Temperature Dependence of Ion Irradiation Induced Amorphisation of Zirconolite
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Materials Division, Australian Nuclear Science and Technology Organisation, P.M.B. 1, Menai, NSW 2234, AUSTRALIA. Materials Science Division, Argonne National Laboratory, 9700 South Cass Ave, Argonne, I1 60439, USA
ABSTRACT The critical dose for amorphisation, Dc, of two end-member zirconolites (CaZrTi 2O7 ) with different stacking fault densities, was measured as a function of irradiation temperature from 20 K to 623 K using the HVEM-Tandem Facility at Argonne National Laboratory (ANL). Below 473 K, the D, values of both samples are identical within experimental error, showing only a small increase in D, from (2.5 to 4.6) x 10"8 ions m2 between 20 K and 473 K. At temperatures above 473 K, the data for the zirconolite containing many stacking faults is bracketed by two data sets from almost crystallographically perfect end-member zirconolites: one collected in this study and one collected in a previous study. The raw Dc versus temperature data from the zirconolites in this and a previous study suggest that the critical temperature above which samples cannot be amorphised and/or recrystallisation is complete, T•, is between 600 and 1000 K. The data sets collected in this study are discussed in relation to a current model. INTRODUCTION Zirconolite is one of the major host phases for actinides in various wasteforms, particularly Synroc [1], for immobilising high level radioactive waste (HLW). Over time, the periodic crystalline matrix is damaged by a-particles and energetic recoil nuclei resulting from a-decay events. The cumulative damage caused by these particles results in amorphisation. Data from natural zirconolites suggest that radiation damage partly anneals over geologic time and is dependent on the thermal history of the material [2]. Proposed HLW containment strategies rely on both a suitably water-resistant wasteform and geologic isolation. Depending on the waste loading, depth of burial, and the repository-specific geothermal gradient, burial could result in a wasteform being exposed to temperatures of between 373-523 K. Consequently, it is important to assess the effect of temperature on radiation damage in synthetic zirconolite. Zirconolite containing wasteforms are likely to be hot pressed at - 1473 K or sintered at - 1623 K [3]. Zirconolite fabricated at temperatures below 1523 K (1250'C) contains many stacking faults and twins [4]. As there have been various attempts to link radiation resistance to structure [5 - 7], it is also pertinent to assess the role of original crystallographic perfection in radiation resistance. In this study, we simulated a-decay damage in two zirconolite samples by irradiating them with 1.5 MeV Kr' ions using the High Voltage Electron Microscope-Tandem User Facility (HTUF) at Argonne National Laboratory (ANL) and measured the critical dose for amorphisation (Dc) at several temperatures between 20 and 623 K. One of the samples had a high degree of crystallographic perfection; the other contained many stacking faults on the unit cell scale. Previous authors proposed a model f
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