The Chemistry of Plutonium Solubility

Doctoral thesis, 2004

When designing a repository for spent nuclear fuel it is necessary to assess the possibility that radionuclides will be transported from the repository by groundwater. A limiting factor for such transport is the solubility of any given radionuclide. Plutonium is present in relatively high amounts in spent nuclear fuel and this thesis discusses the basic principles for plutonium solubility, starting with thermodynamic properties such as hydrolysis, redox equilibria and activity coefficients. The initial theoretical discussion touches upon other chemical properties as well, including colloid formation, sorption and radiolysis effects. The experimental work leading to this thesis was aimed at carrying out solubility experiments with plutonium. Concentration measurements where made using both radiometrical (LSC and Alpha-spectrometry) and non-radiometrical (ICP-MS) methods. Much work has been done on the production and speciation of different oxidation states of plutonium using electrochemistry or chemical separation methods. Even though the aim has been to study plutonium solubility, the results and methods developed or implemented are useful in other chemical investigations of actinides. Finally, the theoretical framework was used to model experimental data from the literature and from some of the attached papers. The model is satisfactory in some cases, but there are also discrepancies, under certain circumstances, between the measured redox potential and the oxidation state speciation in solution. This leads to a suggestion that the plutonium solubility is controlled by a sorption mechanism. It is also shown that Pu(OH)3 solubility data can be modelled using the solubility product of Pu(OH)4 and applying adequate redox potentials to the thermodynamic model.