy, and the glass systems were prone to surface crystallization.2. Glass waste forms xY2O3-(100-x)(12CaO-20Fe2O3-68P2O5) could contain 10 mol% Y2O3. Because Y2O3 that filled the interstice of glass network made network structure more compact, resulting in the increasing of density and hardness. Y2O3 addition in the glass caused the decreasing of P-O-P bond angle, and formed P-O-Y bonds by replacing P-O- and P=O, which leaded to the transition of main structural units and increased the crosslink of the glass network. As Y2O3 content increased, yttrium coordination number (NYO) increased, while the thermal stability of the glass waste forms decreased. After the heat treatment with doped yttrium glass waste forms, addition of Y2O3 leaded to the changes of crystallization phases, which affected the crystallization properties of the waste forms.3. A variety of rare earth oxide glass waste forms showed similar capacity with doped yttrium glass waste forms, and excess rare earth elements resulted in the forming of crystallization phases REPO4. RE2O3 addition in the base glass formed RE-O-P, and cations with higher field strength provided more non-bridging oxygen to depolymerize the glass network. Since the Y-doped phosphate glass compared to La-doped phosphate glass has smaller P-O bond length, which resulted in the change of their hardness. Rare earth element with higher coordination numbers enhanced the crosslink of glass network, which increased its mechanical properties. Thermal stability of different rare earth glasses was improved with increasing field strength values of rare earth ions. Different rare earth glass after heat treatment with the main crystallization phase and their morphology was studied. Its main crystallization phases were FePO4 and REPO4, after acid leaching only crystallization morphology REPO4 was observed by SEM.