Geological disposal of radioactive waste in deep clay formations : 40 years of RD&D in the Belgian URL HADES The option of disposing of radioactive waste deep underground has been studied in Belgium by SCK CEN since the 1970s. This led in 1980 to the construction of the HADES underground research laboratory (URL) in a clay formation the Boom Clay at a depth of 225 m under the premises of SCK CEN in Mol. Over the last four decades many in situ experiments have been conducted in the HADES URL. These have made a significant contribution to ONDRAF NIRAS research development and demonstration (RD&D) efforts demonstrating that disposal in Boom Clay can offer a safe solution for the long term management of high level and or long lived radioactive waste. Moreover the construction of the HADES URL itself is a demonstration that shafts and galleries can be constructed in clay at that depth. However the HADES URL did not only contribute to the Belgian programme. Many of the in situ experiments have been part of international research and the laboratory has provided valuable input to the research programmes of other URLs such as the Meuse Haute Marne URL in France and the Mont Terri rock laboratory in Switzerland. This paper gives a brief overview of the main contributions of the HADES URL to both national and international research into geological disposal Abstract