Phases Formation Rate at Synthesis of Actinide Waste Forms
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Phases Formation Rate at Synthesis of Actinide Waste Forms N.P. Laverov, S.V. Yudintsev, M.I. Lapina, S.V. Stefanovsky 1, S.C. Chae 2, and R.C. Ewing 3 IGEM RAS, Staromonetny 35, Moscow 119017, RUSSIA 1 SIA Radon, 7th Rostovskii per., 2/14, Moscow 119121, RUSSIA 2 KIGAM, 30 Kajung-Dong, Yusung-Ku, Taejon, KOREA 3 University of Michigan, Ann Arbor, MI, 48109-2104, USA.
ABSTRACT Rate of zirconolite (CaZrTi2O7) and pyrochlores (Gd2Ti2O7, CaCeTi2O7, and Gd2Zr2O7) formation by cold pressing and sintering (CPS) was studied. Batches were prepared from CaCO3, ZrO2, TiO2, CeO2, and Gd2O3 milled to size of 20-30 micrometers. Powders were compacted and sintered in air (oxygen) at 800-1600 oC for 0.5-55 hours. Samples were examined with XRD, SEM, and TEM. Phase formation rate was the fastest in Gd-Ti-O and Ca-Ce-Ti-O systems and the reactions were the slowest in Gd-Zr-O system. In zirconolite-based system reaction rate had intermediate value. For three systems at 1400 C equilibrium was reached in 3-5 hours, while in Gd-Zr-O system required about ten time longer reaction time. High formation rate was evidently one of the reasons to select the pyrochlore (Ca,Gd,U,Pu)2(Hf,Ti)2 O7 as a host for Pu in the USA. We have also studied an effect of titanium on pyrochlore formation in the Gd-Zr-Ti-O system. The batches with nominal composition Gd2Ti0.4Zr1.6O7, Gd2Ti0.2Zr1.8O7, and Gd2Ti0.1Zr1.9O7 were compacted and annealed at 1500-1600 oC for 3-98 hours. In most of them along with pyrochlore non-reacted zirconium and gadolinium oxides were found. Their content reduced with temperature and sintering duration increasing. Pyrochlore composition was varied in different parts of the samples. Variation range decreased with rise of temperature, runs duration, and titanium content in the precursors. Partial substitution of Ti for Zr increased rate of pyrochlore formation. However, even at the highest titanium content (0.4 formula units) and temperature of synthesis (1600 oC) too long sintering duration in tens of hours is required to form a single-phase pyrochlore matrix. This makes their production from oxide precursor via CPS route inefficient. Inductive melting in a cold crucible or self-sustaining hightemperature synthesis are more promising methods for fabrication of the zirconate pyrochlores. INTRODUCTION The promising matrices for actinides immobilization are oxides with fluorite-type lattice (cubic zirconia, zirconolite, pyrochlore). These phases have high capacity with respect to actinides and significant chemical durability and radiation resistance [1-7]. Most common method of their preparation is cold pressing and sintering (CPS). CPS was chosen for industrial excess Pu immobilization in titanate pyrochlore in the USA [8-9]. The process includes, in a step-by-step manner: grinding of actinide oxides mixture, addition of matrix components and neutron absorbers, mixture homogenization, compaction at room temperature and pressure of 10-20 MPa, and finally, heat-treatment at 1300-1400 0C for 4 hours. To optimize CPS conditions
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