Intergrowth Structures in Synthetic Pyrochlores: Implications for Radiation Damage Effects and Waste Form Formulation
- PDF / 2,578,296 Bytes
- 8 Pages / 377.46 x 611.1 pts Page_size
- 20 Downloads / 174 Views
ABSTRACT Titanate-based ceramic waste forms are currently under development for the immobilization of excess weapons plutonium. Both Hf and Gd are added to the ceramic formulation as neutron absorbers in order to satisfy a defense-in-depth concept for the waste form. The introduction of significant amounts of hafnium may be responsible for the presence of zirconolite-2M crystals in pyrochlore-based ceramics and the formation of zirconolite lamellae within pyrochlore. The zirconolite grows epitaxially on I 111 }planes of pyrochlore. Although the zirconolite lamellae within pyrochlore are non-cubic, any volume expansion due to radiation damage in the pyrochlore should still be isotropic; in addition, the presence of these intergrowths may allow some stress relief in the ceramic.
INTRODUCTION Titanate-based ceramic waste forms are currently under development for the immobilization of excess weapons plutonium [1]. These ceramics exhibit excellent durability at various physical and chemical conditions [2]. The major phases in these ceramics are pyrochlore [A2Ti 2O], zirconolite [ABTi2 OT], Hf-bearing rutile (TiO 2), and brannerite [BTi O2J, where A = Ca, actinides (ACT), and rare earth elements (REE), and B = ACT, REE, Zr, and Hf. Additional minor phases may occur depending on waste loading; these include uranium oxides and glassy phases. The glassy phases (and rutile) fill interstices between the major phases. Both Hf and Gd are added to the ceramic formulation as neutron absorbers in order to satisfy a defense-in-depth concept for the waste form. Although the capacity of Gd to stop thermal (low energy) neutrons is nearly 500 times greater than that of Hf, the capacity of Hf to absorb thermal and epithermal (medium energy) neutrons is still far greater than any other major element in the ceramic besides Gd [3]. However, the main reason for adding Hf is that it possesses a much lower aqueous solubility than Gd [4]; therefore, Hf should be present in any residual Pu precipitate that may form during possible ceramic corrosion [2]. In this study, a series of plutonium-free ceramic formulations containing various levels of impurities was investigated. In this paper, a particular microstructural feature is described in three formulations: the baseline composition, which contains only Ca, Ti, U, Ce, Gd, and Hf, and two impurity samples. The levels of impurities in these samples are based on anticipated waste streams to be received during the fabrication of the Pu ceramic. The two impurity ceramics examined were a metallic-based impurity sample containing small amounts of transition metals and an extreme oxide impurity sample, which contained Si, Al, Mg, Na, and Ga, as well as a range of transition metal oxides.
EXPERIMENTAL PROCEDURE The ceramic material was prepared at Lawrence Livermore National Laboratory (LLNL) by blending the following: TiO 2 (anatase), U0 2, Gd20 3, Ca(OH) 2, HfO 2, and CeO2 for 1 h at 750'C in air. This mixture was pressed at 83 MPa and then sintered for 4 h in Ar at 1350TC. The cool-down rate was =5
Data Loading...
