[en] Since the end of the 1970s, about 3000 m³ of Eurobitum bituminised radioactive waste has been produced by the EUROCHEMIC/BELGOPROCESS reprocessing facility for the incorporation of precipitation sludges and evaporator concentrates originating from the chemical reprocessing of spent nuclear fuel. Eurobitum is a homogeneous mixture of 60 weigh percent of bitumen and 40 weight percent of waste, of which NaNO3 is the most important component (60-75 weight percent ). The preferred option of ONDRAF/NIRAS for the long-term management of Eurobitum is final disposal in a geologically stable underground clay formation. The Boom Clay is presently being studied as a reference host formation. Due to a combination of favourable properties, the Boom Clay will delay and spread in time the migration of the radionuclides, allowing the majority to decay before reaching the aquifers. Owing to the importance of the clay host formation in the overall repository safety, the processes induced by the emplacement of Eurobitum should not negatively affect the long-term safety functions of this barrier. Basically, two types of disturbances can be distinguished: (1) a mechanical disturbance, caused by the build-up of a pressure in and around the waste, and (2) a chemical disturbance by the release of large amounts of NaNO3 and of water-soluble, organic, potentially complexing molecules due to radiolytic and chemical degradation of the bitumen. The extent of some of these processes will be affected by the continuous evolution of the rheological properties of the bitumen, especially in the presence of oxygen and/or radiation. This ageing results in a harder bitumen, which tends to loose its binding capacity and which becomes increasingly brittle. Fissures and changes in membrane properties of bitumen in terms of oxygen and water diffusion are expected to favour the penetration of oxygen, thus amplifying the ageing deeper inside the product, and the infiltration of pore water, thus possibly affecting the swelling and the leaching of NaNO₃ and radionuclides. The objective of this work is to obtain a semi-quantitative understanding of the ageing of Eurobitum radioactive waste, to be able (1) to estimate the degree of ageing at the time Eurobitum will come in contact with ground water, and (2) to produce, by accelerated ageing techniques, non-radioactive Eurobitum samples that can be used in the studies on water uptake, swelling, and salt leaching