Tome Architecture and management of a geological repository - Andra

6 – C waste repository zoneThe presence of a swelling clay engineered barrier contributes in limiting both the maximumtemperature in the rock at the wall of the shaft and the thermal gradients between cells. The engineeredbarrier characteristics and the spacing between the cells are thermal dimensioning parameters of therepository.6.2.3.3 Limiting chemical disturbancesThe corrosion of the metallic components in the cells, and in particular the over-packs may in the longterm cause interactions with the granite water in the fractures. These chemical modifications may belimited by always placing a swelling clay engineered barrier around the packages6.3 C waste over-packs6.3.1 Selected design principlesThe choice of the constituent metal of the over-pack uses the same analysis as for the clay medium.Thus the reference is non-alloy steel.The corrosion process of non-alloy steel is well known. Concerning aqueous corrosion, a set ofexperimental results and models show that generalised corrosion is the dominant mechanism in themedium and long term. The rate of generalised corrosion, reflecting the thickness of corroded metalover time, can be quantified on the basis of models that have been validated experimentally. Localisedcorrosion, by pitting or fissuring, may be observed on these materials, especially in the presence ofoxygen. However the localised corrosion rate observed experimentally over short periods decreasesfaster than that of generalised corrosion, hence its relative importance reduces with time. Finally, therisks of specific corrosion such as corrosion under stress or hydrogen embrittlement, are of secondaryimportance when compared to other corrosion mechanisms. Iron archaeological analogies over 2000years old back up the assessment of the corrosion processes and provide a time reference for thedurability of the steels over a significant time scale.Furthermore the non-alloy steel corrosion models are tolerant to the water chemistry. They are alsotolerant with respect to composition of the metal, its structural status and surface condition. Thiselement limits the risk of consequent defects degrading the water-tightness and durability of the overpacks,especially at the welds. Moreover, very good welding results are achieved on non-alloy steelsand there is ample feedback on proven welding techniques on heavy thicknesses.They are preferable to passive alloys. Although they offer good corrosion resistance, these materialsare sensitive to environmental conditions and could thus be subject to pitting, which is harder tocontrol over the long term.6.3.2 Description and dimensioningThis section describes the disposal packages comprising a non-alloy steel over-pack that holds a C wasteprimary package. Its design aims at achieving the expected durability performance and is consistent withthe handling processes.6.3.2.1 Description of the disposal packagesTwo disposal packages with standardised dimensions deal with all C waste primary packages:- an external length of 1291 millimetres and a diameter of 615 millimetres for both the C0.1(PIVER vitrified waste) and C0.3 (AVM vitrified waste) reference packages;- a length of 1607 millimetres and a diameter of 550 millimetres for the C0.2 (CSD-V R7/T7/UMo), and C1-C4 reference packages produced at La Hague.Dossier 2005 granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY170/228