Chemical Thermodynamics of Thorium, Volume 11
This volume is the eleventh in the OECD Nuclear Energy Agency (NEA) “Chemical Thermodynamics” series. It is based on a critical review of the thermodynamic properties of thorium, its solid compounds and aqueous complexes, initiated as part of the NEA Thermochemical Database Project Phase III (TDB III). The database system developed at the OECD/NEA Data Bank ensures consistency not only within the recommended data sets of thorium, but also amongst all the data sets published in the series. This volume will be of particular interest to scientists carrying out performance assessments of deep geological disposal sites for radioactive waste.
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Thorium elemental
Nuclear Energy Agency
Thorium metal has a face centred cubic structure, (space group Fm3m , Cu type) at 298.15 K and undergoes a structural change to a bcc-phase (space group Im3m , W??type) at ca. 1630 K. The lattice parameters are 5.0842 Å at 298.15 K [1956JAM/STR] and 4.11 Å at 1723 K [1954CHI]. Rand [1975RAN] critically examined the data of 15 studies of this transformation with the reported transition temperatures ranging from 1605 to 1698 K, with most values lying between 1623 and 1653 K. The probable reason for this seemingly wide range of values is the effect of impurities in the thorium used. It is clear that small amounts of carbon, nitrogen and oxygen, which are difficult to remove from thorium, raise the transformation temperature, whereas additions of niobium, tantalum and zirconium tend to lower the temperature of the phase change. We have therefore given considerable weight to the work of Chiotti and Dooley [1967CHI/DOO] ((1623??????10) K) and Takeuchi et al. [1966TAK/HON] ((1633??????10) K) who both extrapolated the transition temperature to zero carbon content and select for the transformation temperature
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