These conference proceedings examine gas generation, accumulation and migration in underground repository systems for radioactive waste: safety-relevant issues.

Understanding the behaviour of gases in the context of radioactive waste disposal is a fundamental requirement in developing a safety case for the disposal of radioactive waste. Of particular importance are the long-term performance of bentonite buffers and cement-based backfill materials that may be used to encapsulate and surround the waste in a repository, and the behaviour of plastic clays, indurated mudrocks and crystalline formations that may be the host rocks for a repository. The EC Euratom programme funded project, FORGE, has provided new insights into the processes and mechanisms governing gas generation and migration with the aim of reducing uncertainty. This volume brings together papers on aspects of this topic arising from both the FORGE project and work undertaken elsewhere. This has been achieved by the acquisition of new experimental data coupled with modelling, through a series of laboratory and field-scale experiments performed at a number of underground research laboratories throughout Europe.

Purpose ofthe Workshop In the spirit of enhancing developments in science and technology by facilitating international scientific cooperation, the Science Committee of NATO is sponsoring AR W's in several selected priority areas. The objective of this workshop was to discuss what microbial mediated problems have been experienced in the area of nuclear waste management and spent fuel storage. Long term storage of high-level wastes in repositories is just starting in some countries. However, low and medium level wastes have been stored for several decades. In the area of spent fuel interim, storage has been extended at many locations far beyond the intended time. It was a priority of the workshop to examine and discuss what deleterious effects have been observed under these storage conditions or under conditions used in simulated trial tests for predicting material performance under the storage conditions. For example, one chronic problem that was discussed was possibility that microbial influenced corrosion (MIC) could be taking place in the wet storage of spent fuel thereby initiating or accelerating the process of corrosion. Another discussion in the area of waste forms, focused on the presence ofbiofilms which may be breaking down the structure of the waste form and thereby jeopardizing its integrity. The meeting focused on discussing the observations and data collected relating to problems encountered in the storage of these types of wastes, and sharing this information with others that have not monitored their facilities for similar problems.

Many countries are currently exploring the option to dispose of highly radioactive solid wastes deep underground in purpose built, engineered repositories. A number of surface and shallow repositories for less radioactive wastes are already in operation. One of the challenges facing the nuclear industry is to demonstrate confidently that a repository will contain wastes for so long that any releases that might take place in the future will pose no significant health or environmental risk. One method for building confidence in the long-term future safety of a repository is to look at the physical and chemical processes which operate in natural and archaeological systems, and to draw appropriate parallels with the repository. For example, to understand why some uranium orebodies have remained isolated underground for billions of years. Such studies are called 'natural analogues'. This book investigates the concept of geological disposal and examines the wide range of natural analogues which have been studied. Lessons learnt from studies of archaeological and natural systems can be used to improve our capabilities for assessing the future safety of a radioactive waste repository.