The free-fall penetrator is one of the conceptual disposal techniques considered by the Seabed Working Group for the disposal of heat-generating radioactive waste in the deep ocean. However, one aspect of penetrator behaviour that remains unresolved is the degree to which the sediment barrier is disturbed by the penetration event The HOCUS experiments were therefore devised to measure the properties of the sediment filling the entry pathway of the penetrator. This paper describes the experiments which were performed m the Mediterranean in a water depth of approximately 250 m and used a sea floor platform to probe and core the entry pathways of a number of penetrators.

INTRODUCTION

Emplacement of solidified radioactive waste in deep ocean sediments is being considered in several countries as a promising alternative to land-based geologic disposal.

An important issue in assessing the safety of this option is the effectiveness of the sediments to provide a barrier to the migration of radionuclides from the emplaced wastes. In other words it is necessary to determine not only the ACIS Analyse de la Cicatrisation in situ, BRE Building Research Establishment, CEA Commissariat a ?1'Energie Atomique; CEC/JFE Commission of the European Communities/Joint Research Centre, DOE Department of the Environment(UK), DSML Delft Soil Mechanics Laboratory, ENEA. Comitato Nazionale per la ricerca e lo sviluppo della Energia Nucleare e delle Energie. Alternative, ESTG Engineering Studies Task Group, GME Great Meteor East, HOCUS Hole Closure ISMES Istituto Sperimentale Modelli e Strutture, JAERI Japan Atomic Energy Research Institute, NEA Nuclear Energy Agency, SNAP Southern Nares Abyssal Plain, SNL Sandia National Laboratories, SWG, Seabed Working Group properties of the intact sediments but also the changes that the disposal operations may induce

The emplacement option currently receiving the greatest attention from the Seabed Working Group of the Nuclear Energy Agency is the so-called ‘penetrator option’, which would consist of emplacing the waste in the sediments by means of free-falling vehicles or penetrators. It is obvious that the safety of this concept depends to a large extent on the properties of the sediments filling the entry paths of the penetrators.

One aspect of penetrator option that remains unresolved is the degree to which the ‘sediment barrier’ is disturbed by the penetration event Theoretical analyses suggest that an open pathway to the sediment surface is unlikely, because the suction forces generated by the water flowing into the hole would cause the sediments to close behind the penetrator However, the effect of this remoulding process on the properties of the sediments filling the entry pathway can only be assessed experimentally. Consequently, in order to investigate the properties of the sediments disturbed by the penetrators, the ESTG, which is one of the Task Groups of the SWG, organized a series of tests at sea.

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