Geological Repository Layout for Radioactive High-level Long-lived Waste in Argilite

Abstract

In the framework of the 1991 French radioactive waste act, Andra has studied the feasibility of a geological repository in the argillite layer of the Bure site for high level long lived waste. This presentation is focussed on the underground facilities which constitute the specific component of this project. The preliminary underground layout which has been elaborated is based on four categories of data: - the waste characteristics and inventory, - the geological properties of the host argillite, - the long term performance objectives of the repository, - the specifications in term of operation and reversibility. The underground facilities consist of two types of works: the access works (shafts and drifts) and the disposal cells. The function of the access works is to permit the implementation of two concurrent activities: the nuclear operations (transfer and emplacement of the disposal packages into the disposal cells) and the construction of the next disposal cells. The design of the drifts network which matches up to this function is also influenced by two other specifications: the minimisation of the drift dimensions in order to limit their influence on the integrity of the geological formation and the necessity of a safe ventilation in case of fire. The resulting layout is a network of 4 parallel drifts (2 of them being dedicated to the operation, the other two being dedicated to the construction activities). The average diameter of these access drifts is 7 meters. The link between the surface and the underground is ensured by 4 shafts. The most important function of the disposal cells is to contribute to the long term performance of the repository. In this regard, the thermal and geotechnical considerations play an important role. The B wastes (intermediate level wastes) are not (or not very) exothermic. Consequently, the design of their disposal cells result mainly from geotechnical considerations. The disposal packages (made of concrete) are piled up in big cavities the diameter of which is about 10 meters and the length of which is about 250 meters. On the other hand, the design of the C waste disposal cells (vitrified waste) is mainly derived from their thermal power (about 500 W after a 60 year period of interim storage). The disposal cell is a tunnel the diameter of which is about 0,70 m and the length of which is about 40 m. The number of the disposal packages (made of steel) per cell, the spacing between two adjacent canisters within a given cell and the spacing between two adjacent cells are adjusted to limit the peak of temperature in the host formation at 100°C. The disposal cells are also characterized by favourable design factors which would facilitate the potential retrieval of the wastes. The whole underground layout would represent a surface area of several km 2 .