Corrosion processes at the GGG40 steel–bentonite interface

Spheroidal graphite cast iron (GGG40/0.7040) is used as an overpack material for the reference Pollux 10 container, a prototype for the storage of fuel elements in different types of host rocks (Hassel et al., 2019). Corrosion processes are expected to be triggered after some decades, as the contacting bentonite, which is used as fill material, becomes soggy because of the decay of the fission heat generation and the intrusion of pore water as the temperature decreases under the condensation point (King and Padovani, 2011). The distinct electrochemical reactivities of the graphite, ferrite, pearlite, and cementite phases exposed by this material introduce local galvanic elements which influence the topographic evolution of degradation (Spence, 2005). The aim of this work is to elucidate the corrosion dynamics of active and passive areas by following the chemical evolution at the interface of cast iron–bentonite during the first stages after its saturation with geological pore water. Polarization experiments and electrochemical impedance spectroscopy were applied to monitor the corrosion process in a bentonite cell, where GGG40 steel is put into contact with a light compacted Wyoming bentonite slurry 1 : 10 of bentonite to Opalinus Clay pore water (Fig. 1a). Experiments were performed at 30 and 50 ∘C for longer than 3 months. The surface chemistry and morphological changes were investigated by local XPS, SEM-EDX, and TEM (X-ray photoelectron spectroscopy, scanning electron microscopy with energy dispersive X-ray, and transmission electron microscopy, respectively). These experiments were complemented with corrosion studies performed in pore water under different temperatures, hydrostatic pressures, and pH and with and without dissolved oxygen. The polarization curves indicate a constant corrosion rate of GGG40 steel in saturated bentonite after 30 d. SEM micrographs reveal a preferential dissolution of the ferrite phase around the graphite spheres and the ferrite lamellae contained in the pearlitic eutectic. A strong localized dissolution of ferrite along the graphite boundary can be also observed; this is a typical case of crevice corrosion caused by the unhindered access of oxygen to the graphitic cathodic areas (Wang et al., 2022). The surface chemical studies indicate the accumulation of iron oxides, which can be attributed to a hydrated magnetite and the formation of iron silicates (Zhang et al., 2021). TEM pictures of a cross-sectional lamella, including part of graphite sphere, show the formation of a silicate film covering the corroding surface with an irregular adherence (Fig. 1b). The initial relatively high corrosion rate of GGG40 steel can be attributed to the dissolution of the more active ferrite with the formation of poorly passivating iron oxides and silicates. The system is driven towards the dissolution of pearlite at more positive electrode potentials. The bentonite slurry limits the access of oxygen to the graphitic cathodic areas, reducing the corrosion rate by 1 order of magnitude in comparison with that in aerated pore water. A surface enrichment of cementite with superior passive properties and the neutralization of the local elements by approaching the corrosion potential of graphite (Kadowaki et al., 2019) is also expected. Thus, the consumption of oxygen and the transport limitation of the cathodic reaction by bentonite forecast a considerable reduction in the degradation of the container after a sacrificial corrosion phase

The effect of temperature and fuel surface area on spent nuclear fuel dissolution kinetics under H2 atmosphere

In this work we present the results of two spent nuclear fuel leaching experiments in simulated granitic groundwater, saturated with hydrogen under various pressures. The results show a large impact of the dissolved hydrogen already at 1 bar H2 and room temperature on the release of both the uranium and of the fission products contained in the fuel matrix. Based on the results of this study and on published data with fuel from the same rod, the importance of the oxidative dissolution of spent fuel under repository conditions as compared to its non-oxidative dissolution is discussed. The XPS-spectra of the fuel surface before the tests and after long-term leaching under hydrogen are reported and compared to reduced UO2 and SIMFUEL surfaces. The overall conclusion is that in spite of the unavoidable air contamination, hydrogen pressures of 1 bar or higher counteract successfully the oxidative dissolution of the spent nuclear fuel. The stability of the 4d-element metallic particles during fuel leaching under such conditions is also discussed, based on data for their dissolution. The metallic particles are also stable under such conditions and are not expected to release their component metals during long-term fuel leaching.\ua0\ua9 2020 The Author

Corrosion of steel in contact with bentonite under conditions relevant for nuclear waste disposal

