HE-E Experiment: Laboratory test in a THM cell with the Sand/Bentonite mixture.

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 engineered barrier system (EBS) is likely to be controlled by the effective thermal conductivity of the dry buffer. In particular, the temperature evolution of the EBS and surrounding host rock was simulated using reference data for the thermal properties of HLW, bentonite backfill and Opalinus Clay. The results showed that the canister 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 started 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 increased almost linearly to its maximum value in a period of one year after which the temperature was held constant. 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 1.81 g/cm3, water content 10.3%). The two sections are fully symmetric apart from the granular material filling the rest of the gallery: whereas section 1 is filled with pure MX80 bentonite pellets, section 2 is filled with a 65/35 granular sand/bentonite mixture. The characteristics of both materials are described below: · The granular bentonite (B) has been adopted in 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 was 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. · The sand/bentonite (S/B) mixture (having a higher thermal conductivity) was selected by GRS and provided by MPC (Limay, France). The components were 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 an average 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, an average value of 1.5 g/cm3 was taken. 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, i.e. the interaction of the water coming from the host rock and the thermal gradient generated by the heat emitted by the wastes in the canisters. With the aim of complementing 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. From the end of the PEBS project (February 2014), the laboratory tests went on under contract with the Mont Terri consortium.