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ABSTRACT: Twelve research teams in the European Community participated in a benchmark exercise for rock-mechanics computer codes about disposal of radioactive waste in salt. Two cases were considered: firstly, a complex hypothetical problem; secondly a laboratory model test. This work was carried out within the Commission of the European Communities' R&D programme on "Management and Storage of Radioactive Waste". RESUME: Douze equipes de la Communaute Europeenne ont participe à un exercice d'intercomparaison de codes de calcul geomecaniques concernant l'evacuation de dechets radioactifs dans le sel. Deux cas ont ete examines: d'abord, un problème theorique complexe; ensuite, un essai de laboratoire sur maquette. Ce travail a ete realise dans le cadre du programme de R&D de la Commission des Communautes europeennes sur "la gestion et le stockage des dechet8 radioactifs". ZUSAMMENFASSUNG: Zwölf Forschungsgruppen der Europaischen Gemeinschaft haben an einem Benchmark fuer felsmechanische Computerprogramme teilgenommen, das die Endlagerung radioaktiver Abfalle im Salz betraf. Zwei Probleme sind betrachtet worden: erstens, ein theoretisches Problem; zweitens, ein Modellversuch im Labor. Diese Arbeit wurde im Rahmen des F + E Programms der Kommission der Europaischen Gemeinschaften ueber "Bewirtschaftung und Lagerung radioaktiver Abfalle" durchgefuehrt. 1 INTRODUCTION Among the various studies relating to geological disposal of radioactive waste, the calculation of mechanical stresses in the host formation is of great importance. This investigation is necessary because of the two following requirements: first, the design and construction of stable underground repositories for the operational phase, i.e. the deposition of waste; then, the prediction of the large-scale mechanical behaviour of the host rock after repository closure, mainly under the influence of heat emission from high-level waste. Owing to the complexity of the geometrical arrangements Considered, and of the rheological behaviour of geological, media, these calculations are carried out by computer codes, generally using the finite element method. For about ten years, the Commission of the European Communities has been participating in the development of such calculation tools, in the framework of its R & D programme about "Management and Storage of Radioactive Waste". A next useful step was then to perform, at Community level, an exercise which would allow to assess the capabilities of these tools. In a first step, the exercise should focus on the viscoplastic calculation of stresses in salt, as this material was the subject of the largest body of geo-mechanical research, both theoretical and experimental. At the end of 1984, the Commission launched the Community Project COSA ("Comparison of Computer COdes for SAlt"), which brought together the main European teams having expertise in this field. The project was completed by the middle of 1986 (1). This exercise was the first one of this kind at Community level; it was given the following quantitative objectives: assessing the numerical accuracy of the codes involved, and verifying their capabilities to adequately replicate real world's situations. More qualitatively, it was hoped that increased exchanges would take place between the various teams, and that the methodology used for the exercise would be tested so that it could be improved for further steps. Similar projects were already performed outside the Community, particulary in the USA in the framework of the "WIPP Benchmark" managed by the SANDIA laboratories (2). For COSA, one tried to avoid duplications with these predecessors; however, profit was taken from this experience. 2 IMPLEMENTATION OF THE EXERCISE Ten European computer teams participated in the COSA project; the British engineering firm ATKINS R&D, Epsom, acted as scientific secretary and impartial co-ordinator on behalf of the Commission. These teams are: In Belgium: The company FORAKY, together with the Centre d'Etude de l'Energie Nucleaire (CEN/SCK) and the Laboratoire de Genie Civil of the University at Louvain-la-Neuve (LGC). In Germany: The Rheinisch-Westfalische Technische Hochschule (RWTH), Aachen; The Gesellschaft fuer Strahlen- und Umweltforschung (GSF), Braunschweig; The Kernforschungszentrum Karlsruhe (KfK), Karlsruhe. In Denmark: The Engineering Department of the RISØ Laboratory (RISØ), Roskilde. In France: The Laboratoire de Mecanique des Solides of the Ecole Polytechnique (LMS), Palaiseau; The Centre de Mecanique des Roches of the Ecole des Mines (EMP), Fontainebleau; The Laboratoire d' Analyse Mecanique des Structures, Departement des Etudes Mecaniques et Thermiques, of the CEA-Saclay (CEADEMT) In Italy: The Istituto Sperimentale Modelli e Strutture (ISMES), Bergamo. In the Netherlands: The Energieonderzoek Centrum Nederlands (ECN), Petten. The codes participating in the exercise are listed in Table 1; those between brackets were specifically used for thermal calculations.