Integration of the findings on the impact of irradiation, dry density and particle size on the microbial community: EURAD - D15.9

The results obtained in Task 4 of ConCorD are discussed in this report. From this, it is clear that the high initial temperatures and gamma radiation during the saturation phase have potential to exhaust most microbial activity in the bentonite zones long-term subjected to the temperatures of at least 90 °C. As a consequence of the suppression of microbial activity in the buffer, biofilm formation on the canister surface does not need to be considered. However, further microbiological studies combined with mathematical modelling of special temperature evolution in DGR are necessary to predict the size and spacious distribution as well as long-term persistence of such microbially depleted zones in the bentonite buffer around the metal canisters. When the saturation is reached, microbial activity will be further suppressed by compacted bentonite where the threshold dry density depends on the bentonite (their physical-chemical properties and the indigenous microbial communities). However, the diffusion reactor experiments and oedometer experiments investigating this within ConCorrD are still ongoing and no final conclusion can be made. But the current results indicate that the diffusion reactor and oedometer can be used as a standard method to investigate threshold densities for buffer candidates. It was shown by batch microcosm experiments that the Calcigel bentonite is a good candidate to study the impact of dry density on microbial activity and corrosion because this bentonite has a low sulphate content and contains natural sulphate-reducing bacteria that can under suitable conditions contribute (production of sulphides) or prevent (consumption of formed hydrogen) the corrosion. It is expected, that the only form of microbial induced corrosion that needs to be considered is that due to the diffusion of aggressive metabolic by-products (invariably sulphide) to the canister surface from locations at which microbial activity is deemed to be possible. In accordance with this assumption, the compacted bentonite samples from UGR that were supplemented with SRB and IRB showed highest alteration of the metal surface (at 1.6 g/cm3). This shows that introduced microorganisms can affect the corrosion even in bentonite compacted at a high dry density. The non-irradiated compacted bentonite samples without additional bacteria showed that the metal surface was covered with copper oxides and the coupons had a low corrosion rate (studied in detail in Task 3). Results from UGR/CIEMAT also showed that the used porewater (see also Task 3) and the way of compaction (first saturated and then compacted or first compacted and then saturated) can have an influence on the corrosion. Project title: European Joint Programme on Radioactive Waste Management Project Type: European Joint Programme (EJP) EC grant agreement No. 847593 Work Package Title: CONtainer CORrosion under Disposal conditions