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Chemical Thermodynamics of Thorium

image of Chemical Thermodynamics of Thorium

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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Miscellaneous compounds and complexes

Nuclear Energy Agency

The early identification of SrThO3(cr) by Naray-Szabo [1947NAR] has been questioned (see discussion below), but Purohit et al. [2000PUR/TYA] have suggested that it is formed by combustion of a sol-gel containing Th(NO3)4, Sr(NO3) and citric acid. They indicate that its structure is a monoclinic distortion of the ideal perovskite lattice, with a??= (6.319??????0.004) Å, b = (3.240??????0.001) Å and c = (4.928??????0.003) Å, ?? ??= (117.38??????0.05)°. The only thermodynamic data for SrThO3(cr) are measurements by Ali?? (Basu) et al. [2001ALI/MIS] of the pressure of Sr(g) obtained by heating SrThO3(cr) in a tungsten cell from 1677 to 2419 K by weight-loss Knudsen effusion. X-ray examination of the residue after partial decomposition showed that the condensed phases were different in different temperature ranges. As well as SrThO3(cr), W(cr) and ThO2(cr), the tungstate Sr2WO5(cr) was present from 1677 to 2047 K, but SrWO4(cr) was found in experiments from 2138 to 2419 K. Since SrWO4 would be molten at these higher temperatures, Energy Dispersive X-ray analysis was carried out to determine whether there was any solubility of thorium species in this melt. Less than 10% ThO2 was detected, but it was not clear whether this was due to dissolution of e.g. of SrThO3 or from interference from neighbouring ThO2 particles. Thus SrWO4(l) was treated as a pure phase in the subsequent analysis, as was ThO2(cr), although the solubility of SrO in ThO2(cr) at 2273 K is reported to be as high as 13 mol%. The error from these assumptions was estimated by the authors to be 2.6 kJ·mol–1. The authors discuss and dismiss the possible loss of oxygen to form phases such SrWO3(cr) containing lowervalent tungsten 

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