Technical Report NTB 85-26

Modelling of Solute Transport in the Near Field of a High Level Waste Repository

The effects on solute transport of the bentonite backfill in the storage tunnel of an underground repository for high-level radioactive waste have been studied by means of mathematical models.


The study concerns a hypothetical site, whose characteristics, however, with respect to geology, hydrology and geochemistry, have been derived from field observations. The models used to simulate migration processes are described, together with justifications for assumptions and with verification of methods.


The results, presented as time histories of release fluxes up to at least one million years, demonstrate the delaying effect of the bentonite on both solubility-limited and highly soluble nuclides. The time required for the mass transport leads to the effective elimination of many nuclides with shorter half-lives before any significant release to the geosphere.


Calculations have also been made to establish a conservative estimate of the rate of supply of corrosive sulphide ions to the metallic waste containers. Results and consequences of these computations are also discussed.