Architecture and management of a geological repository - Andra

5 – Repository Modules• Particular case of the resaturation periodDuring resaturation of a structure, flows in the argilite converge towards this structure. If theresaturation periods of adjacent structures are different, they can lead to transfer of water from onestructure to the next. If the cells reach saturation first, flows can occur towards the as yet unsaturatedaccess drift and are liable to transport any radionuclides released by the packages.The disposal concept presented here comprises a number of factors which contribute to limiting thepossibility of convective transport in the cell: hydraulic closure of the cell, cell dimensions limiting theintercepted water flow, elongation limiting the axial rate of the water for a given intercepted flow.• Production and fate of gases (corrosion hydrogen) in the C cellsWhen decided on as part of reversible management, closure of the structures leads to them beinggradually resaturated by the water from the argilites and that of the surrounding rock, if there wasdesaturation during the operating phase. Given the low permeability of argilites, this is a slow process.It is accompanied by the production of hydrogen as a result of corrosion of the metal components,which delays the return to complete saturation.The very slow corrosion rates (a few microns/year, or probably even a few tenths of a micron/year)lead to a period of several thousand years (about 5,000 years) for the main hydrogen production phase.The modelling conducted on the basis of experimental work with samples and a borehole led to anassessment of the properties of hydrogen transfer in the argilites. It shows that hydrogen transfer takesplace first of all by dissolution, and then by diffusion through the Callovo-Oxfordian. When the wateris saturated with hydrogen, this then follows two-phase migration, principally through areas with thelowest gas inlet pressures (close to 2 to 3 MPa for the initial ruptured/damaged zone, as compared with5 to 7 MPa for sound argilites). If hydrogen production is faster than its migration, this is expressed ingaseous form and the gas pressure can reach the porosity threshold of the argilites (micro-fissures) orof the swelling clay engineered components (cell plugs). Opening up of micro-fissures enables the gasto escape and the pressure may drop. It was found on the samples and during a borehole test that themicro-fissures close up after the gas passes, without altering the properties of the rock (the waterpermeability of the argilite remains unchanged) ([62] and [6] - Tome 2). Finally, when the pressurehas dropped enough for the hydrogen to be no longer able to enter the porosity of the variouscomponents, it slowly evacuates in dissolved form.5.2.1.4 Module separationThe design of the C waste disposal zones involves division into about ten subassemblies, in order tominimise the consequences of an altered situation (failure or intrusion). These subassemblies areisolated from each other by seals in their access drifts.The sealing of each cell also contributes to limiting the impact of any such altered situation.5.2.1.5 ReversibilityReversibility is designed to maintain flexible management of the repository packages, cells andmodules. At the beginning of the disposal process, this flexibility is comparable to storage; thepackages can be simply retrieved over a period of from one to a few centuries.The option of retrieving the packages depends on the durability of the packages, cells and accessdrifts. This covers the integrity of the disposal packages, the components of the cell containing them,as well as ensuring that a minimum functional clearance is maintained around the packages to enablethem to be retrieved over this same period of centuries. The integrity of the various elements for aperiod of at least a century implies a mechanical design taking account of the evolution of thematerials and, in particular in the case of C cells, of corrosion of the metal parts. Similarly,maintaining the access drifts implies mechanical sizing of the concrete lining for an identical period.Finally, the modularity of the C waste repository zone provides it with overall management flexibility.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM192/495