THM analysis of a large‐scale heating test incorporating material fabric changes

Abstract

Engineered barriers are basic elements in the design of repositories for the isolation of high-level radioactive waste. This paper presents the thermo-hydro-mechanical (THM) analysis of a clay barrier subjected to heating and hydration. The study focuses on an ongoing large-scale heating test, at almost full scale, which is being carried out at the CIEMAT laboratory under well-controlled boundary conditions. The test is intensely instrumented and it has provided the opportunity to study in detail the evolution of the main THM variables over a long period of time. Comprehensive laboratory tests carried out in the context of the FEBEX and NF-PRO projects have allowed the identification of the model parameters to describe the THM behaviour of the compacted expansive clay. A conventional THM approach that assumes the swelling clay as a single porosity medium has been initially adopted to analyse the evolution of the test. The model was able to predict correctly the global THM behaviour of the clay barrier in the short term (i.e. for times shorter than three years), but some model limitations were detected concerning the prediction of the long-term hydration rate. An additional analysis of the test has been carried out using a double structure model to describe the actual behaviour of expansive clays. The double structure model explicitly considers the two dominant pore levels that actually exist in the FEBEX bentonite and it is able to account for the evolution of the material fabric. The simulation of the experiment using this enhanced model provides a more satisfactory reproduction of the long-term experimental results. It also contributes to a better understanding of the observed test behaviour and it provides a physically based explanation for the very slow hydration of the barrier.

A coupled THC model of the FEBEX in situ test with bentonite swelling and chemical and thermal osmosis

“…The retention curve for bentonite is similar to that of Villar et al (2008) and Sánchez et al (2011) and is given by:…”

Generic Argillite/Shale Disposal Reference Case

“…The exponent n ranges from 3 to 4.5 (ENRESA 2000), but most models (Zheng et al 2011a;Sánchez et al 2012) have used 3. The relative permeability curve for MX-80 bentonite is the same as in equation (1), with n ranging from 2 to 4 (Hökmark 2004).…”

“…ENRESA (2000) lists a quite large range of  and P 0, but in some modeling works (Zheng et al 2011a;Sánchez et al 2012),  is 0.18 and P 0 is 2×10 7 Pa; Villar et al (2008) used the same value of  but a slightly different value for P 0 (2.8×10 7 Pa). The parameters  and P 0 for MX-80 bentonite are from Hökmark (2004).…”

Use of a Dual-Structure Constitutive Model for Predicting the Long-Term Behavior of an Expansive Clay Buffer in a Nuclear Waste Repository

“…These aspects include accumulation of irreversible strain upon suction cycles (Day, 1994;Al-Homoud et al, 1995;Tripathy et al, 2002) or effects of microstructural swelling on water permeability. The effect of microstructural swelling on water permeability has been observed to have a significant effect on resaturation of expansive soil barriers, for example at the mock-up test performed at CIEMAT, Madrid (Spain), which reproduces a large-scale heating test of a bentonite buffer (Sánchez et al, 2012). …”

“…This causes the microporosity to invade the macroporosity as the expansive soil hydrates, which may reduce its permeability by several orders of magnitude (Olivella and Gens, 2000;Alonso and Hoffmann, 2007). This permeability reduction as the liquid saturation degree increases can explain delays in the saturation time that have been observed at both mock-up and in situ experiments involving expansive soil barriers (Sánchez et al, 2012). Therefore, the use of a dual structure constitutive model is required to properly simulate the THM behavior of expansive soils used as buffer material to backfill the space between the canisters containing nuclear waste and the tunnel walls.…”