Design bases of the Belgian repository for vitrified heat producing radioactive wastes

On the basis of experimental data gathered during the first years of the research programme centred on the Boom clay at Mol, a preliminary safety analysis of a repository was carried out as part of the Performance Assessment of Geological Isolation Systems (PAGIS) project. The conclusions were as follows: ''the calculated maximum radionuclide concentrations ... are very sensitive to three parameters related to the clay layer: the effective thickness; the diffusion coefficient; the retardation factor.'' On the other hand, ''the very efficient confinement (... provided by the clay layer ...) masks the contributions of the engineered barriers''. As a consequence, the design of the repository is being based on the following two main principles, also stressed in PAGIS: ''the mined repository should be kept as flat as possible to avoid that a considerable fraction of the thickness of the clay layer should be disturbed by the repository construction; the strong influence of the clay layer emphasizes the importance of the limitation of near field effects which could damage the first meters of the clay layer around the repository galleries.'' These conclusions and principles have been translated into a set of design criteria. (author). 1 ref., 4 figs