Abstract
This thesis examines the practical engineering problems associated with high-activity radioactive waste disposal. It includes a critical review of all the available disposal options, but focuses on concepts involving deep underground burial in purpose-built repositories. The construction and waste containment properties of crystalline, argillaceous and saliferous host rocks are examined and some inherent uncertainties in predicting their performance as natural barriers to radionuclide migration are described. It is shown that repository construction can introduce anomalous migration paths via peripheral zones of disturbance, rock/backfill separation planes and structural linings unless special preventative measures are taken. For jointed rock masses, it is shown that repository excavations may short-circuit natural flow paths. Repository design proposals from the major nuclear power-producing countries are reviewed, with particular emphasis on crystalline rock repository systems. It is shown that although the international proposals demonstrate the broad feasibility of repository construction, they generally fail to achieve optimum solutions in terms of waste containment. For jointed rocks, an optimisation study is described which demonstrates how adjustment of pre-disposal strategies for conditioning high-level wastes, altering the construction sequence, and manipulating the emplacement configuration can effect reductions in repository size to mitigate against the adverse effects of discontinuities. It is shown that applying the minimum repository size philosophy to other host rocks can also produce benefits in terms of waste containment and construction costs, and some radical design alternatives are proposed incorporating relevant principles. The role of engineered barriers, in the form of high-integrity backfills and waste unit claddings, is examined. Potentially suitable backfills are identified by reference to geochemical stability and radionuclide containment properties, and cost implications are evaluated. Parametric sensitivity studies are performed to demonstrate the influence of waste unit and backfill properties on radionuclide containment, and some new practical design concepts and emplacement techniques are suggested.