Keywords: finite element modelling, disposal drift, backfill material, excavation disturbed zone, waste repository, rock salt.

Repository concepts for disposal of high-level radioactive waste from reprocessing and spent- fuel elements are based on multiple barriers for protection of the environment against the radioactive nuclides. According to these concepts [1], the containers with heat generating radioactive waste will be placed in long parallel drifts at a depth of about 850 m below the surface in a rock salt formation. The remaining space between the containers and host rock will be filled with dry crushed salt. At the time of emplacement, the generated heat power in a cask will be up to 7.5 kW, depending on the interim storage period. It is expected that in response to the thermally induced creep closure of the excavations the backfill material will compact sufficiently to serve as an efficient seal for the radioactive waste.

The construction of deep underground structures disturbs the initial stress field of the surrounding rock. This effect can generate micro-cracks and degrade the hydro-mechanical properties of the rock salt. For the long-term performance of an underground repository in rock salt the evolution of the Excavation Disturbed Zone (EDZ) and the hydro-mechanical behaviour of this zone represent an important issue with respect to the integrity of the geological and technical barriers. In this contribution, attention will be focused on the mathematical modelling of thermomechanical processes in the near field of a disposal drift for heat-generating waste in a planned underground repository.

The objective of this paper is to give an overview of the actual repository concepts focusing on long-term thermomechanical effects related to the drift emplacement of heat- generating waste. This includes the modelling of the following processes:

• during the excavation the development of significant deviatoric stresses and potential fracturing of the rock salt,
• the stress relaxation due to thermally induced creep strain rates causing a further extension of the disturbed zone in the surrounding rock and
• a rapid reduction of the deviatoric stresses after consolidation of the crushed salt due to increasing the support effect of the backfill.

The numerical investigations have been performed with finite element codes specially developed for the repository design taking into account the non-linear, temperature and time dependent behaviour of rock salt. Recently, a new material model which is able to describe the dilatant creep strain has been implemented. According to this model, the total strain rate is given as the sum of elastic, thermal and viscoplastic creep rates. The viscoplastic flow function depends on mean stress, deviatoric stress and volumetric strain. The material parameters have been evaluated from few laboratory tests and in situ measurements [2]. For dry crushed salt as backfill a constitutive model based on a viscoplastic formulation considering volumetric and deviatoric strain rates under hydrostatic and shear stress conditions was used [3]. The capabilities of these codes to solve complex problems were demonstrated by comparison of the calculation results with measurements of a large-scale in situ experiment [4]. In this paper the most relevant parameters in the constitutive equations are varied in order to examine the sensitivity of the model response.

Finally, the numerical results suggest that under assumed conditions the volume closure of the disposal drift and the compaction of the backfill material are mainly determined by the temperature increase and the high lithostatic pressure at the repository level. After a few years, the compaction pressure in the backfill increases and therefore supports substantially the rock around the drift. A relevant relaxation and redistribution of the stresses takes place. Furthermore, due to thermally induced creep strain rates of the rock salt the self-healing process of the micro-cracks starts and the spatial extension of the EDZ will reduce over the first hundred years. Further work is in progress to extend the model for a more reliable analysis of this effect.

1

Bechthold, W., et al., "System analysis for a dual-purpose repository", Final Report, EUR 14595 EN, Brussels, 1993.

2

Bechthold, W., et al., "Backfill and Material Behaviour in Underground Repositories for Radioactive Waste in Salt (BAMBUS II Project)", Final Report, 2004, (in preparation).

3

Pudewills, A., Krauss, M., "Implementation of a viscoplastic model for crushed salt in the ADINA program", Computers and Structures, vol. 72, pp. 293-299, 1999. doi:10.1016/S0045-7949(99)00006-1

4

Pudewills, A., Droste, J., "Numerical modeling of the thermomechanical behavior of a large-scale underground experiment", Computers and Structures, vol. 81, pp. 911-918, 2003. doi:10.1016/S0045-7949(02)00427-3

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