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EXAFS

STUDY

OF

(La 1 _XCex)

2

Zr 2 O7

HIROTO YOKOI* ,TSUNEO MATSUI* ,HIDEO OHNO**, AND KATSUMI KOBAYASHI*** *Department of Nuclear Engineering, Faculty of Engineering, Nagoya University,Furo-cho, Chikusa-ku, Nagoya 464-01, Japan. **Office of Synchrotron Radiation Facility Project, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan. ***Photon Factory, National Laboratory for High Energy Physics, Tsukuba, Ibaraki 305, Japan.

ABSTRACT The local structures around Zr atoms such as coordination number of oxygen and cations, and the Zr-O and Zr-Ln (Ln; La, Ce) interatomic distances in the (Lal-iCe )2r70 pyrochlore compounds (X = 0-0.25), where Ce is a simulation of actinides,were studied by means of extended X-ray absorption fine structure (EXAFS) spectrometry. The local structure determined by EXAFS in this study is discussed in relation to the dissolution behavior of (LaX Ce )2Zr 207 (leaching rates of Zr, La and Ce) in an acid solution (pH=l) at3 3K reported previously. The difference in the local structure before and after leaching test was also Investigated.

INTRODUCTION The development of the safe disposal technique of the high-level nuclear waste (HLW) in addition to that of the advanced inherently-safe reactor is necessary for public acceptance. The transuranium (TRU) elements, due to their long half-life, in HLW should be conditioned so as to minimize a release from the final repository to the biosphere. There are two suitable matrix forms for fixation of fission-products; one is a borosilicate glass matrix and the other is a ceramic matrix. For fixation of TRU elements, the latter seems to be better than the former because of its thermodynamic stability for long time. The pyrochlore structure containing Zr as a main constituent element with an ideal composition M2Zr 2 07 (M = tri-and/or tetravalent cations) is considered to be a promising host ceramic for solidification (i.e. immobilization) of high level nuclear waste, especially lanthanide and actinide elements for two reasons; low leaching rate in water[l] and acid solution[2] and confinement of considerable amount of actinide and lanthanide elements in its crystal structure[3,4]. The information on the local crystal structure around cations in the pyrochlore structure is important since the thermal stability and dissolution behavior of the constituent elements in water (and/or acid solution) are thought to be related to their local crystal structure (i.e. the coordination number and the bond length). The solubility limit of Ce, Nd and Sr in lanthanum zirconate (La 2 ZrZO7 ) with a pyrochlore structure was recently determined from the compositional dependence of the lattice constant by Hayakawa and Kamizono[4], but the structural study on (LalXM )2Zr 2 07 (M = lanthanide and actinide elements) has not been performed yet. Extended X-ray absorption fine structure (EXAFS) spectrometry is important method to determine the local structures around the specified atoms in various materials and has been recently applied only for Gd2 Zr 2O7 wi