Annual Report 2015 / Institute for Nuclear Waste Disposal. (KIT Scientific Reports ; 7725)

The contributions collected in this report provide a representative overview of the scientific outcome of INE research activities in 2015. The structure of the report follows widely the organization of the institute according to research topics: basic research towards understanding geochemical reactions of radionuclides on a molecular scale and applied studies on radionuclide retention in multi-barrier system under real repository conditions

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

Bentonite nanoparticle stability and the effect of fulvic acids: Experiments and modelling

In this study, the critical coagulation concentration (CCC) for FEBEX bentonite colloids is determined by colloid coagulation studies under variation of pH, electrolyte concentration, and fulvic acid (GoHy-573FA) content. For CaCl$_{2}$ electrolyte solution, a pH-independent Ca-CCC of 1 mmol L$^{-1}$ is found. In the case of NaCl background electrolyte, a pH-dependent Na-CCC can be determined with 15 ± 5 mmol L$^{-1}$ at pH 6, 20 ± 5 mmol L$^{-1}$ at pH 7, 200 ± 50 mmol L$^{-1}$ at pH 8, 250 ± 50 mmol L$^{-1}$ at pH 9, and 350 ± 100 mmol L$^{-1}$ at pH 10, respectively. The addition of 1 mg L$^{-1}$ dissolved organic carbon in the form of fulvic acid (FA) increases the Ca-CCC to 2 mmol L$^{-1}$. An association of FA with FEBEX bentonite colloids as surface coating can clearly be identified by scanning transmission X-ray microscopy (STXM). The experimental bentonite stability results are described by means of an extended DLVO (Derjaguin–Landau–Verwey–Overbeek) approach summing up hydration forces, short-range Born repulsion, van der Waals attraction, and electrical double layer repulsion. The measured zeta (ζ)-potential of the bentonite colloids is applied as platelet face electrokinetic potential and the edge electrokinetic potential is estimated by the combination of silica and alumina ζ-potential data in the ratio given by the FEBEX bentonite structural formula. Adjusting the montmorillonite face electrokinetic potential by a maximum of ±15.9 mV is sufficient to successfully reproduce the measured stability ratios. Due to the uncertainty in the ζ-potential measurement, only semiquantitative calculations of the stability ratio can be given

Radionuclide geochemistry: solubility and thermodynamics in a HLW repository

Deep geological disposal is the internationally favoured option to isolate high-level nuclear waste (HLW) from the biosphere and to minimise the potential radiological risk for future generations. Potentially contacting aqueous solutions such as groundwater may, however, lead to the corrosion of the solid casks containing the nuclear waste, and the formation of aqueous radionuclide systems in the near-field of the emplacement rooms. As dissolved species, radionuclides can in principle further migrate into the far-field and finally reach the biosphere on medium and long timescales. Like all chemical species, the radionuclides are subject to fundamental (geo)chemical laws. Relevant reactions that control retention and release, and hence, the migration behaviour and fate of radionuclides in a repository, are solubility equilibria, formation of soluble complexes, redox reactions, sorption on and incorporation into mineral surfaces, transport phenomena etc. These processes depend directly on the (geo)chemical boundary conditions, and, consequently, can differ greatly for various host rock systems such as clay rock, rock salt, and crystalline rock. Many of the radionuclides in HLW are heavy metals that are sparingly soluble under various repository-relevant conditions, e.g. actinides, lanthanides, transition metals, so that only partial dissolution (mobilisation) from the solid waste matrices is expected. This underlines the importance of evaluating the radionuclide solubility within a geochemically based safety assessment for repositories as it provides reliable upper-limit concentrations of the mobile, potentially migrating radionuclide fraction in the near-field. In this contribution, we discuss relevant aspects related to the topic radionuclide solubility and thermodynamics in a HLW repository. This includes a summary of recent laboratory studies on the solubility behaviour and speciation of key radionuclides in repository-relevant solutions, which are an important basis for obtaining (geo)chemical information and models, and the corresponding fundamental thermodynamic constants on aqueous radionuclide systems. National and international thermodynamic database projects, where quality-assured thermodynamic data (solubility products, complex formation constants, and ion-interaction parameters) are evaluated and compiled, e.g. the Nuclear Energy Agency Thermochemical Database (http://www.oecd-nea.org, last access: 1 November 2021) or the Thermodynamic Reference Database (http://www.thereda.de, last access: 1 November 2021), are highlighted and the main remaining uncertainties discussed. The experimental information and the quantitative thermodynamic data are applied within a generic case study to demonstrate the impact of different geochemical solution conditions representing different host rock systems considered as HLW repositories in Germany on the solubility and speciation of selected radionuclides

Annual Report 2014 / Institute for Nuclear Waste Disposal. (KIT Scientific Reports ; 7709)

The contributions collected in this report provide a representative overview of the scientific outcome of INE research activities in 2014. The structure of the report follows widely the organization of the institute according to research topics: basic research towards understanding geochemical reactions of radionuclides on a molecular scale and applied studies on radionuclide retention in multi-barrier system under real repository conditions