One of the most important current civil engineering challenges is to design underground repositories for the disposal of radioactive waste. This subject is currently being addressed in several countries and, in the future, will be of concern to other countries. In this paper, the rock mechanics contribution to this design challenge is outlined. To begin, the nature of the disposal problem and the information required are described. Then different aspects are individually addressed: in situ rock stress; the properties of the intact rock, including thermal properties, fractures and hydrogeological aspects. The reproducibility of numerical modelling and associated technical auditing are included because of the need to ensure that all aspects of the design have been suitably checked. Finally, future directions are discussed, bearing in mind the 437 nuclear reactors currently operating throughout the world, especially in terms of the current and potential Underground Research Laboratories that are required for in situ experiments to support the design premises.
The purpose of this paper is to provide an overview of the rock mechanics contribution to the design of an underground repository for the disposal of radioactive waste and to describe some of the key design aspects within this context. In this first section, the nature of the disposal problem and the information required to design a radioactive waste repository are outlined.
The design and construction of an underground repository for the disposal of radioactive waste is a unique engineering problem for four main reasons.
The primary purpose of the repository is to ensure that unacceptable quantities of radionuclides do not escape from the radioactive waste to the biosphere. Thus, the function of the repository is somewhat different to other engineering objectives=in that essentially nothing should happen, as opposed to the continued excavation of ore in a mine or the movement of people in a transportation tunnel.
The design life of a conventional underground opening varies from a few days (as in a longwall mining operation) to the ~120 years for a civil engineering project such as a metro system. However, the design life of a radioactive waste repository is of the order of hundreds of thousands of years. In other words, the regulator will wish to see confirmation in the design that the repository will safely contain the waste for up to a million years. Because it is not possible to predict the behaviour of engineered components for that length of time, the long term safety is ensured by the multi-barrier system, i.e., not only of the engineered canister containing the waste but also the surrounding rock mass.
Unlike a civil engineering project where the location is usually fixed by its function, e.g. a transport tunnel, or a mine which is located where the orebody is, a radioactive waste repository can, in principle and apart from political considerations, be established in a wide variety of locations and at a wide variety of depths.