Research Into Effects of Repository Heterogeneity
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243 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
NAMMU Calculations Calculations can be performed to examine the role of waste packaging as a barrier to radionuclide release. A model has been constructed to represent a stack of seven identical cylindrical waste packages (i.e. drums) within a repository vault. The arrangement of drums in this waste stack is based on the specification for the disposal of 500 litre drums in stillages within the repository vaults as for the Nirex 97 assessment [2]. The model was two-dimensional, with all waste stacks assumed identical. Angular variations in the properties of the system relative to an axis running vertically through the centre of the waste drums were ignored. It was also assumed that the direction of groundwater flow in the vicinity of the waste drums was vertically upwards based on the results of hydrogeological modelling as reported in Reference [2]. A schematic of the model is provided in Figure 1(a). The model also considered the case that the containers had corroded, such that it is possible for groundwater flow to have access to the drum contents.
(a
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grout
(b)
backfill surrounds packages
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vent
Figure 1. (a) Schematic of NAMMU model of part of waste stack (intact containers). (b) Schematic of radionuclide migration and water flow simulated using NAMMU. Broken arrows indicate path of possible radionuclide diffusion. Solid arrows represent direction and position of significant groundwater flow. Dots represent boundary of significant flow. Simulations of radionuclide migration for this model were conducted using the NAMMU computer program [5]. NAMMU (Numerical Assessment Method for Migration Underground) is a software package for modelling groundwater flow and transport processes in porous media. It applies a finite-element method to solve the governing equations for radionuclide migration. In the present context it was applied to calculate the flux of a radionuclide from the top of a waste stack. An appropriate groundwater volume flux through the repository was selected based on the Nirex 97 assessment [2]. Also: * for simplicity the radionuclide was taken to have a sufficiently long half-life that radioactive decay need not be considered in the calculations. It was also assumed to be non-sorbing and that the aqueous concentration was unaffected by solubility limitation; * the concentration of the radionuclide in the system was expressed as a fraction of the initial concentration within the waste packages (hence calculated fluxes were in units of yf' rather than moles yrt'). All of the waste packages were assigned the same initial inventory;
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the backfill and in-drum regions were assigned appropriate transport properties [4] but these were modified in a simple fashion to take account of the possibility of cracking. Table I provides a summary of these data; changes in the properties of the radionuclide, containers, backfill and waste encapsulant (e.g. through continued exposure to the groundwater) were not simulated; c
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