Study of Processing Hllw by Super High Temperature Method. Part II*. Reducing Reactions and Formation of Complex Oxides
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STUDY OF PROCESSING HLLW BY SUPER HIGH TEMPERATURE METHOD PART I *. REDUCING REACTIONS AND FORMATION OF COMPLEX OXIDES IN THE SIMPLIFIED HLLW Masayosi UNO, Youji KADOTANI, Chic MIYAKE and Misato HORIE** Department of Nuclear Engineering, Faculty of Engineering, Osaka University Yamadaoka 2-1, Suita, Osaka 565 Japan **PNC, Tokai-Mura, Ibaraki 319-11 Japan ABSTRACT The reducing reactions of platinum metal oxides and the formation of the complex oxides were studied to estimate the process at high temperatures. Heat treatment of Re207 with TiN causes reduction up to the sublimation point of Re207 and exhibits the highest value of the recovery among the reducing agents used in the present study. Reducing reactions of the platinum metal oxides with the reducing agents lower the melting temperature. The simplified complex oxides contain the complex oxides of the representative elements of FP and the metallic elements of the reducing agents, which contribute to a decrease of melting temperature, neodymium zirconium oxides and neodymium barium oxide which may transform to the other phases during heating. Uranium may dissolve in the phases in which uranium is identified by EPMA but cannot detected by X-ray diffraction. INTRODUCTION In the proposed super high temperature processing technique, high level liquid waste (HLLW) generated in the Purex process is first calcined at 70000 and then reduced and melted above 1600 0C with a reducing agent such as BN, AIN, Si3N4 and TiN. Elements with high standard free energy of oxide formation (platinum metals and transition metals) are reduced and form the metal or alloy phase which separates from the oxide phase of stable oxides of alkaline earth elements, rare earth elements, Zr and actinidesll,2,31. Recovery of platinum metals has a great influence on reducing the costs associated with super high temperature process. The recovery of Re as a surrogate for Tc that has a low sublimation point was also found to be possible. In the present study, the reducing process for the platinum metal oxides and rhenium oxide was studied by thermal gravimetry and high temperature X-ray diffraction methods. The information on the chemical states of the elements in the complex oxides is very important since the oxide phase is treated as a compact high active waste (HAW) without additives. However, the formation of several complex oxides of many elements in HLLW may make it difficult to identify the compounds formed in the oxide phase. In this study, simplified complex oxides that contain one from each element group were produced and the constituent compounds in the products were identified.
*Part I is included in ref. 3.
Mat. Res. Soc. Symp. Proc. Vol. 353 0 1995 Materials Research Society
1340
EXPERIMENTAL In the study of reducing reactions for the platinum metal oxides, RuO2, Rh203, PdO and Re207 as a surrogate for Tc oxide, were used. The selected elements in the simplified complex oxides were Ba, Nd and U, as the representative of alkaline earth elements, rare earth elements and actinides, res
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