[en] The objective of this report is to review and discuss the scientific and technical knowledge which supports the development of geological disposal systems. It is expected that the report will provide Member States with an impartial view on the scientific basis and rationale of the geological disposal concept and the mechanisms to realize disposal and assess its safety. It should also provide some indication of the R and D requirements regarding some of the open issues mentioned in Section 1.1. A large part of existing knowledge is generic in nature and is derived from the earth sciences and from underground engineering work. However, much specific knowledge has been derived from the characterization of potential repository sites, from studies in underground research laboratories and from the operation of underground repositories for the disposal of various types of waste, including hazardous wastes and low and intermediate level radioactive wastes (LILWs). Examples of LILW disposal facilities include those at Asse and Morsleben in Germany. Relevant knowledge has also been derived from natural analogue studies, from deep excavations and from hydrogeological and geochemical investigations carried out for a variety of purposes. This report focuses on the identification of, and practical means to obtain and use, the necessary information at each stage of the development of a geological repository for solid radioactive wastes. The emplacement of the wastes can be carried out in different ways and various repository designs are possible. The different types of geological environment that have been considered for the disposal of radioactive wastes can contribute in different ways to the overall objective of ensuring containment of the radionuclides for the necessary period of time. Geological repositories have the greatest potential for ensuring the highest level of waste isolation, and are considered applicable to the disposal of the most demanding categories of radioactive waste, including high level waste, spent nuclear fuel and other long lived radioactive wastes. Building, operating and closing a geological repository for long lived radioactive wastes, including closure of all underground excavations, require that considerable technical and scientific information be used in every aspect of the conceptual approach, design, engineering and safety assessment of such a facility. Some Member States have made the decision that practically all radioactive waste containing non-negligible quantities of radionuclides, regardless of their half-lives, should be placed in geological repositories. Such repositories specifically designed for the disposal of LILW, for example the proposed Konrad facility in Germany, are also within the scope of this report. Historic programmes in some countries, notably the former Soviet Union and the United States of America (USA), have involved the injection of liquid wastes or sludge into deep geological formations. These practices are not dealt with in this report. The status of the scientific understanding and adequacy of knowledge in regard to the disposal of radioactive wastes in geological repositories is described in four sections. The second section deals with the geological disposal concept and the different functions that a repository is expected to assume in different periods of its life cycle. The third and fourth sections describe the processes relevant to the containment of the radionuclides in the repository and other processes which might affect the long term integrity of the different barriers. They also address the site or concept specific aspects of several important issues and indicate the areas which are likely to require further work within particular geological disposal programmes. The fifth section highlights the central role of the safety case and discusses the use of safety/performance assessments (SA/PAs) in the decision making process during repository development. This section emphasizes the need to build a consensus on the conclusions of the safety case. The final section includes short statements and a discussion of important general issues. These are put in the perspective of what has been learned, and what scientific and technical knowledge would need to be acquired at a proposed site and for a specific disposal concept in order to achieve a reasonable assurance of safety