An example of the coupled thermal hydraulic processes in the FEBEX type repository

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Problem description

This example is based on the The TASK D_THM1 of the DECOVALEX-THMC project. The TASK D_THM1 of the DECOVALEX-THMC project studies the coupled thermal hydraulic and mechanical (THM) processes in the FEBEX type nuclear waster repository. In the FEBEX type repository, nuclear waster canisters are stored in the drifts excavated in the deep rock mass of granite, which is fully water saturated, and they are sealed with bentonite. Initiallly, the betonite, the sealing material, is partially saturated. TASK D_THM1 defines a 2D model about the FEBEX type repository \cite BirEtAl:2008. In this example, TASK D_THM1 is simplified in order to test the staggered scheme for TH process in ogs6. The simplifications are
  1. resizing the domain to an area that can represent the near field of an installed nuclear water canister,

  2. assuming the the betonite is fully saturated from the beginning,

  3. ignoring the mechanical process.

With such simplifications, the geometry of the present example is illustrated in the following figure:

In the above figure, the domain in the annulus sector represents the sealing material, bentonite. A heat power, which is generated by the nuclear waste with one million year variation, is applied onto the inner arc of the annulus sector. On the top boundary, the boundary conditions are p = 4.3 ⋅ 106 Pa, T = 294 K. While on the bottom boundary, the boundary conditions are set as p = 4.7 ⋅ 106 Pa, T = 310 K. The initial conditions are given as p = 4.7 ⋅ 106 Pa, T = 298 K.

The material properties are shown in the following table:

Material properties
Property Value Unit
Bentonite
Density kg/m3 1600
Porosity - 0.01
Thermal conductivity W/(mK) 3
Specific heat capacity J/(kgK) 3
Saturated permeability m2 2.0 ⋅ 10−21
Granite
Density kg/m3 2700
Porosity - 0.41
Thermal conductivity W/(mK) 3
Specific heat capacity J/(kgK) 900
Saturated permeability m2 10−17

Solution

As the reference results, the temperature and pressure distributions in the domain at the time of 18 years are shown in the following figure, in which the thermal convection effective can be seen clearly.

Reference

Birkholzer, J. and Rutqvist, J. and Sonnenthal, E. and Barr, D. ( 2008 ): DECOVALEX-THMC Project, Task D: Long-term permeability/porosity changes in the EDZ and near field due to THM and THC processes in volcanic and crystalline-bentonite systems. SKI


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