Conceptual design of geological repositories for high level wastes

The various types of geological formation in Japan can be roughly divided into two groups: massive, fractured, hard rocks and soft to medium hard, multilayered, homogeneous rocks. The former are represented by granitic rocks and the latter include Tertiary mudstone, shale and sandstone. In designing geological repositories numerous points must be studied, such as mechanical stability of tunnels in deep formations; seismic stability, which is a prerequisite for nuclear related facilities; the effect of thermal discharge from nuclear wastes; and public safety from radioactive substances dissolved in groundwater. The paper discusses the design of geological repositories in terms of their operational functions by assuming the unit capacity of a system to be 10,000 waste forms. On the basis of a general assessment of mechanical and seismic stabilities, fundamental approaches and procedures in designing a system to overcome thermal problems are proposed. Numerical analyses revealed that under the waste disposal programmes presently contemplated in Japan, problems can be avoided if solidified wastes are stored for about 30 years in interim storage facilities and then buried at a suitable depth, depending on the specific characteristics of the host rock. (author)