Keywords: thermal simulation, spent nuclear fuel, repository, decay heat, finite element method.

The heat produced by nuclear decay has an important effect on the stability of the properties of construction materials for spent nuclear fuel repositories. The temperatures are mainly over 100 degrees Celsius and could lead to irreversible changes in the bentonite. 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 from the heat producing spent nuclear power [1].

In spite of the fact that simulations of thermal processes are almost routine in many technical problems, it is not the case for the spent nuclear fuel repositories. In this paper, we analyse the influence of boundary conditions on the spatial temperature distribution. Boundary conditions represent the interaction of the model considered with its surroundings. Inappropriately selected boundary conditions can have an undesirable effect on the results of the model. It is useful to try several choices of boundary conditions and discuss their influence on the required results. In our case, the required results are the temperatures in the vicinity of the canisters in the scale of centimetres or metres, which can be over 100 degrees Celsius. Hence the selection of appropriate boundary conditions is one of the crucial tasks in thermal dimensioning of the repository's model.

The model represents an infinite periodic structure of tunnels and boreholes. It is a good model for local temperature analysis in the vicinity of the canisters [2]. The single segment of the periodic model consists of one quarter of a borehole with appropriate parts and granitic massif with appropriate dimensions. On the top and the bottom area of the segment, we define the three alternative boundary conditions: prescribed temperature, so-called far-field elements, and zero heat flow.

We can compare the effect of different above boundary conditions on the bottom and top area of the segment. Different types of boundary conditions on the bottom and top areas do not have an effect on the maximal temperature and the temperature distribution in the vicinity of the borehole. The investigated boundary conditions have a significant effect on the rate of temperature decrease and the temperature distribution in the scale of tens of metres from the canister.

The analysis shows the influence of the boundary conditions on the temperature distribution in the model of the repository. The most important result for thermal dimensioning is the maximum temperature achieved. Various types of boundary conditions do not have effect on the maximum temperatures in our model. The temperature distribution in the immediate vicinity of the borehole is similar in all the cases analyzed. A different situation is in the area of tens of metres from the canister. Here the temperature varies greatly, dependent on the boundary conditions selected.

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

M. Hokr, J. Novak, "Thermal simulations of high-level waste repository", in "Proceedings of 8th ECMS 2007", Liberec, Czech Republic, 2007.

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