Preliminary geotechnical results from the Mock-Up-CZ experiment

Abstract

The issue of the disposal of radioactive waste is one of the most pressing challenges of our age, for which, in most countries, the deep repository concept is generally considered to be the most suitable final solution. In order to make such a repository both safe and reliable, intensive research is underway worldwide. The construction of physical models is one approach to the study of the engineered barriers for deep geological repositories; one such experiment, Mock-Up-CZ, has been performed at the Centre of Experimental Geotechnics, CTU in Prague. The Mock-Up-CZ experiment simulated the vertical placement of a container with radioactive waste, an approach that is in line with the Swedish KBS-3 system. The physical model consisted of a barrier made up of bentonite blocks, powdered bentonite backfill, a heater and hydration and monitoring systems. The basic material used in the experiment consisted of a mixture of Czech bentonite from the Rokle deposit (85%), quartz sand (10%) and graphite (5%). The first phase of the experiment commenced on 7th May 2002, during which the heater was switched on, with no water input. After 6 months the second phase commenced in which water was introduced through the hydration system. This phase ended on 2nd January 2006 when the heater was switched off. After allowing time for cooling, the dismantling phase commenced (30th January 2006). The dismantling process was conducted according to a very detailed project schedule and was extensively documented. The dismantling project included not only the dismantling itself but also included a detailed plan for subsequent multidisciplinary research on the samples taken from the experiment. It was hoped that the research would reveal any changes which might have affected the materials under investigation. This paper focuses on just one part of the research—the geotechnical element which includes the results of density distribution changes (the experiment was made up mainly of highly compacted blocks), water content distribution, permeability, changes in liquid limits and swelling pressure as well as thermal conductivity. It was reasoned that changes in such properties might well indicate changes in the structure and/or composition of the material in question. Conditions within the experiment were not homogeneous, the main reasons being that, firstly, the temperature of the heater was lower at the bottom of the experiment and, secondly, a thermal gradient existed between the central axis and the outer surface. In order to correlate the results with conditions inside the experiment, temperature distribution “maps” were employed. The use of this technique in the testing of the samples allowed the team to more easily identify the various changes and trends taking place. At the present time it is clear that the surroundings of the upper 1/3 of the heater, where a large number of changes in the bentonite were observed, were the most active area of the experiment.