Corrosion Resistance of Candidate Overpack Materials in Deep Argillaceous Disposal Environments

Abstract

In situ corrosion testing and in situ monitoring of the near field chemistry are performed as part of the Belgian experimental program for the qualification of an overpack material for long term HLW containment in deep clay formations. Several test facilities are now in operation and some of the first experimental data are presented and discussed in terms of the program objectives.

History of corrosion studies on container materials for the final disposal of VHLW and SF conducted in Belgium

“…It was believed that this would allow definition of safety margins accounting for certain corrosion rate abnormalities and thus determination of the extra wall thickness of the metallic overpack needed to ensure its long-term integrity. [17,18] Simultaneously, immersion and laboratory experiments (complementary to the in situ corrosion experiments) were conducted to investigate the influence of several parameters such as temperature, type of backfill (Boom clay, bentonite), presence of aggressive species (Cl − , SO 4 ), oxygen content (oxic vs. anoxic conditions), and so forth on pitting corrosion. In situ experiments (e.g., Control Experiment with Radiation of the Belgian Repository for Underground Storage [CERBERUS]) were launched to evaluate the effect of γ-irradiation and radiolysis products on the corrosion rate.…”

“…Zn-coated cast iron operated at 13°C, 50°C, and 150°C, were installed in the clay formation at Terhaegen. [17,19,20,22,25,26] The tests in a humid clay atmosphere were carried out under dynamic conditions either in a corrosion chamber (50°C) or in inert furnaces (at 150°C and 300°C). The atmosphere used was a synthetic one simulating the atmosphere expected in the boreholes of a geological clay formation corresponding to pyrolysis of the clay at 300°C.…”

“…Figure 7 illustrates the set-up used to perform the corrosion tests under γ radiation. [22,[27][28][29] The metallic samples were exposed to either synthetic interstitial clay water or clay water slurries for periods of up to 42 days at 90°C under oxidizing or reducing conditions (by using either air or argon as cover gas above the test media). The synthetic test solutions used in these studies corresponded with interstitial clay water in equilibrium, either with fully oxidized clay or with undisturbed clay as removed from the geological host formation.…”

“…The metals were tested in three different conditions: as-received, heat-treated, and welded. [22,[27][28][29] F I G U R E 5 Experimental setup used to perform corrosion experiments in a humid clay atmosphere. [24] T A B L E 3 Composition of the interstitial clay water (in g/L) used in the screening studies.…”

“…These in situ tests were designed in the early 1980s, on the basis of the experience gained with similar "field" experiments carried out in a clay quarry at surface level (at Terhaegen, in Belgium). [17,18,30] The HADES URL has played an important role in the corrosion research of the metallic overpack carried out in Belgium. It provided the opportunity to gain a better understanding of the corrosion processes in clay because it allowed the study of the corrosion behavior of candidate overpack materials under conditions that approached "real" disposal conditions as closely as possible.…”