Ground water flow through an underground repository for nuclear waste located 500 m below the ground surface as proposed by Swedish authorities may be considerably reduced by surrounding the repository with a zone of constant potential like Faraday's cage in electricity. The zone consists of tunnels and boreholes between them.

INTRODUCTION

It is proposed that nuclear waste from Swedish power plants be permanently stored in an underground repository. This should consist of a system of storage tunnels located 500 metres below the ground surface; and the nuclear waste should be enclosed in special canisters surrounded by compacted bentonite. The canisters are calculated to last for many thousands of years. However, if the canisters should be destroyed, radioactive material would be transported with groundwater to the biosphere in wells, rivers and lakes, or the sea. Therefore, the requirement is that the rock as the last barrier should have such very low permeability that the movement of the groundwater should take many hundreds or thousands of years from the storage to the biosphere.

The Swedish Government has approved this project and accepted the judgement by the State Nuclear Power Safety Board that acceptable rock for a storage of nuclear waste is available. However, many people have protested against this decision, and different opinions exist about rock quality and the long-term effect of the storage.

HYDRAULIC CAGE

In primary rock such as granite etc. there exist small micro-cracks along mineral particles. Therefore, it is true that groundwater flow will occur in such rock but only to a very small extent. The permeability of such as homogeneous considered rock is very low, in fact so low that the rock can be considered to be impervious, and that the ground water flows in open cracks only. If the cracks are distributed evenly in different directions and the distance between the cracks is not too great, we may calculate the groundwater flow by using Darcy''s formula:

• q = k.S m3/s per m2 area (1)

where k = coefficient of permeability, m/s
• S = hydraulic gradient, m/m

The velocity of the water in the system of cracks is

• v = q/n = 1/n. k. s m/s (2)

where n = effective porosity.

The effective porosity is not equal to the total porosity of the rock. The porosity of. the "homogeneous" rock should not be included, and neither cracks running laterally to the direction of the groundwater flow. The porosity, n, is difficult to determine at the site, but may be assumed to be between 0.001 and 0.0001 for very good hard rock.

The formulae show that the intensity of the groundwater flow is proportional to the hydraulic gradient. A method to reduce the groundwater flow and its velocity and thereby to reduce the requirement regarding the permeability of the rock is to reduce the hydraulic graient around the repository.

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