Lanthanide Orthophosphates as a Matrix for Solidified Radioactive Defense and Reactor Wastes
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LANTHANIDE ORTHOPHOSPHATESAS A MATRIX FOR SOLIDIFIED RADIOACTIVE DEFENSE AND REACTOR WASTES M. PETEK, M. M. ABRAHAM, AND L. A. BOATNER Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830 INTRODUCTION Previous investigations have shown that the lanthanide orthophosphates (i.e., analogs of the mineral monazite) are promising candidate waste forms for the storage of high-level actinide or TRU wastes [1]. In the present work, the properties of lanthanide orthophosphates loaded with simulated Savannah River defense wastes and simulated reprocessed light water reactor wastes have been investigated with the goal of evaluating the potential of these materials as advanced primary waste forms. Pure LaPO 4 was selected as the host matrix for the incorporation of either Savannah River defense waste (SRW), or reprocessed reactor fuel waste as represented by the PW-4b composition. Mixed rare-earth phosphate is more economical than any of the pure lanthanide phosphates and was also used as a host for SRW. The formation and precipitation of the composite phosphates was accomplished by adding ammonium hydrogen phosphate and urea to nitric acid solutions of combined rare-earth and calcined oxides of the respective simulated waste form [2]. The compactability of the resulting product was studied as a function of the metal-to-urea ratio during precipitation, waste content and composition, and sintering temperature. Selected properties such as density, particle size, crystallographic phases, and leachability of the resulting materials were examined. MATERIALS AND METHODS Powdered, solidified waste phosphate samples were prepared by first combining the simulated waste form and the corresponding rare-earth oxides or nitrates in a nitric acid solution. Ammonium hydrogen phosphate and urea were added, and the mixture was calcined to form a fine powder as described by Abraham et al. [2]. The resulting powder was either hot-pressed, or coldpressed and sintered, and the density of the compacted cylindrical bodies was determined from their geometrical dimensions and weight. Lanthanum phosphate was used as the host material for incorporating the simulated PW-4b waste form and for the simulated composite SRW without aluminum removal [3]. In addition, a commercially-available lanthanide nitrate solution** was used to prepare waste forms with a mixed rare-earth phosphate matrix for the composite SRW. The composition of the solid material contained in this nitrate solution was 66% La, 24% Nd, 8.2% Pr, 0.7% Ce, and 1.1% other rare earths. X-ray diffraction measurements were used to determine the presence of phases other than the monoclinic monazite phase in the compacted material. Scanning electron microscopy with an X-ray energy spectroscopy capability was used to examine the relative distribution of various elements within different phases. *Research sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract W-7405-eng-26 with Union Carbide Corporation.
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