Long-term THM tests reports: Isothermal infiltration tests with materials from the HE-E

Abstract

A common design of a high-level radioactive waste (HLW) disposal system consists of the wastes encapsulated within steel canisters that are emplaced within horizontal tunnels, with the space between the canisters and the surrounding rock filled with a bentonite-based material. In the early post closure period the buffer is expected to experience the maximum temperature. In this phase the buffer is largely unsaturated and the thermal evolution of the EBS is likely to be controlled by the effective thermal conductivity of dry buffer. In particular, the temperature evolution of the engineered barrier system and surrounding rock was simulated using reference data for the thermal properties of HLW, bentonite backfill and Opalinus Clay. The results showed that the surface temperatures would reach a maximum value of ~150°C within a few years after emplacement (Johnson et al. 2002). These anticipated temperatures at the canister surface, in the bentonite and at the bentonite-host rock interface were scaled down in time and space to meet the specifications of the HE-E experiment, which is being carried out in the framework of PEBS (Gaus et al. 2011). The HE-E experiment targets the period immediately after repository closure when the temperatures are maximal and the moisture content is low but increasing. The HE-E experiment is a 1:2 scale heating experiment considering natural resaturation of the EBS and a maximum heater surface temperature of 140°C. Heater temperature is foreseen to increase almost linearly to its maximum value in a period of 1 year after which the temperature will be held constant for the years to follow. The experiment is located at the Mont Terri URL (Switzerland) in a 50-m long non-lined horizontal microtunnel of 1.3 m diameter excavated in 1999 in the shaly facies of the Opalinus Clay. The test section of the microtunnel was characterised in detail during the Ventilation Experiment (ENRESA 2005). The detailed design of the experiment is described in Teodori & Gaus (2011). The experiment consists of two independently heated sections (Figure 1), where the heaters are placed in a steel liner supported by MX80 bentonite blocks (dry density 1.8 g/cm3, water content 11%). The two sections are fully symmetric apart from the granular filling material: whereas section one is filled with pure MX80 bentonite pellets, section 2 is filled with a 65/35 granular sand/bentonite mixture with the characteristics described below: · granular bentonite (B) and bentonite blocks are used in one section of the test, corresponding to the Swiss disposal concept. It is the same as the one used for the ESDRED project, mixture type E (sodium bentonite MX-80 from Wyoming). The material is described in detail in Plötze & Weber (2007). Once emplaced its water content was 5.9% and the dry average density was 1.46 kg/m3. · sand/bentonite (S/B) mixture (having a higher thermal conductivity) and bentonite blocks are used in the other section. The sand/bentonite mixture was provided by MPC (Limay, France). The components are 65 % of quartz sand with a grain spectrum of 0.5 – 1.8 mm and 35 % of sodium bentonite GELCLAY WH2 (granular material of the same composition as MX-80) of the same grain spectrum, which was obtained by crushing and sieving from the qualified raw material. Water content was 13 % for the bentonite and 0.05 % for the sand, giving a total water content of the mixture in the range of 4%. There is some uncertainty about the actual emplaced density of the mixture, and values as low as 1.26 g/cm3 have been given. However, based on the tests performed to check the emplacement technique, a value of 1.5 g/cm3 has been taken for the calculations and the laboratory tests. A heater system, capable of representing the temperature curve of the anticipated heat production in the canisters (up to a maximum of 140°C), was switched on the 28th June 2011. During the experiment the temperature, humidity and the water saturation are monitored through a system of sensors on the heater surface within the liner, in the bentonite and in the surrounding host rock. The performance of tests at different scales, in both the laboratory and the field, is very useful to observe the thermo-hydro-mechanical processes taking place in the engineered barriers and the geological medium. They also provide the information required for the verification and validation of mathematical models of the coupled processes and their numerical implementation. The laboratory tests in cells are particularly helpful to identify and quantify processes in a shorter period of time and with less uncertainty regarding the boundary conditions than the in situ tests. In the tests in cells the sealing material is subjected simultaneously to heating and hydration in opposite directions, in order to simulate the conditions of the clay barrier in the repository. With the aim of helping to understand the information provided by the HE-E in situ test, CIEMAT undertook, in the framework of the PEBS project, the performance of two tests in cells simulating the conditions of the sealing materials used in the two sections of the in situ test (Villar et al. 2012). To complement the knowledge gained with these TH tests, infiltration tests at room temperature were performed in a large scale oedometer. This would help telling apart the effects caused by the thermal gradient from those purely hydraulic. This report presents the results of the five isothermal tests performed with the materials used in the HE-E experiment.