Keywords: thermal simulation, spent nuclear fuel, repository, multiscale model technique, finite element method.

Even with the rapid development of new technologies some amount of high radioactive waste remains from nucler power plants or from different industries which will be necessary to isolate from the biosphere for thousands years. The mix of technological and nature barriers is proposed for the safe deposition of those radioactive waste.

The heat produced by the nuclear decay of radioactive waste has an important effect on the stability of the spent nuclear fuel repositories. Mainly temperatures over 100 degrees Celsius could lead to irreversible changes in the engineering barrier of the repository. For purposes of insulation, the canisters with radioactive waste are surrounded by a barrier of bentonite clay. The role of the bentonite is to provide a good insulation for water flow and radionuclide migration and a sufficiently high heat conductivity to cool the canister with heat producing spent nuclear power [1].

It is a really complicated task to develop a finite element model for the analysis of a spatial temperature distribution in the repository. We need more details about temperature distribution in the vicinity of the borehole on the one hand, and we have an extensive surounding of the host rock on the other hand. If we use a local model of the vicinity of the borehole, we can analyse the spatial temperature distribution in the borehole, but we have to specify appropriate boundary conditions on its boundaries. Inappropriately selected boundary conditions can have an undesirable effect on the results of our model. On the other hand, if we make a simplified global model of the repository, including a ground surface, we can determine boundary conditions exactly, but it is difficult to determine local temperatures in the vicinity of the borehole in the scale of centimetres or metres.

Hence the one of the possible solutions is a combination of the local and simplified global model of the repository. Because we are not able to to specify sufficiently acurate boundary conditions for the local problem, we solve the global model. Based on the results of the global problem, we can specify boundary conditions for the local problem.

The main achievement of this work is a presentation of a simple multiscale model technique for chosing appropriate thermal boundary conditions for a spent nuclear fuel repository model. It is shown, that so called far-field elements in ANSYS [2] can substitute the multiscale approach in this application. The multiscale technique is more time consuming than a far-field element aproach, but far-field element can not be used for coupled thermo-mechanical problems. Hence the multiscale approach is almost the only technique for coupled models.

1

IAEA, "Scientific and Technical Basis for the Geological Disposal of Radioactive Wastes", Int. Atom. En. Ag., Technical Reports Series No. 413, February 2003. URL

2

J. Novak, M. Hokr, "Finite Element Simulations of the Thermal Conditions in a High-Level Waste Repository", in B.H.V. Topping, L.F. Costa Neves, R.C. Barros, (Editors), "Proceedings of the Twelfth International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, United Kingdom, paper 130, 2009. doi:10.4203/ccp.91.130

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