Carbon steel is considered by many countries as a potential canister material to encapsulate high-level nuclear waste (HLW) before final disposal in a deep geological repository. Depending on the repository concept, compacted bentonite may be used as backfill material. Groundwater will eventually migrate through the barriers and induce steel corrosion. Information on corrosion rates (i.e., service lifetime of the container) and corrosion mechanisms are of high importance for the Nuclear Disposal Safety Case. A low-alloyed carbon steel and a spring steel of comparable composition, though with higher Si content, were selected in this study. The higher Si content of the spring steel is expected to reduce substantially the corrosion rate by forming of a layer of iron silicates better protecting the surface than for the carbon steel. This hypothesis was tested by performing corrosion experiments in closed vessels under anoxic and water saturated conditions at room temperature (RT) and at 50°C. Coupons were polished and the MX-80 bentonite was pre-equilibrated with synthetic Grimsel pore water prior to use. After 3 months of reaction time, and cooling down to RT where necessary, pH and redox potential (Eh) were measured in-situ and the composition of ultra-centrifuged pore water determined by ICP-OES and IC. Coupons were analyzed by various techniques and corrosion rates were determined from weight loss measurements. Relatively comparable pH values were measured for the carbon steel (pH 8.40) and the spring steel (pH 8.14) at RT, and lower values were measured in experiments conducted at 50°C. Eh values were not much affected by temperature, but were significantly lower for the carbon steel (around -380 mV). The composition of the pore water in all experiments were relatively comparable. The corrosion rate for the spring steel was larger than for the carbon steel and values increased with temperature. No presence of corrosion products on the surface of both steel coupons could be detected by XRD analysis. However, morphological changes could be seen at the surface of both coupons by SEM, and a change in chemical composition of the exposed surfaces was evidenced by SEM-EDX analysis. Elemental compositions point to the presence of a thin layer of Fe-silicate covering the coupons. Complementary information on elemental composition and oxidation state of the new-formed mineral was provided by surface sensitive XPS analysis. Overall, the investigated spring steel is less corrosive than carbon steel under elevated temperature conditions while at room temperature the investigated carbon steel has more corrosion resistance compared to spring steel. The increase of corrosion rate with temperature agrees with reported studies and is expected to decrease with reaction time (e.g., doi.org/10.5006/1.3287691) owing to the development of a more compact alteration layer at the surface of coupons. This assumption is being investigated in experiments with longer reaction time and the comparison of results from both materials will enable to conclude whether Si present in the steel has a significant impact on the corrosion resistance

How well suited are current thermodynamic models to predict or interpret the composition of (Ba,Sr)SO₄ solid-solutions in geothermal scalings?

In this study, we report results of the analysis of a particularly interesting scaling sample from the geothermal plant in Neustadt-Glewe in northern Germany, which contained 19% Galena (PbS) and 81% of a heterogeneous assemblage of (Ba,Sr)SO₄ crystals with varying compositions, 0.15 < X$_{Ba}$ < 0.53. A main fraction of the sample (~56%) has a barite content of X$_{Ba}$ ≈ 0.32. We try to relate the solid composition of the (Ba,Sr)SO₄ solid-solution to the conditions at the geothermal plant concerning temperature, pressure, and solution composition, and discuss it with respect to the challenges in modelling the composition of (Ba,Sr)SO₄ solid-solutions on the basis of thermodynamic mixing models. We note that considerable uncertainties are related to the description of (Ba,Sr)SO₄ formation by means of thermodynamic models. The scaling composition observed in this study would be in line with endmember solubilities as predicted by the PhreeqC-Pitzer database for 70 °C and an interaction parameter, a0 = 1.6. According to such a model, the scaling heterogeneity would reflect bimodal precipitation behaviour due to various degrees of depletion of the brine with respect to X(Ba)($_{aq}$). Minor fluctuations in X(Ba)($_{aq}$): 0.0017 < X(Ba)($_{aq}$) < 0.0042 explain the full range of observed solid compositions. The choice especially of the interaction parameter seems to some extent arbitrary. This knowledge gap strongly limits the interpretation of (Ba,Sr) SO₄ compositions. Thus, it is not possible to distinguish between kinetic and thermodynamic effects on partitioning or to use the solid-solution composition to draw conclusions about the precipitation conditions (e.g. Temperature)

