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The long-term and safe storage of high-level radioactive waste (HLW) in general, remains a challenge world-wide. The deep geological repository (DGR) is a concept of applying multi barriers to store the HLW and to ensure the long-term safety. Bentonite is proposed as a potential material for sealing the space between the canister containing the HLW and the surrounding host rock. To investigate the microbial diversity and metabolic activity of naturally occurring microorganisms in Bavarian bentonite B25 as well as their influence on the used canister materials (copper & cast iron), we conducted anaerobic microcosm experiments, incubating Bavarian bentonite B25, synthetic pore waters (synthetic Opalinus Clay pore water or diluted cap rock solution) and metal plates (copper or cast iron coupons) up to 400 days at 37 °C. We also added H2 or lactate to stimulate microbial activities in certain microcosms. Geochemical analyses showed a decrease of Eh, potassium and sulfate as well as an increase of Fe(II) in microcosms that contained synthetic Opalinus Clay pore water, H2 and cast iron. Statistical analysis indicated that these observations have no significant correlation in shaping microbial communities in the respective microcosms. Moreover, SEM images provided strong evidence that no clear corrosion on cast iron and copper was observed that is due to microbial activity. Overall, this study shows that B25 bentonite may be an ideal barrier material for a DGR due to its negative microbial effects, which prolong the entire lifespan of a DGR.

1 Introduction Bentonite is considered as buffer and sealing material in a multi-barrier system for a deep geologic repositories (DGR) of nuclear waste and spent fuel [1]. Another part of the engineered barrier system is the containment of the radioactive waste. Cast iron is often taking into account for the construction of the containers as a candidate material [2]. But the cast iron components are fairly unstable, can corrode to insoluble corrosion products and react with the bentonite buffer matrix. Anaerobic corrosion together with microbially influenced corrosion are dominant forms of corrosion in the a DGR and the interactions at the metal/bentonite interface determines the performance of bentonite-based radioactive waste barriers [3]. The aim of the current study was to characterize the surface damage associated with corrosion of the cast iron and to compare the potential of the indigenous microorganisms present in different bentonites to influence the corrosion of cast iron. 2 Results Three types of bentonite (B25, Calcigel, MX-80) were chosen for mesocosm-experiment setup as described in [4]. All three bentonites have different smectite content and an indigenous microbial community. The mesocosms with cast iron coupons, artificial Opalinus clay porewater and bentonite were incubated in N2/CO2 atmosphere for 271 days at 30 °C. Some of the mesocosms were supplemented with 5 mM sodium lactate and hydrogen (to a 0.5 bar of total pressure) to stimulate microbial activity. After the incubation period the content of the mesocosms was divided and subjected to different analysis, including geochemical analysis (as e.g. ICP-MS, ion and high-performance liquid chromatography), DNA isolation and amplification of the intergenic spacer to determine the microbial community structure, SEM-EDX and RAMAN spectroscopy to characterize the surface damage of the cast iron coupons. The black precipitates were visible in the mesocosms containing Calcigel with lactate as substrate and for all the substrate-containing samples with MX-80. The obtained geochemical data confirmed the differences in the different microcosms by demonstrating unequal levels of sulphate and lactate consumption. Moreover, surface analysis of the cast iron coupons showed that corrosion rate and metabolite accumulation are also dependent on the bentonite type. In addition different microbial community structure was observered by intergenic spacer analysis (RISA) depending on the conditions applied and used bentonite. Therefore, the used bentonites varied in respect to reactivity and microbial activity. Overall, the results show the importance of selection of suitable bentonite for DGR to adjust microbial implications and possibly faster corrosion rate of the metal containers. We acknowledge funding by the BMBF (Grant 02NUK053B) and HGF (Grant SO-093). References [1] P. Sellin and etc., The Use of Clay as an Engineered Barrier in Radioactive-Waste Management – A Review, Clays and Clay Minerals 61(6), pp. 477-498 (2014). [2] F. King. Container Materials for the Storage and Disposal of Nuclear Waste, Corrosion 69(10), pp. 986-1011 (2013). [3] S. Kaufhold and etc. About the Corrosion Mechanism of Metal Iron in Contact with Bentonite, ACS Earth Space Chem. 4, 5, pp. 711–721 (2020). [4] N. Matschiavelli and etc., The Year-Long Development of Microorganisms in Uncompacted Bavarian Bentonite Slurries at 30 and 60 °C, Environ. Sci. Technol., 53, 17, 10514–10524 (2019)

Copper and cast iron are potential materials for the storage canisters of high-level radioactive waste. We designed slurry-experiments for analyzing the microbial influence on the corrosion process of these metals. These slurry experiments contain the Bavarian B25 bentonite, synthetic Opalinus Clay pore water or diluted cap rock solution as well as copper- or cast iron plates in various combinations. During an incubation time of 400 days under anaerobic conditions at 37 °C cast iron plates corrode very fast. The respective metal surfaces show the formation of iron oxides and –carbonates which could form a passivating film that protects the cast iron from further corrosion.

Every year 12,000 metric tonnes of high-level radioactive waste (HLW) are produced worldwide. For the long-term storage of this highly radiotoxic waste, a deep geological disposal by using multiple barriers is favored. Bentonite is proposed as a potential material for sealing the space between the canister containing the HLW and the surrounding host rock. In order to investigate the microbial diversity and metabolic activity of naturally occurring microorganisms as well as their time-dependent evolution, we conducted anaerobic microcosm experiments containing bentonite and a synthetic Opalinus Clay pore water solution. During the one-year incubation at 30 and 60 °C, lactate- or H2-stimulated microcosms at 30 °C showed the dominance and activity of strictly anaerobic, sulfate-reducing and spore-forming microorganisms. The subsequent generation of hydrogen sulfide gas in the respective set ups, led to the formation of fractures and iron-sulfur precipitations. In microcosms that incubated at 60 °C, thermophilic bacteria dominated, independent from the availability of substrates. In the respective microcosms, no significant metabolic activity was detected and there was no change in the analyzed bio-geochemical parameters. Our results show that indigenous microorganisms evolve in a temperature- and substrate-dependent manner. Potentially formed metabolites could affect the dissolution behavior of minerals and ions within the bentonite as well as the corrosion process of the canister material and require further investigations.

We show how to formulate two-point boundary value problems to compute laminar channel, tube, and Taylor-Couette flow profiles for some complex viscoelastic fluid models of differential type. The models examined herein are the Pom-Pom Model [McLeish and Larson 42:81-110, (1998)] the Pompon Model [Öttinger 40:317-321, (2001)] and the Two Coupled Maxwell Modes Model (Beris and Edwards 1994). For the two-mode Upper-Convected Maxwell Model, we calculate analytical solutions for the three flow geometries and use the solutions to validate the numerical methodology. We illustrate how to calculate the velocity, pressure, conformation tensor, backbone orientation tensor, backbone stretch, and extra stress profiles for various models. For the Pom-Pom Model, we find that the two-point boundary value problem is numerically unstable, which is due to the aphysical non-monotonic shear stress vs shear rate prediction of the model. For the other two models, we compute laminar flow profiles over a wide range of pressure drops and inner cylinder velocities. The volumetric flow rate and the nonlinear viscoelastic material properties on the boundaries of the flow geometries are determined as functions of the applied pressure drop, allowing easy analysis of experimentally measurable quantities