Tome Architecture and management of a geological repository - Andra

5 – B waste repository zoneConcrete disposal packages need to be given additional confinement performance for the mostradioactive B waste (B1, B5), for the activation products and long-lived fission products ( 94 Nb, 93 Zr,135 Cs, etc.) in the form of low permeability. Furthermore the packages need to last at least severalmillennia.Certain types of waste produce hydrogen. The hydrogen is generated by the radiolysis of materials,primarily the organic matter contained in certain technological waste or embedding materials such asbitumen. Thus the disposal package options studied for the waste in question provide for permeabilityto this gas.5.2.2.3 Thermal criterion of designThe concern for compactness and cell temperature criteria must be compatible from a dimensionalangle.There is a dual requirement for limiting temperature:- to control alteration of the disposal package concrete;- to control radionuclide behaviour inside the cell.A temperature threshold that affects the chemical change processes of the material (carbonation,hydrolysis) emerges from studying the physical and chemical properties of the cementitious materialsbeing considered. As long as temperature remains below this threshold, the physical and chemicalproperties, especially permeability, change in a reversible manner or remain constant. Above thisthreshold, some cement paste hydrates become unstable: major mineralogical modification wouldtherefore lead to irreversible changes in concrete physical and chemical properties. Additionally thereare uncertainties surrounding the chemical properties at these temperatures. A maximum temperatureof 70°C has been adopted.The temperature is limited to below 50-60°C on the basis of the available data, and below 80°C with acontrolled margin of uncertainty, to ensure that the right conditions are in place to control radionuclidesolubility and sorption behaviour.Thus in terms of dimensioning, 70°C is the temperature limitation criterion in the disposal tunnels.Furthermore the temperature must be below 30°C for bituminised waste (type B2), to maintain therheological properties of the embedding bitumen.5.2.3 Limiting disturbances to the granite by the repositoryIncreasing the depth generally leads to reducing the hydraulic gradients in the structures, which islikely to enhance the robustness of the repository in the event of seal failure. By limiting the hydrogeologicaland hydro-geochemical disturbances, it is a potential parameter for adapting thearchitecture of a B waste repository to a granite site.The other thermal or chemical disturbances likely to affect the properties of a B waste repository aremanaged either by dimensioning the structures and the various disposal tunnel components or bymanaging the disposal process.5.2.3.1 Limiting chemical disturbancesExcavating and operating a disposal tunnel and its long term evolution induce chemical disturbances atthe structure walls. An initial disturbance is caused by introducing air into the tunnel during theoperating phase. Once the repository is closed and water returns to the tunnels, gradual alteration ofthe concretes leads to alkaline disturbance at the rock wall.Installing the tunnels in "blocks" of very low permeability and slightly fissured granite ensures that theoxidising or alkaline disturbances essentially affect the granite rock itself rather than the granite fracturenetwork.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY118/228