ABSTRACT

Abstract. A novel principle for a waste repository, based on a backfilled thermosyphon as the functional unit, is proposed. Such a unit reduces temperatures and temperature gradients in the vicinity of a waste package and in the repository near field, reducing leach rates from packages and thermomechanical disturbance of the rock mass. Preliminary analyses confirm the mode of performance of the system, and reveal some interesting features of material transport and sealing in backfill and rock fissures. The thermosyphon combines attractively with the prospect of developing a near-field geochemical trap with chemical specificity for radionuclides. It is proposed that a thermosyphon repository may provide superior performance of engineered and natural barriers compared with conventional designs. The achievement of the ability to predict accurately the performance of the thermosyphon, relative to other isolation schemes, will represent a benchmark in the understanding of the absolute performance of any repository.

Introduction

The heat emitted by high level nuclear waste, and the inevitable entry of groundwater to the immediate vicinity of waste packages, are key issues in the ultimate isolation of immobilized nuclear wastes in a deep subsurface repository. The scope of the problem and some current conceptual designs for repositories have been reviewed by St. John (1982). After a repository has been closed and fills with groundwater, fluid transport may occur toward the ground surface, due to the buoyancy forces caused by heating. Entrainment of radioactive waste in convective groundwater flow is the most credible mechanism for waste transport to and dispersion in the biosphere (Pigford, 1984). The rationale of waste isolation schemes, and associated field studies to examine component issues in the performance of a geologic repository, re described by Witherspoon, Cook and Gale (1981). A comprehensive review of the status of investigations into the many aspects of the coupled mechanical-thermal-hydrological-geochemical problem posed by deep sub-surface isolation of high level waste was conducted by de Marsily (1985) and other authors at the same meeting.

A repository, the engineered barriers and the natural barriers must perform satisfactorily over a geologic time scale. Thus, in an Australian research programme on waste isolation, the key scientific principle being applied is that the engineered system should emulate processes known to occur in natural geologic systems. Particular attention is given to the roles of water as a thermomechanical and geochemical agent in the performance of the engineered and natural components of the containment system. A logical corollary to this philosophy is that only naturally occurring materials, demonstrably stable on a geologic time scale, can be used in construct- ion of waste packages, engineered barriers and repository sealing.

Novel Operating Principles for a Repository In current conceptual designs for a repository, the operating principle is that, should radio- nuclides be released into the groundwater, the time for waste transport from the repository to the surface is sufficient to provide decay and dilution to acceptable levels of radiation in the biosphere. An example of conventional repository design is illustrated in Figure 1. Determination of the acceptability of a particular isolation design therefore depends on analysis of the performance of the various elements of a set of redundant barriers.

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