Mechanisms of selenium removal by partially oxidized magnetite nanoparticles for wastewater remediation

Magnetite nanoparticles are a promising cost-effective material for the remediation of polluted wastewaters. Due to their magnetic properties and their high adsorption and reduction potential, they are particularly suitable for the decontamination of oxyanion-forming contaminants, including the highly mobile selenium oxyanions sele-nite and selenate. However, little is known how the remediation efficiency of magnetite nanoparticles in field applications is affected by partial oxidation and the formation of magnetite/maghemite phases. Here we char-acterize the retention mechanisms and capacity of partially oxidized nanoparticulate magnetite for selenite and selenate in an oxic system at different pH conditions and ionic strengths. Data from adsorption experiments showed that retention of selenate is extremely limited except for acidic conditions and strongly influenced by competing chloride anions, indicating outer-sphere adsorption. By contrast, although selenite adsorption ca-pacity of oxidized magnetite is also adversely affected by increasing pH, considerable selenite quantities are retained even at alkaline conditions. Using spectroscopic analyses (XPS, XAFS), both mononuclear edge-sharing (2E) and binuclear corner-sharing (2C) inner-sphere selenite surface complexes were detected, while reduction to Se(0) or Se(–II) species could be excluded. Under favourable adsorption conditions, up to ~pH 8, the affinity of selenite to form 2C surface complexes is higher, whereas at alkaline pH values and less favourable adsorption conditions 2E complexes become more dominant. Our results demonstrate that magnetite can be used as a suitable reactant for the immobilization of selenite in remediation applications, even under (sub)oxic conditions and without the involvement of reduction processes

241^{241}Am Migration in a Sandy Aquifer Studied by Long-Term Column Experiments

The migration behavior of $^{241}$Am(III) in a sandy aquifer was studied under near-natural conditions by long-term column experiments of more than 1 year duration. Columns with 50 cm length and 5 cm in diameter were packed with aeolian quartz sand and equilibrated with two different groundwaters having an original dissolved organic carbon concentration (DOC) of 1.1 and 7.2 mg·dm$^{-3}$, respectively, from the Gorleben site (Lower Saxony, Germany). In each experiment, 1 cm$^{3}$ of Am-spiked groundwater ([Am] = 0.2 to 2 μmol·dm$^{-3}$) was injected into the column. The flow rate of the groundwater was adjusted to 0.28 m·d$^{-1}$. A small colloid-borne Am fraction was found to elute together with tritiated water. After 414 and 559 days, respectively, the experiments were terminated. Whereas the nonsorbing tracer of tritiated water would have covered a distance of about 350 m in that time period, the maximum of the Am activity was detected between 32 and 40 mm column length. Applying selective dissolution analysis to the sand surface, Am was found to be preferentially bound to iron hydroxide/oxide sites. From this Am distribution, a retardation factor R of about 104 was determined and compared to static batch experiments. The Am breakthrough was calculated for the conditions of the column experiment

Design of bimetallic Au/Cu nanoparticles in ionic liquids: Synthesis and catalytic properties in 5‐(hydroxymethyl)furfural oxidation

In alloyed nanoparticles, synergistic electronic and/or geometric effects may enhance the catalytic properties compared to their monometallic counterparts. Herein, we address the synthesis of bimetallic Au/Cu nanoparticles with different compositions by wet chemical reduction in ionic liquids. The nanoparticles were successively supported on carbon. The ionic liquid could be recycled after synthesis. Annealing of the carbon-supported NPs at 400 °C led to NPs of the ordered intermetallic L1$_{0}$ AuCu phase. The nanoparticle-derived catalysts were characterized by X-ray diffraction analysis, transmission electron microscopy, X-ray photoelectron spectroscopy and optical emission spectroscopy with inductively coupled plasma. Oxidation of biomass-derived furans is a prominent process for biomass transformation into value-added chemicals. Herein, the oxidation of 5-hydroxymethyl-2-furfural (HMF) to 2,5-furandicarboxylic acid (FDCA) was chosen as a model reaction to evaluate the effect of Cu addition and intermetallic structure on the catalytic performance. Particularly Au/Cu nanoparticles with an Au/Cu ratio of 3 : 1 showed very high conversion to FDCA