Quantitative X-Ray Diffraction Analysis of Titanate Waste Forms and its Application to Damage Ingrowth
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QUANTITATIVE X-RAY DIFFRACTION ANALYSIS OF TITANATE WASTE FORMS AND ITS APPLICATION TO DAMAGE INGROWTH T.J. WHITE* AND H. MITAMURA** *Electron Microscope Centre, The University of Queensland, Brisbane, Australia, 4067 **Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken, 319-11, Japan. ABSTRACT Quantitative powder X-ray diffraction was used to analyse the phase assemblages of titanate 244 nuclear waste ceramics doped with Cm and accumulating a-decay doses up to 1.24 x 1018 alpha-decay-events g-l. Among the Cm-bearing phases, perovskite is more susceptible to damage than zirconolite. Phases devoid of curium (hollandite and freudenbergite) experience a flux of ionizing radiation, and may also undergo radiation-induced structural degradation. The analysis demonstrated that at a dose of 1.24 x1018 alpha-decay-events g-1 approximately 20 wt% of the waste phases remain aperiodic after annealing for 2 hours at 800oC. INTRODUCTION The longevity of crystalline high-level nuclear waste forms is controlled fundamentally by the structural changes which accompany the accumulation of a-recoil damage. Information concerning damage ingrowth is often obtained by powder X-ray diffraction (XRD). Until recently, diffraction data was treated only in a semiquantitative fashion. In particular, although unit cell dilation and loss of crystallinity were monitored, the susceptibility of different phases to radiation damage or the extent of structural recovery during subsequent thermal treatment could not be satisfactorily gauged. These limitations have been overcome by the development of methods for full profile analysis', where the entire pattern is fitted, step by step, to a pattern calculated using models for the crystal structures and peak profiles. This paper reports preliminary information on the structural changes in radiation damaged titanate waste forms derived from quantitative XRD. Full profile analyses were undertaken of a non-radioactive, prototypical titanate ceramic without waste, together with as-damaged and annealed ceramics doped with 0.69 and 0.91 wt% 244Cm. The intense a flux experienced by the latter samples effectively compressed up to 33000 years of radiation damage (corresponding to a dose of 1.24 x 1018 a-decay-events g-1) into a period of 2 years. We demonstrate that estimates of phase proportions and crystallinity can be obtained, and that full profile analysis provides a useful means of quantifying radiation damage effects in polyphase ceramics. However, further study is required to correctly account for the effects of preferred orientation and X-ray absorption. METHODOLOGY X-ray diffraction methods have been used for the determination of phase abundance in polycrystalline mixtures for many years [1]. Most of these methods use empirical relationships between integrated peak intensities, with relatively few studies devoted to the direct determination of phase abundance by analysis of the full diffraction profile. However, it has recently been demo
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