Architecture and management of a geological repository - Andra

6 – Overall underground architectureOver the scale of a century, there is little interaction between the modules. On the other hand, afterseveral centuries, the temperature tends to become progressively homogenous in the repository zone,with a slight gradient from the core of the zone towards its perimeter. On a scale of tens of thousandsof years, the distribution of temperature in and around the C waste repository zone returns close to theinitial natural geothermal gradient (Figure 6.3.2 and Figure 6.3.3).Because of the interactions between modules that occur over time, the thermal period, defined by atemperature at the core of the packages that is higher than approximately 50 °C, depends on thedensity of the placement of the packages in the repository on the scale of the repository zone. Thisdensity brings the distances between modules into play.A more rapid drop in temperature will be observed in the part of the repository where C0 typepackages are placed (see Chapter 3) due to the lower exothermicity of these packages; the distancebetween this part of the repository and other modules taken into account ensures thermalindependence.6.3.2.2 Thermal independence of different repository zonesThe thermal independence of the B waste repository zone from the C waste and, where applicable, thespent fuel repository zones has also been verified. The influence of such a zone situated 250 metersfrom a B waste disposal cell is less than 4 °C. The corresponding thermal peak is reached severalthousand years after placement in the repository.Within B waste, while B2 type packages (bituminised waste) are very slightly exothermic, they arevery sensitive to temperature. Thus it has been verified that the distance between cells containing B2packages and cells containing other packages will keep the temperature of B2 packages below 30°C.Figure 6.3.4, for example, shows the increase in temperature above the initial temperature of the rock(equal to 22°C) in a cell of B2.1 packages the end of which would be situated 100 meters from a cellof slightly exothermic B packages (here a B3 reference package has been adopted, the heat rating ofwhich decreases more slowly over time compared to that of the other reference packages, and, as aresult, it is more likely to have greater influence of the neighbouring cells).Temperature increase (°C)Core of the cellRepository chamber wallCell wall3 m cell wall6 m cell wall Mid distance between cells (30 m)54.543.532.521.510.501 10 100 1000 10000Time (year)Figure 6.3.4Increase in temperature in a cell of B2.1-type waste as a function of timeIn this configuration, an initial temperature peak occurs less than ten years after the packages areplaced in the repository. This peak is caused by the thermal power emitted by the B2.1 packages and isvery low. After several centuries, a second peak can be caused by the heat flux emitted byneighbouring slightly exothermic packages [10].DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM266/495