Crystal chemistry of sodium zirconium phosphate based simulated ceramic waste forms of effluent cations (Ba2+, Sn4+, Fe3+, Cr3+, Ni2+ and Si4+) from light water reactor fuel reprocessing plants

A novel concept of immobilization of light water reactor (LWR) fuel reprocessing waste effluent through interaction with sodium zirconium phosphate (NZP) has been established. Such conversion utilizes waste materials like zirconium and nickel alloys, stainless steel, spent solvent tri-butyl phosphate and concentrated solution of NaNO₃. The resultant multi component NZP material is a physically and chemically stable single phase crystalline product having good mechanical strength. The NZP matrix can also incorporate all types of fission product cations in a stable crystalline lattice structure; therefore, the resultant solid solutions deserve quantification of crystallographic data. In this communication, crystal chemistry of the two types of simulated waste forms (type I—Na_1.49Zr_1.56Sn_0.02Fe₀.₂₈Cr_0.07Ni_0.07P₃O₁₂ and type II—Na_1.35Ba_0.14Zr_1.56Sn_0.02Fe₀.₂₈Cr_0.07Ni_0.07P_2.86Si_0.14O₁₂) has been investigated using General Structure Analysis System (GSAS) programming of the X-ray powder diffraction data. About 4001 data points of each have been subjected to Rietveld analysis to arrive at a satisfactory structural convergence of Rietveld parameters; R-pattern (R_p) = 0.0821, R-weighted pattern (R_wp) = 0.1266 for type I and R_p = 0.0686, R_wp = 0.0910 for type II. The structure of type I and type II waste forms consist of ZrO₆ octahedra and PO₄ tetrahedra linked by the corners to form a three-dimensional network. Each phosphate group is on a two-fold rotation axis and is linked to four ZrO₆ octahedra while zirconium octahedra lies on a three-fold rotation axis and is connected to six PO₄ tetrahedra. Though the expansion along c-axis and shrinkage along a-axis with slight distortion of bond angles in the synthesized crystal indicate the flexibility of the structure, the waste forms are basically of NZP structure. Morphological examination by SEM reveals that the size of almost rectangular parallelepiped crystallites varies between 0.5 and 1.5 μm. The EDX analysis provides the analytical evidence of immobilization of effluent cations in the matrix. The particle size distributions of the material along selected reflecting planes have been calculated by Scherrer's formula.