Temperature Dependence of Amorphization for Zirconolite and Perovskite Irradiated with 1 Mev Krypton Ions
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TEMPERATURE DEPENDENCE OF AMORPHITZATION FOR ZIRCONOLITE AND PEROVSKITE IRRADIATED WITH 1 MeV KRYPTON IONS T.J. WHITE*, R.C. EWING**, L.M. WANG**, J.S. FORRESTER*** & C. MONTROSS***
*Ian Wark Research Institute, The University of South Australia, Warrendi Road, The Levels, SA 5095, Australia "**Departmentof Earth and Planetary Sciences, The University of New Mexico, Albuquerque NM 87131, USA ***Department of Mining and Metallurgical Engineering, The University of Queensland, Brisbane QLD 4075, Australia ABSTRACT
A transmission electron microscope investigation was made of zirconolites and perovskites irradiated to amorphization with 1 MeV krypton ions using the HVEM-Tandem Facility at Argonne National Laboratory. Three specimens were examined - a prototype zirconolite CaZrTi2 OT, a gadolinium doped zirconolite Ca 0.75Gd0.50Zr 0.75Ti20 7 and a uranium doped zirconolite Ca,. 75U0.5oZr 0.75Ti207 . The critical amorphization dose D, was determined at several temperatures between 20K to 675K. D. was inversely proportional with temperature. For example, pure zirconolite requiring lOx the dose for amorphization at 475K compared with gadolinium zirconolite. Using an Arrhenius plot, the activation energy E. for annealing in these compounds was found to be 0.129 eV and 0.067 eV respectively. The greater ease of amorphization for the gadolinium sample is probably a reflection of this element's large cross section for interaction with heavy ions. Uranium zirconolite was very susceptible to damage and amorphised under 4 keV argon ions during the preparation of microscope specimens. In each sample, zirconolite coexisted with minor perovskite, reduced rutile (Magneli phases) and zirconia. These phases were more resistant to ion irradiation than zirconolite. Even for high gadolinium loadings, perovskite (Ca0 ..Gdo.2TiO3) was 3-4 times more stable to ion irradiation than the surrounding zirconolite crystals.
INTRODUCTION
Two elements of radiation damage ingrowth are of particular importance in assessing the long term durability of ceramic nuclear waste forms. First, the intrinsic radiation resistance of the crystalline phases must be understood, at least in a qualitative sense, to avoid the selection of materials which are highly unstable towards irradiation. Amongst those compounds known to be resistant it is important to determine if there is a threshold dose where physical properties degrade catastrophically. This knowledge can be used to more safely select nuclear waste loadings appropriate to specific polyphase assemblages. Second, the capacity of a ceramic to repair by self annealing the displacement damage induced by alpha-recoil events will mitigate the effects of irradiation, and in deep geological repositories, might be used to advantage to maximise waste loadings. Zirconolite has long been identified as one of the most versatile and durable minerals available for the stabilisation of transuranic elements and rare earth fission products [1]. Its chemistry is diverse [2], and mineral specimens have quantitatively retai
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