Preliminary geotechnical results from the Mock-Up-CZ experiment

摘要

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