A review of the retention mechanisms of redox-sensitive radionuclides in multi-barrier systems

Abstract The deep geological disposal concept is widely accepted by the scientific community for the storage of high activity level nuclear waste. It uses a multi-barrier system to isolate radioactive waste from the hydrosphere and biosphere for hundreds of centuries. The multiple barriers include, from the waste to near- and far-field: metal (e.g., iron or copper) and/or concrete canisters/casks containing the radioactive waste, cement, clay (e.g., smectites) buffer and/or backfill materials, and naturally occurring host rocks (e.g., claystone and granite). The mobility of radionuclides (RNs) is a key issue regarding the safety assessment of nuclear waste repositories, especially for the soluble and mobile RNs, such as 129 I, 36 Cl, 235, 238 U, 79 Se, and 99 Tc. Among them, 235, 238 U, 79 Se, 99 Mo, and 125 Sb are some of the redox-sensitive RNs whose mobility largely depends on their speciation, i.e. , their oxidation states. The interactions of these redox-sensitive RNs with the components of each of the barriers is complex, and it needs to be fully understood for a correct safety assessment. Much progress has been made in the recent years in getting a fundamental understanding of the migration of redox-sensitive RNs in geological media. Here, an overview of the major achievements is presented. In general, the electron donors in repositories can be steel (i.e., zero valent iron), steel reaction products when in contact with groundwater (producing H 2 ), steel corrosion products (e.g., magnetite, green rust, and ferrous oxyhydroxides), Fe(II)-sulfides (e.g., pyrite, chalcopyrite, and mackinawite), Fe(II)-bearing clays (e.g., Fe-bearing smectites) in claystone, or Fe(II)-bearing mica minerals (e.g., biotite) in granites. All these phases can sorb redox-sensitive RNs and drive reductive immobilization processes. For U and all the redox-sensitive RN oxyanions, the resulting reduced products are the most stable and least soluble phases, such as FeSe 2 for Se or UO 2 for U, which may be favorably formed in the presence of a high solubility of electron donors and fast reaction kinetics. However, with slow reaction kinetics, metastable reduced mixed species in intermediate oxidation states such as Se(0) and hyperstoichiometric uranium oxides are produced. In order to characterize the RN-bearing phases and the uptake and reduction pathway, powerful molecular-scale tools such as X-ray photoelectron spectroscopy (XPS) or X-ray absorption spectroscopy (XAS) are commonly used. Here, we provide a comprehensive perspective on the studies addressing interactions of redox-sensitive RNs with the above-mentioned potential barrier.