The effectiveness of the subseabed disposal option for heat-generating wastes depends in part on the integrity of the barrier provided by the sediment, once the near-field barrier has been breached. This chapter describes a study of diffusive migration through three types of deformation which may be present in the sediment and which may provide pathways for the migration of activity to the sediment surface. These three potential routes are

  1. the projectile pathway (in the case of penetrator disposal)

  2. hydraulic fractures which may result from the differential expansion of sediment and pore fluid during radiogenic heating, and

  3. the naturally occurring faults, which have been found in some areas of Atlantic abyssal Plains

The investigation was divided into three experimental areas

  1. the determination of diffusion coefficients for the migration of a conservative ionic species through sediment samples containing the deformations and held under the effective stress experienced at 30 m beneath the seabed,

  2. autoradiography of similar, but unstressed samples using both conservative and non-conservative radioactive species; And

  3. scanning electron microscopy of projectile pathways produced during accelerated physical modelling of the emplacement technique.

The techniques used in the first two experimental areas are described briefly and the implications of the results are discussed in terms of predicted breakthrough times at the sediment surface

INTRODUCTION

The aim of this study was to investigate the effect on diffusion of the pathway left by the projectile and any other deformations induced by the emplacement of a hot projectile within the seabed sediment. It was supposed that nuclear waste containers would be breached after about 103 years and that by the time any rupture of the canister occurred there would be no gradient of pressure or temperature within the near field.

The experimental work consisted of three sections

  1. Autoradiography at atmospheric pressure of sediment samples which contained the possible preferred migration pathways, that is model penetrator pathways, hydraulic fractures simulating those cracks which might be produced by the expansion of pore water around a hot penetrator, or shear planes emulating the faults which have been revealed during seismic surveying of some areas of the abyssal seabed

  2. Experiments to measure the rate of diffusion of sodium iodide through intact sediment samples under the effective stress encountered 30 m below the seabed and to compare these with rates of diffusion through samples containing possible preferred pathways

  3. Scanning electron microscopy of the pathways produced during geotechnical centrifuge tests at 100 g by model penetrators fired into a tub of sediment experiencing the same stress conditions as subseabed sediments (Pooroshasb, 1987) This was an attempt to provide a measure of the size of the disturbance around a penetrator pathway with regard to orientation. The details of this work can be found in Gronow (1986) and are not given here

Keywords:
coeffiaent, projectile pathway, sediment, migration pathway, kaolin, concentration, fracture, diffusion, pathway, apparent diffusion coeffiaent
Subjects:
Reservoir Characterization, Formation Evaluation & Management
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1989. The Society for Underwater Technology
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