Evaluation of Solubility and Speciation of Actinides in Natural Groundwaters
- PDF / 418,357 Bytes
- 10 Pages / 420.48 x 639 pts Page_size
- 71 Downloads / 187 Views
ALUATION OF SOLUBILITY AND SPECIATION OF ACTINIDES IN NATURAL GROUNDWATERS
Marc R. Schweingruber Swiss Federal Institute for Reactor Research, CH-5303.W~renlingen,
Switzerland
INTRODUCTION Groundwater circulation is in the long term probably the most important mechanism for the transport of radionuclides from deep underground repositories to the biosphere. The fate of leached long-lived nuclides, such as the actinides, is of particular interest to
repository safety analysis. Both experi-
mental and theoretical studies should lead to a comprehensive description of their behaviour in the geosphere. First quantitative prognoses on concentrations of such elements in aqueous solution and the pattern of their complexation may be obtained by a theoretical speciation analysis based on the premises of chemical equilibrium. Results of this kind can form a basis to justify boundary conditions and parameter choices in geospheric transport models. The application of speciation models to nuclide 1-5
migration scenarios is worldwide in progress
This paper summarizes first modeling efforts undertaken to evaluate the speciation and solubility of U, Th, Np and Pu in two selected Swiss groundwaters. Starting from a rather general box model,
two simple case studies will
be defined. The first could be visualized as pertaining to the far-field, the second to the near-field of a repository. For both cases the required model parameters are specified. Some emphasis is laid upon the effect of kinetically hindered solid phase precipitation.
MODEL As a simplification, the geosphere can be divided into a series of boxes with characteristic, uniform internal properties. The boxes' dimensions will, of course, strongly depend both on the processes to be modelled and on the scale of the whole system under consideration. Figure 1 depicts the structure of such a box chosen to model actinide speciation and solubility in groundwater. A well-mixed, mobile aqueous phase contacts a set of non-aqueous phases, which
680
may interact with the solution. If
the system is
not in
overall chemical equi-
librium, some reactions might change the composition and/or the speciation of the aqueous phase. Asymptotically,
partial equilibria will be reached by rever-
sible reactions which are fast in comparison to water residence time.
The
leaching of an actinide source embedded in the box ( e.g. uranium ore body, clearly induced by a non-equilibrium situation and may
waste repository ) is
lead to partial or complete dissolution of phases initially present as well as to the formation of secondary solids. In principle, the following parameters must be defined to model the consequences of actinide leaching with this approach:
(1) temperature and pressure;
(2) (4)
chemical composition and masses of non-aqueous phases ( with respect to
solution volume );
(5)
(3)
elemental composition of the inflowing solution;
water residence time;
thermodynamic and kinetic characterization of both the
formation of dissolved species and the precipitation/dissol
Data Loading...
