Upscaling of modelling results from small scale to one-to-one scale

Description

The feasibility study of a radioactive high-level and intermediate-level long-lived waste repository in a Callovo-Oxfordian claystone formation (COx) is currently under completion in France. Thermo-Hydro-Mechanical (THM) behavior of the COx is of great importance with regard to the design and safety calculations. Concerning the high-level waste, the heat emitted from the waste causes a pore-pressure increase within the surrounding rock due to the differential thermal expansion of the pore water and the solid skeleton. The low permeability of the COx and its relative rigidity prevent the discharge of the induced pressure build-up.

To investigate the THM response of the COx to a thermal loading, an important research program has been conducted at Andra since 2005 through laboratory and in situ experimentations. The purpose of Task E under DECOVALEX-2019 is upscaling THM modeling from small size experiments (some cubic meters) to real scale cells (some ten cubic meters) and to the scale of the waste repository (cubic kilometers).

For that purpose, two in situ tests performed in the Meuse/Haute-Marne underground research laboratory will be interpreted and modeled in the frame of the task:

  • the TED experiment, a small-scale heating experiment focused on the claystone THM behavior of the undisturbed rock mass in the far field
  • the ALC experiment, a one-to-one scale heating experiment especially focused on the interaction between the surrounding rock and the support (steel casing in this case) in the near field

Following this work, the impact of the behavior of one single cell at the repository scale (considering several parallel cells) will be addressed. This application should address some key technical challenges, such as the variability of the THM parameters over such a surface and the determination of appropriate boundary conditions when 2D modeling is used to represent a series of parallel cells.

Experimental Data

The TED experiment lasted 3 years between 2010 and 2013. It involved three heaters at a depth of 490m, in three parallel boreholes at a separation of about 2.7m. The three heaters were 4m long and were installed at the end of 160mm diameter and 16m long boreholes, drilled from a main drift and parallel to the maximum horizontal stress. This arrangement represented a similar configuration to high-level nuclear waste cells with parallel micro tunnels, but at a smaller scale. The TED experiment was heavily instrumented with 108 temperature sensors in the rock mass, 69 temperature sensors in the 3 heater boreholes, 18 piezometers, 2 extensometers and inclinometers, and 10 temperature sensors recording the temperature at the level of the main drift. The temperature measurements made during the TED experiment showed that the rock has an anisotropic thermal conductivity; at the same distance from the heater, the temperature increase is higher in the bedding plane than in the perpendicular direction. Observations of pore pressure also showed that its evolution depended on the location with respect to the bedding plane; following a power increase, the pore pressure increased faster in the direction parallel to bedding than in perpendicular direction.

illustration
Layout of the TED in situ experiment

The ALC experiment is an ongoing in situ test. The experiment is a full-scale representation of a single high-level waste cell in Callovo-Oxfordian claystone. It aims at verifying the construction feasibility of a high level waste cell representative of the 2009 reference concept and at understanding the THM behavior of the COx and of the interface between the rock mass and the casing under thermal loading. The ALC cell is a steel-cased borehole with a total length of 25m, divided into a head part (6 m long) and a body part (19m long), the heated part being located between 10 and 25m deep. The excavation diameter in the body part is 750mm. The steel casing in the body part and the steel insert in the head part are instrumented with strain gauges, total pressure sensors, temperature sensors, and relative humidity sensors. The heaters have temperature sensors and strain gauges. Moreover, the ALC experiment includes a peripheral instrumentation in the rock mass with 6 monopacker and multipacker boreholes for liquid pressure measurement, 2 temperature measurement boreholes and 1 borehole with strain sensors. Finally, strain gauges, displacement sensors, inclinometers and extensometers, and a temperature/relative humidity sensor have been installed in the main drift.

illustration
Layout of the ALC in situ experiment

Approach

Task E adopts a step-by-step approach, from small-scale to full-scale study. It is structured into four main steps and includes a benchmark test (step 1), an interpretive exercise (step 2), a blind prediction (step 3a) and an application (step 4).

  • Step 1: Simple 3D THM modeling benchmark
  • Step 2: Interpretative modeling of the TED experiment (back analysis)
  • Step 3: Modeling of the ALC experiment
    • Step 3a: Predictive modeling of the ALC experiment using the reference values for the rock mass parameters determined in step 2
    • Step 3b: Interpretative modeling of the ALC experiment
    • Step 3c: Interpretative modeling of the ALC experiment focus on the support of the micro tunnel
  • Step 4: Prediction at the repository scale/ modeling of an area with several high level waste cells

Participating Groups

  • France: Andra
  • Germany: BGR (UFZ Leipzig)
  • UK: RWM (Quintessa Ltd)
  • USA: Department Of Energy (Lawrence Berkeley National Laboratory)

Further Information

For further information, please contact the task leader, Darius Seyedi.

References

  1. Armand G., Leveau F., Nussbaum C., de La Vaissiere R., Noiret A., Jaeggi D., Landrein P. Righini C. (2014) Geometry and properties of the excavation induced fractures at the Meuse/Haute-Marne URL drifts. Rock Mech. Rock Eng., 47, 21-41. doi:10.1007/s00603-012-0339-6.
  2. Conil N., Armand G., Garitte B., Jobmann M., Jellouli M., Filippi M., De La Vaissière R., Morel J. (2012) In situ heating test in Callovo-Oxfordian claystone: measurement and interpretation, In Proceeding of the 5th Int. meeting of Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, Montpellier, October 22-25.
  3. Garitte B., Gens A., Vaunat J., Armand G. (2014) Thermal Conductivity of Argillaceous Rocks: Determination Methodology Using In Situ Heating Tests. Rock Mech. Rock Eng., 47:111–129