Architecture and management of a geological repository - Andra

5 – Repository ModulesFigure5.3.11Transverse sections through CU1 spent fuel cells• Components and functions of the usable part of the cellThe metal lining has the following three functions : to physically support the argilite, protect personneland, during excavation, act as a thrust tube. The larger diameter of CU1 cells leads to their thicknessbeing increased to 30 mm, as against 25 mm for CU2 cells. The metal lining is perforated to allowwater to pass through it and re-saturate the swelling clay bufferThe clay engineered barrier consists of swelling clay (approximately 70%) and sand (approximately30%) in the form of prefabricated rings.Three major phases can be identified and analysed in the light of the functions fulfilled by the claybuffer or to which it contributes. These functions are the constitution of a diffusive barrier (protectingthe packages from aqueous alteration) and the dissipation of the heat released by the packages (and itstransfer to the argilite) [13].In phase 1, during the first decades, the unsaturated clay buffer does not need to fulfill its role as adiffusive barrier (the container prevents the water from coming into contact with the fuel). However,the clay buffer must allow the still very high heat flow from the packages to be evacuated (thetemperature peak occurs a few decades after placing the packages).In phase 2 (up to approximately 10 000 years), the container still protects the fuel (by preventingwater from coming into contact with them). The clay buffer, by being re-saturated, is able to fulfil itsrole as a diffusive barrier. The buffer continues to dissipate the heat produced by the waste to theargilite. However, the heat flow rate is gradually reduced as the heat rating of the packages decreases.In the longer term, in phase 3, (beyond 10 000 years), the container loses its watertight integrity andthe water can come into contact with the spent fuel assemblies. The clay buffer (re-saturated to over95%) then fulfils its function as a diffusive barrier, limiting the possibility of transport around thespent fuels. The buffer’s thermal role gradually diminishes.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM240/495