CRITICAL ISSUES IN MODELING THE LONG-TERM HYDRO-THERMOMECHANICAL PERFORMANCE OF NATURAL CLAY BARRIERS

Performance assessment of deep repositories for heat-generating radioa ctive waste requires the capability of predicting reliably the evoluti on of the system during a time period commensurate with the hazardous life of the waste. In many repository designs clay barriers represent important elements of the waste isolation system. In order to provide reasonable assurance that clay barriers will ensure long-term waste is olation, it is essential to understand their behaviour under a variety of conditions. Due to the variability of argillaceous materials, to t he complexity of the phenomena that might take place in a waste reposi tory and to the longevity of the required isolation, an adequate under standing of the behaviour and the capability to model the evolution of the clay barriers are not easy tasks. The factors that need to be und erstood and modelled include stress evolution, long-term strain or cre ep, thermal effects on solid skeleton, on interstitial fluids and on m ineralogy. The difficulty of the task is increased by the facts that m any effects are coupled? that their rates must be extremely low, in or der to be realistic, and that the time period to be modelled defies th e possibility of direct experimental observation. Several critical iss ues are identified and discussed briefly, such as: constitutive law to describe the thermo-mechanical behaviour of the clay skeleton, modell ing of the fluid phase in clays and its response to heating, thermal f racturing and healing. Strategies are suggested for a rational approac h to the experimental investigation of some relevant processes. The st udy of suitable natural analogues, for example the thermo-metamorphic halo occurring at Orciatico in Tuscany, could provide valuable insight s in the thermal effects of heating clay barriers. It is conceivable t hat models describing a variety of relevant phenomena, such as dehydra tion, fracturing and permeability changes could be tested through the study of the Orciatico analogue. In the end performance assessments of clay barriers would benefit through improvements in modelling: this w ould involve progress in understanding the basic phenomena and their c oupled nature, improved conceptual and mathematical models and increas ed reliability for their calibration/validation. The improved understa nding of phenomena requires additional experimental activities on vari ous levels: molecular, microscopic, macroscopic, medium scale and in s itu.