Sodium Zirconium Phosphate [NZP] as a Host Matrix for High Level Radioactive Wastes
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Sodium Zirconium Phosphate [NZP] as a Host Matrix for High Level Radioactive Wastes Yoshimi SEIDA, Mami Yuki, Kazunori Suzuki, and Toshio Sawa* Nuclear Chemical Engineering Center Institute of Research and Innovation Takada 1201, Kashiwa, 277-0861 Japan * Hitachi co.ltd. ABSTRACT Various elements (Cs, Sr, Ba, Zr, Ru, Pd, Ce, Nd, Gd, Fe, Cr, Ni, Mo and Te) in the simulated high-level radioactive wastes generated from commercial PUREX reprocessing were immobilized by a ceramic solidification using sodium zirconium phosphate, NaZr2(PO4)3 as a host matrix. The convertibility of the elements to the specific M, A and X sites in NZP crystal structure was determined with consideration of stoichiometry, charge balance and ion size of each element. Small disk samples of NZP waste form containing the elements were prepared by the sol-gel synthesis followed by calcination and compression sinteration at high temperature. The physicochemical structures such as produced phase in the waste forms and dispersion of the embedded elements in the NZP waste forms were investigated by means of XRD and SEM/EDX. Chemical behavior of the embedded elements and the limit of substitution of the NZP structure for the elements were investigated by the series of analysis. Moreover, in order to increase the limit of substitution of NZP, the effectiveness of pretreatment with heating the sol-gel products at 473-773K was experimentally investigated. The capability of embedding of NZP for the elements in HLW has been discussed. INTRODUCTION Sodium zirconium phosphate, NaZr2(PO4)3 (abbreviated as NZP) and its family compounds have been considered as a potential host matrix for several elements in high-level radioactive wastes (HLW), because of their capability of embedding a wide range of elements into the crystal structure of NZP [1]. The structure of NZP is generally represented by the formula MA2(XO4)3. NZP has a three dimensional anionic framework of [An2(XO4)3](2n-9) which has a structure in which AO6 octahedra share apexes with XO4. It has been reported that Na, Zr and P at the respective M, A and X sites in the parent NZP are substituted by various cations and/or anions [3]. There are few studies that examined the capability of embedding the elements in HLW generated from the commercial PUREX reprocessing into NZP structure [2-5]. Sol-gel method is one of the potential synthesis methods of NZP that are applicable to the solidification of HLW. By the sol-gel method, larger amount of substitution with the HLW elements in the NZP structure can be expected because the elements can be homogenously mixed with materials of the matrix in the state of aqueous solution at the first stage of synthesis. In this study, main elements in the simulated HLW from PUREX reprocessing were embedded in NZP, and the behavior of chemical immobilization was investigated in order to estimate the capability of embedding and/or incorporating the elements of NZP.
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EXPERIMENTAL Composition of NZP containing the elements in HLW Table 1 shows the elements
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