Ceramic Immobilisation Options for Technetium
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Ceramic Immobilisation Options for Technetium Martin C. Stennett*, Daniel J. Backhouse, Colin L. Freeman and Neil C. Hyatt. Department of Materials Science and Engineering, The University of Sheffield, Sheffield, S13JD, United Kingdom. ABSTRACT Technetium-99 (99Tc) is a fission product produced during the burning of nuclear fuel and is particularly hazardous due to its long half life (210000 years), relatively high content in nuclear fuel (approx. 1 kg per ton of SNF), low sorption, and high mobility in aerobic environments. During spent nuclear fuel (SNF) reprocessing Tc is released either as a separate fraction or in complexes with actinides and zirconium. Although Tc has historically been discharged into the marine environment more stringent regulations mean that the preferred long term option is to immobilise Tc in a highly stable and durable matrix. This study investigated the feasibility of incorporating of Mo (as a Tc analogue) in a crystalline host matrix, synthesis by solid state synthesis under different atmospheres. Samples have been characterised with X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray absorption spectroscopy (XAS). INTRODUCTION One of the main challenges facing the nuclear industry is the issue of waste management. All waste from the nuclear fuel cycle must be rendered passively safe so that it can either be placed in interim storage or sent for disposal. This is very challenging for some radionuclides, such as 99Tc. 99Tc is produced during nuclear fission of 235U, with a yield of 6.06 %, and during fission of 239Pu, with a yield of 5.9% [1]. This means that for every tonne of enriched 235U that is used in a nuclear reactor, approximately 1 kg of 99Tc is produced [2]. There are two main reasons for the difficulty in dealing with 99Tc; firstly, it has a long half-life of approximately 210000 years, and secondly, it is present in spent fuel rods as the technetium (VII) oxide, Tc2O7. Tc2O7 is water-soluble and forms the anionic pertechnetate species (TcO4-) in solution which is extremely mobile in the environment. An obvious end-route for 99Tc is for it to be immobilised or encapsulated in a vitreous or ceramic host matrix. There are however difficulties in accomplishing this due to the high volatility of technetium species, particularly Tc2O7. One possible solution to this issue is immobilising the 99Tc in its Tc4+ oxidation state. Technetium (IV) oxide is much less volatile than Tc2O7, and has a sublimation temperature of approximately 900°C, in contrast to the melting and boiling points of Tc2O7, which are 119.5°C and 311°C, respectively [3]. Tc4+ can potentially be immobilised by incorporation into the structure of a durable ceramic phase which has suitable sized crystallographic sites. In this work Gd2Ti2O7, which crystallises with the pyrochlore structure was explored as a suitable host for Tc [4]. The pyrochlore structure has an ideal formula A2B2O7 and adopts cubic symmetry, with space group Fd3-m. The A-site can be occupied by tri- and tetravalent lanthanides and
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