Practical designs for clay based engineered barrier systems for heat emitting radioactive wastes

Many of the present designs of repositories for radioactive wastes derive from generic feasibility studies which emphasize post-closure safety. These include little (or no) treatment of the practicality of safe and quality-assured construction of engineered barriers under the conditions (humidity, dust, etc.) and requirements (emplacement rate, remote handling, etc.) of an operational underground facility. Indeed, as soon as attempts are made to demonstrate such concepts in-situ at full scale, considerable practical problems are encountered and, in many cases, additional engineering components are introduced (liners, borehole caps, grouts, rock-bolts, drainage systems, etc.) which could be detrimental to - or at leas t complicate - the long-term safety case. As the discrepancy between the idealized concepts illustrated in performance assessment and the actual systems which are shown to be feasible grows, there is a critical need for design rationalization. Such a process needs to include careful balancing of factors influencing safety during the operational phase - which should not be compromised - with those which contribute to potential hazards which occur only in the distant future. Apart from such almost philosophical considerations, the robustness of the EBS construction procedure to possible operational perturbations needs serious consideration. Even if closed and sealed repositories are very insensitive to disruptive events such as earthquakes, hurricanes, industrial actions and terrorist actions, the operational system may be more vulnerable to perturbation. Designs should be introduced which, to the greatest extent possible, not only fail safe, but are also easy to remedy (or reverse) in case the assurance of EBS quality is lost. This paper will expand on ideas for a second generation of clay-based EBS designs, which are both practical and safe. Associated requirements for R and D and performance assessment model development will also be outlined, with a particular focus on issues directly related to highly compacted bentonite as a key component of the EBS. (authors)