ABSTRACT

One alternative to close a radioactive waste repository in rock salt is the construction of a "Cross Section Closure (CSC)" sealing barrier. The proposed material for the CSC is highly compacted bentonite. To investigate the swelling behaviour of a highly compacted bentonite a large number of laboratory tests were performed. In addition, large scale model test were carried out to demonstrate the development of a nearly homogeneous and isotropic swelling pressure. The results of the large scale model tests were interpreted numerically based on the models for stress-strain behaviour and water uptake which are implemented in the finite element codes FEST03 and HYDOPO. The investigations show a good agreement of the results of the model tests and the corresponding analyses and show, that the theoretical models are capable of describing the behaviour of sealing structures based on highly compacted bentonite.

ZUSAMMENFASSUNG

Eine Möglichkeit fuer den Verschluß eines Endlagers fuer radioaktive Abfalle im Steinsalz ist die Konstruktion einer Querschnittsabdichtung aus hochverdichtetem Bentonit. Zur Untersuchung des Quellverhaltens von hochverdichtetem Bentonit wurden eine große Anzahl von Laborversuchen durchgefuehrt. Um die Entwicklung eines nahezu homogenen und isotropen Quelldrucks nachzuweisen, wurden darueber hinaus auch Modellversuche in größerem Maßstab durchgefuehrt. Die Ergebnisse dieser Modellversuche wurden im Hinblick auf die Wasseraufnahme und das Spannungs-Verformungsverhalten mit Hilfe numerischer Berechnungen mit den finite Element Programmen FEST 03 und HYDOPO interpretiert. Die Untersuchungen zeigen gute Übereinstimmungen zwischen Versuchs- und Berechnungsergebnissen. Die numerischen Modelle sind somit geeignet, das Verhalten von Abdichtungsbauwerken aus hochverdichtetem Bentonit zutreffend zu beschreiben.

RÉSUMÉ

Une possibilite pour fermer un depôt final de dechets radioactifs dans sel gemme est la construction d'un etanchement de coupe transversale, fabrique de bentonite fortement compacte. Pour etudier le comportement de gonflement de bentonite fortement compacte, de nombreux essais en laboratoire ont ete executes. En outre, des essais sur modèle sur une plus grande echelle ont aussi ete executes afin de prouver le developpement d'une pression de gonflement à peu près homogène et isotrope. Les resultats de ces essais sur modèle etaient interpretes en vue de l'absorption d'eau et du comportement contraintes-deformations par le moyen de calculs numeriques avec les programmes d'elements finis FEST03 et HYDOPO. Les etudes montrent de bonnes conformites entre des resultats d'essai et de calcul. Les modèles numeriques sont ainsi capables de decrire justement le comportement d'ouvrages d'etanchement fabriques de bentonite fortement compacte.

Introduction

The tunnels and shafts of underground repositories for nuclear or chemical waste, placed in geological barriers, need to be sealed after the deposition of waste in the underground cavities. Several demands on the sealing, like very low hydraulic and gas conductivity and high resistance against chemical attack or nuclear radiation, are made. The sealing also has to bear the overburden pressure.

At present, several shutdown conceptions for radioactive waste repositorys are being discussed. In a repository for radioactive waste, the rate of water infiltration into the near field has to be restricted in order to reduce potential migration of radionuclides to the biosphere. In the case of natural geological barriers, e.g. a salt dome, groundwater flow is determined by the characteristics of the host rock. For the engineered barrier system it is possible to restrict water flow rates to acceptable levels by selecting suitable sealing materials. Highly compacted bentonite is recognized to be an economical and reliable material that is relatively easy to handle and has a low permeability.

The suggested CSC concept is illustrated in Fig. 1 (Wittke, 1996). Its main component is a spherical masonry dam, which will be built of highly compacted bentonite bricks.

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