REE and TRU Incorporation into Monazite Structure Ceramics
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REE and TRU Incorporation into Monazite Structure Ceramics S. I. Rovnyi1, G. M. Medvedev1, A. S. Aloy2, T. I. Koltsova2 and S. E. Samoylov2 1
Production Association Mayak, Ozyorsk, Chelyabinsk Region, Russian Federation RPA V.G. Khlopin Radium Institute, St. Petersburg, Russian, E–mail: [email protected]
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ABSTRACT One of the high levels of actinide, and in particular Cm, waste streams at the Russian radiochemical Production Association (PA) Mayak was generated during spent fuel reprocessing. Using oxalate precipitation, the rare earth elements (REE) and transuranic elements (TRU) settled out in the form of oxalate residues. Due to in high REE contents in this residue, the mineral-like matrix based on (REE)PO4 solid solution, with monlclinic monazite structure have been proposed to use as a suitable ceramics form for final actinide immobilization. For this purpose the synthetic REE oxalates were first transformed into REE orthophosphates in a thin-film evaporator (TFE). Then the (REE)PO4 powder was compacted both by either hot uniaxial pressing (HUP) or cold uniaxial pressing followed by sintering (CUP) . This ceramic with the monazite structure has a high density and exhibits chemical durability by leaching. INTRODUCTION High-level waste (HLW) produced at the Production Association (PA) Mayak facility during the reprocessing of spent nuclear fuel (SNF) are exposed to a separation stage for the purpose of 90Sr/137Cs recovery using a “pure-strip” solution. The rest of the HLW contains primarily a mixture of rare earth elements (REE) and transuranic elements (TRU) with an increased amount of 241Am and 244Cm. To meet the requirements, which regulate the management with high alpha-bearing waste, the REE and TRU concentrate were later treated using oxalic acid to precipitate REE and actinides. Taking into account that the oxalate residue is high in REEs it was decided to explore the opportunity of fabrication monazite-type ceramics [1] based on the REE-rich residue and to study the chemical and physical properties of these ceramics. EXPERIMENTAL RESULTS Oxalate precipitation of REE and TUE Approximately 117m3 of “pure-strip” solution have been accumulated at the PA Mayak facility after the separation of 90Sr/137Cs from HLW processing. The average composition of the “strip” solution is as follows: (by element, g/l): Nd – 0.5, Sm – 0.3, La – 0.2, Pr – 0.15, Gd – 0.03, Y – 0.05, U – 0.5, Pu – 0.005, Al– 5.0, Fe – 2.5, Si – 2.0, Mn – 0.7, Cr – 0.5, Ni – 0.4, Ca – 0.2, and Mg – 0.1 determined by ICP-MS method. The oxalate precipitation procedure [2] represents a unique situation for REE and TRU selective concentration in a small volume of residue. Based on experiments done on a laboratory scale with a real and simulated solutions it was shown that residues obtained accumulated to a level containing 90% cerium and lanthanum, 95% praseodymium, promethium, neodymium and
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samarium, 98% yttrium, gadolinium, europium, americium and curium, 88% plutonium relatively to their initial content in solution. Production
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