Architecture and management of a geological repository - Andra

Furthermore, equipment and liner maintenance operations in the accessible connecting drifts can becarried out depending on the regular visual inspections and results of the dedicated monitoring of thesestructures.In addition, in the event that the packages are retrieveed, it is technically possible to dismantle the sealand restore the access drift, as indicated in Section 10.4.• Evolution of the cellStopping the ventilation fairly quickly induces a change in the physical conditions within the cells:consumption of the oxygen in the air and, for certain cells, heating of the packages and cell oraccumulation of radiolysis gases. In addition, it stops the de-saturation kinetics in the rock and thustriggers the cell and seal re-saturation process.This re-saturation process represents an important phase in the evolution of the cell, even if thisevolution is very slow due to the very low permeability of argilite. The rock and liner re-saturationprocess is accompanied by an increase in the relative humidity of the air in the cell, which is thusalways in equilibrium with the moisture conditions of the cell wall concrete. The increase in therelative humidity contributes simultaneously to the increase by capillary action of the degree ofsaturation of the concrete in the disposal packages. However, due to the slow re-saturation of thecells 135 [8], over a period of several centuries, there will be no liquid water in the spaces between thepackages and liner concrete. The concrete of the disposal packages remains unsaturated.In the cells containing slightly exothermic packages (essentially type B1 and B5 packages) the heat isno longer evacuated by ventilation. The temperature of the packages, then of the liner and the rockincreases. The maximum temperature of the packages remains below 60 °C [10]. It is reached a littleunder ten years after the ventilation is stopped. The temperature of the packages then slowlydecreases, initially by approximately 1°C per decade, then more and more slowly. The temperature ina cell containing non-exothermic waste always remains below 30°C. Monitoring of the temperaturemakes it possible to check that the temperatures reached fully meet the laid down criteria (seechapter 5) and to establish its condition at the time of a potential waste package retrieval.In cells in which packages emit gases through radiolysis, closing the cell and, in particular, stoppingthe ventilation, lead to the trapping and gradual accumulation of these gases. In view of the very slowdecrease in their production [62], the duration of the ventilation before sealing only slightly modifiesthe gas accumulation kinetics in the cell. When they accumulate in a cell, these gases become slightlypressurised. As long as the connecting drift is open and ventilated, the gas pressure difference betweenthe two ends of the access drift to the cell promotes the transfer of these gases through the unsaturatedcell. These small quantities of gas can be diluted in the ventilation air in the connecting drifts or, ifnecessary, collected. In addition, this accumulation of gas contributes to the slowing down of thealready very slow process of cell re-saturation.The absence of water from the cell for several centuries considerably limits the chemical degradationof the liner, of both disposal packages and primary packages (by carbonation, hydrolysis, oxidationand corrosion). Changes to the cell therefore result above all from the very slow and gradual increaseof the thrust of the argilites on the liner during their re-saturation. This thrust, which is very small oreven stopped so long as the rock was de-saturated, nevertheless remains sufficiently moderate for theliner and filling concrete to remain intact for several centuries (see chapter 5) [57]. The durability ofthe lining and the maintaining of the functional clearances between the packages and the wall do nottherefore deteriorate after closing the cell. Similarly, as long as the lining remains intact, the functionalclearances between the packages should not have changed very much or at all.Under these conditions, the disposal packages remain intact, as they have no loading applied to themother than the weight of subjacent packages. The absence of any notable damage to the disposalpackages and primary packages thus prevents any dissemination of radioactive materials within thecell.135 It is estimated that the cell will take thousands if not tens of thousands of years to be re-saturatedDOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM373/495