Technical Report NTB 90-48

A source term model STRENG has been devised to analyse the diffusive release of radionuclides from vitrified high level waste in a repository with cylindrical geometry. The model includes the dissolution of the glass waste matrix, the diffusive transport of the radionuclides through a bentonite clay buffer, the effects of solubility limits, as well as the radioactive decay and ingrowth.

The model requires as input the following parameters:

• Radioactive decay constants and decay chain structure
• Initial inventories of the radionuclides in the repository
• Canister failure time
• The elemental solubility limits for the waste
• Details of the physical properties of the repository
• Details of the physical and chemical properties of the glass matrix
• The retardation factors of the elements in the bentonite buffer
• A list of times at which the output is required

From the above data the model produces as output:

• The flux of radionuclides into the host rock
• The inventory of the radionuclides remaining in the glass matrix
• The rate at which radionuclides are released from the glass matrix 
• The inventory of precipitated species in the pore-water when the glass dissolves

STRENG utilises user-friendly menu screens for the pre- and post-processing of the data used. Within this framework it provides a quick view graphical output of the flux to the geosphere together with the facility to store these results in a form more suited to a professional graphics package. In order to form part of a larger modelling effort, STRENG produces output in a form desired by Nagra for their subsequent groundwater flow/transport codes for modelling the geosphere transport.