Formation Rate and Compositions of the Actinide Hosts with Garnet Structure
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Formation Rate and Compositions of the Actinide Hosts with Garnet Structure Tatiana S. Ioudintseva1 and Soo-Chun Chae2 IGEM RAS, Staromonetny 35, Moscow 109017, RUSSIA 2 KIGAM, 30 Kajung-Dong, Yusung-Ku, Taejon, KOREA 1
ABSTRACT Garnet phases have been considered as a durable crystalline waste form for hosting actinide. The garnet-structure phases with stoichiometries of Ca2,5Ce0,5Zr2Fe3O12, Ca2CeZrFeFe3O12, and Ca1,5GdCe0,5ZrFeFe3O12 were synthesized through cold pressing and sintering in air and oxygen to determine the optimum parameters for the formation of actinide waste forms. Cerium (Ce) was used as an imitator of plutonium due to its similarity in oxidation state and ionic radii. Gadolinium (Gd) plays a major role as an absorber of neutrons that prevents nuclear chain reaction. It also serves as imitators of the trivalent actinides. Ce-garnet or (Ce,Gd)-garnet is chemically analogous to the garnets with plutonium and/or trivalent actinides. The results of XRD and SEM-EDS examination of the products of experiments reveal that equilibrium state was reached at the temperatures of 1300 oC and 1200 oC for 1 and 5 hours heating, respectively.
INTRODUCTION Fluorite and garnet type oxides are promising matrices for actinide wastes. These have high isomorphic capacities of actinides and accompanying elements, such as Zr, REE (pyrochlore, zirconolite, garnet), Mn, Fe, Al (garnet), and have both good radiation and chemical durability. In addition, the matrix properties also require the ability of their large-scale fabrication. The cold pressing and sintering (CPS) method is efficient route to actinide matrices production. The method is used to prepare various crystalline hosts for actinide immobilization [1-4]. Phase relation and kinetics of zirconolite and pyrochlore formation [4] and synthesis of Th-loaded garnets [5] were studied previously. Precursor with (Ca2.5Th0.5)Zr2Fe3O12 composition yielded single-phase garnet ceramic, whereas (Ca2Th)ZrFeFe3O12 formulation produced garnet at 1050 o C and one with thorianite (ThO2) at 1200 oC (Table I). It took only 3 hours to form the garnet empirically. The examination of composition with SEM-EDS should be performed to confirm whether the synthetic garnet composition is consistent with initial batch composition on the basis of stoichiometry. The role of garnet-structure matrices as a durable waste form for hosting actinide was first proposed in Russia [6]. The samples, analogous in composition to andradite, Ca3Fe2Si3O12, were produced by inductive cold crucible melting. However, no evidence of the garnet formation exists. Later, matrices containing up to 80 vol.% of garnet [7] were obtained using the same method. The contents of REE and Zr in garnets exceed 20 wt%, but the ones of actinides in these phases was not investigated. Garnets as potential matrices for actinide waste were also being studied in the Radium Institute [8,9]. These scientists selected the (Y,Gd)-(Al,Ga) garnet to investigate its possibility of a potential matrices. Ce and U oxides were substit
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