Speciation of iron(II/III) at the iron-cement interface: a review.

Steel is used as reinforcement in construction materials and it is also an important component of cement-stabilized waste materials to be disposed of in deep geological repositories for radioactive waste. Steel corrosion releases dissolved Fe(II/III) species that can form corrosion products on the steel surface or interact with cementitious materials at the iron-cement interface. The thermodynamically stable Fe species in the given conditions may diffuse further into the adjacent, porous cement matrix and react with individual cement phases. Thus, the retention of Fe(II/III) by the hydrate assemblage of cement paste is an important process affecting the diffusive transport of the aqueous species into the cementitious materials. The diffusion of aqueous Fe(II/III) species from the steel surface into the adjacent cementitious material coupled with the kinetically controlled formation of iron corrosion products, such as by Fe(II) oxidation, decisively determines the extension of the corrosion front. This review summarises the state-of-the art knowledge on the interaction of ferrous and ferric iron with cement phases based on a literature survey and provides new insights and proper perspectives for future study on interaction systems of iron and cement.

Carbon-14 release and speciation during corrosion of irradiated steel under radioactive waste disposal conditions.

Carbon-14 is a key radionuclide in the safety assessment of deep geological repositories (DGR) for low- and intermediate-level radioactive waste (L/ILW). Irradiated metallic wastes generated during the decommissioning of nuclear power plants are an important source of 14C after their disposal in a DGR. The chemical form of 14C released from the irradiated metallic wastes determines the pathway of migration from the DGR into the environment. In a long-term corrosion experiment with irradiated steel simulating the hyper-alkaline, anoxic conditions of a cement-based DGR, total inorganic (TI14C2) and organic 14C contents (TO14C) in the liquid and gas phases (TG14C), as well as individual 14C-bearing carbon compounds by compound-specific radiocarbon analysis (CSRA), were quantified using accelerator mass spectrometry (AMS). The AMS-based quantification allows the determination of 14C in the pico- to femtomolar concentration range. An initial increase in TO14C was observed, which could be attributed partially to the release of 14C-bearing oxygenated carbon compounds. In the long term, TO14C and the TI14C remain constant, while TG14C increases over time according to a corrosion rate of steel of 1 nm/yr. In solution, 14C-bearing carboxylic acids (CAs) contribute ~40% to TO14C, and they are the main 14C carriers along with 14C-bearing carbonate (14CO32-). The remaining fraction of TO14C (~ 60%) is likely due to the presence of as yet non-identified polymeric or colloidal organic material. In the gas phase, 14CH4 accounts for more than 80% of the TG14C, while only trace amounts of 14CO, and other small 14C-bearing hydrocarbons have been detected. In a DGR, the release of 14C will be mainly in gaseous form and migrate via the gas pathway from the repository near field to the surrounding host rock and eventually to the environment.

Processes leading to reduced and oxidised carbon compounds during corrosion of zero-valent iron in alkaline anoxic conditions.

The Swiss disposal concept foresees that carbon-14 (14C) is predominantly released from irradiated steel disposed of in a cement-based repository for low- and intermediate-level radioactive waste. To predict how 14C migrates in the cementitious environment of the repository near field and subsequently in the host rock, knowledge about the carbon speciation during anoxic steel corrosion in alkaline conditions is therefore essential. To this end, batch-type corrosion experiments with carbon-containing zero-valent iron (ZVI) powders subject to oxidative pre-treatments were carried out in NaOH solution at pH 11 and 12.5. Alkanes and alkenes (C1-C7) were identified in the gas phase and produced on the iron surface by a Fischer-Tropsch type mechanism. The kind of oxidative pre-treatment has an effect on the production rate of hydrocarbons (HCs). In the liquid phase, carboxylic acids were identified and produced during the oxidative pre-treatment of the ZVI powders. They are released instantaneously from the oxide layer upon contact with the alkaline solution. The kind of oxidative treatment and the exposure time to oxic conditions directly influence the amount of carboxylic acids accommodated in the oxide layer.

Determination of the ¹4C content in activated steel components from a neutron spallation source and a nuclear power plant.

The (14)C content in activated steel components from the Swiss Nuclear Power Plant (NPP) Gösgen and the Spallation Neutron Source SINQ at the Paul Scherrer Institute is determined using a wet chemistry digestion technique and liquid scintillation counting for (14)C activity measurements. The (14)C activity of an activated fuel assembly steel nut from the NPP is further compared with theoretical predictions made on the basis of a Monte Carlo reactor model for this NPP. Knowledge of the (14)C inventory in these activated steel materials is important in conjunction with future corrosion studies on these materials aimed at identifying the (14)C containing organic compounds possibly formed in the cement-based near field of a repository for radioactive waste.