Radionuclide Migration Studies on Tonalite
- PDF / 1,088,547 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 79 Downloads / 172 Views
RADIONUCLIDE MIGRATION STUDIES ON TONALITE.
P. HOLTTA*, M. SIITARI-KAUPPI*, M. HAKANEN* and A. HAUTOJARVI** * University of Helsinki, Department of Radiochemistry, **
Unioninkatu 35, SF-00170 Helsinki, Finland Technical Research Centre of Finland, Nuclear Engineering Laboratory, P.O. Box 208, SF-02151 Espoo, Finland.
ABSTRACT Migration of water, chloride, sodium, and calcium in tonalite was studied, using dynamic column and static through-diffusion methods. Autoradiography of rocks impregnated with 14 "C-methylmethacrylate was introduced in order to determine the spatial porosity distribution, as well as to identify and visualize the migration pathways of non-sorbing radionuclides in tonalite matrix at the mm-cm scale. The migration routes of sorbing radionuclides and the 45 sorptive minerals in tonalite were determined by autoradiographic methods, using Ca as a tracer. Transport of radionuclides was interpreted, using models for hydrodynamic dispersion with diffusion into the rock matrix. In tonalite, porous minerals were distributed homogeneously in matrix and, therefore, retardation capacity of the rock matrix was found to be high.
INTRODUCTION In the Finnish nuclear waste disposal concept, the geosphere, represents a natural barrier, isolating radionuclides from the biosphere. Radionuclides, released from underground repositories in crystalline rock, can reach the biosphere only with groundwater flow in the fractures. To assess the safety of these repositories, the retardation characteristics of the bedrock need to be known. The migration of sorbing radionuclides is delayed both by interaction with the fracture surfaces and fracture filling materials, and by diffusion into the microfissures of the rock. Non-sorbing radionuclides are dispersed and retarded significantly only by dilution, due to diffusion into the rock matrix. In order to predict the radionuclide transport in rock fractures and matrix, it is necessary to know the different effects of physical phenomena on the transport of solutes, as well as the properties of rocks. Migration of radionuclides in single rock fractures has been studied in a number of laboratory experiments, e.g. see KBS Report 84-01 and AECL Report TR-449 [1,2]. Elution curves have been interpreted, using models in which radionuclides are transported by advection-dispersion, a matrix diffusion included [3]. Residence-time distribution curves have also been explained by the channelling of water flow [4]. Hydrodynamic dispersion, caused by a velocity field and transverse molecular diffusion, has been demonstrated to dominate the transport behavior of non-sorbing radionuclides in short columns with wide fractures. The effects of matrix diffusion were seen convincingly in the experimental elution curves only in a very porous column material. In dense rock matrix, diffusion effects would dominate only at very low-flow velocities (< 10' ms'). [5] In this work, transport of non-sorbing and sorbing radionuclides was studied, using tonalite fracture columns with intact and altered surfa
Data Loading...
