Emanation Thermal Analysis Study of Brannerite
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Emanation Thermal Analysis Study of Brannerite V. Balek1, V. Zelenák1*, Z. Málek1, E. R.Vance2 and J. Subrt3, 1 Nuclear Research Institute CZ- 25068 Rez, Czech Republic 2 Australian Nuclear Science and Technology Organization, Private Mailbag 1, Menai (Sydney), NSW 2234, Australia 3 Institute of Inorganic chemistry, Academy of Sciences of the Czech Republic, CZ-25068, Rez, Czech Republic * On leave from the Faculty of Sciences, University of Kozice, Moyzesova 11, SK-04154 Kozice, Slovak Republic ABSTRACT Radon emanation thermal analysis (ETA) during ”step by step” heating to selected temperatures up to 1200oC in argon and subsequent cooling yielded detailed information on the annealing of radiation damage and macroscopic defects in a natural brannerite. Thermogravimetry showed mass losses in the temperature ranges 230-315, 570-760 and 8401040°C. Mass spectrometry of the evolved gases indicated M/Z = 44, i.e. CO2. Crystallization of initially amorphous brannerite was indicated by ETA in the temperature range 900-1020°C, in agreement with the X-ray diffraction results. From the radon diffusion activation energy values determined in ETA, the degree of structural ordering was assessed at different temperatures. INTRODUCTION Brannerite is a minor phase in titanate-based ceramics designed for the geological immobilization of surplus Pu [1, 2]. The brannerite was found to incorporate neutron absorbers such as Gd and Hf in its structure as well as Pu and U. The crystal structure is monoclinic [3, 4] and both the U and Ti are in distorted octahedral co-ordination. The crystal chemistry of brannerite has been studied recently in some detail [5, 6]. Other important questions for the radioactive waste immobilization application are the aqueous dissolution behavior and radiation damage resistance of brannerite. Brannerite is normally found as an amorphous mineral and its transformation to crystallinity on annealing at temperatures of ~1000oC has been studied by X-ray diffraction (XRD), thermogravimetry (TG), differential thermal analysis (DTA) and scanning/transmission electron microscopy [4, 5]. In the present paper, Rn emanation thermal analysis (ETA) has been used to see if further information on the annealing behavior of brannerite could be obtained. The ETA results are discussed in comparison with those of the more common techniques mentioned above. Since ETA is not a well-known experimental method, we shall here describe the basic theory. ETA [7, 8] involves the measurement of radon release rate from samples previously labeled by exposure to 228Th and 224Ra. The decay products, 224Ra and 220Rn, were incorporated into the sample to a maximum depth of 80 nm due to the recoil energy (85 keV / atom). 220 Rn was formed by the spontaneous alpha decay of 224Ra.
Some of the 220Rn formed by α-decay of 224Ra is directly released from the sample by recoil. Other 220Rn is trapped in lattice defects, such as vacancy clusters, grain boundaries and pores, and can be released from the sample by thermal diffusion. Thus the defects in the
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