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

Highlights

• Sorption mechanisms of redox-sensitive RNs on geochemical barriers are reviewed.

• Reductive immobilization is an efficient radionuclide removal pathway.

• Fe and S bearing minerals act as key reductants in repositories.

• Non-redox sorption dominates radionuclide uptake on cement and Fe-free clays.

• Minerals with reducing power will create potential reactive interfaces.

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 ¹²⁹I, ³⁶Cl, ^{235, 238}U, ⁷⁹Se, and ⁹⁹Tc. Among them, ^{235, 238}U, ⁷⁹Se, ⁹⁹Mo, and ¹²⁵Sb 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 H₂), 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 FeSe₂ for Se or UO₂ 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.

Introduction

Geological disposal was defined in a 1995 Collective Opinion of the Nuclear Energy Agency (NEA) Radioactive Waste Management Committee document entitled “The Environmental and Ethical Basis of Geological Disposal” (Vuori, 1995). Nowadays, deep geological disposal is the widely preferred solution for the final disposal of intermediate-level long-lived (ILW-LL) and high-level waste (HLW), at 250 m–1000 m depth in mined repositories. In contrast, intermediate-level short-lived (ILW-SL) and low-level waste (LLW) will be stored in near-surface repositories at ground level, or in caverns below ground level (at tens of meters depth).

The International Atomic Energy Agency (IAEA) estimates that low- and intermediate-level radioactive waste has been worldwide generated to a cumulative volume of 7.3 × 10⁶ m³, and the produced cumulative volume of HLW reached 8.3 × 10⁵ m³ until the early 2000s (IAEA, 2008). However, as of 31 December 2013, 0% of HLW is present in disposal sites, while in contrast ∼80% of LL/IL-SLW and ∼20% of ILW is stored in disposal sites (IAEA, 2018). Thus, the disposal of HLW is required, and a sustainable way for LLW and ILW disposal should be also found in the long term.

Mined repositories constitute the most widely accepted deep geological disposal concept, generally relying on a multi-barrier system to isolate the waste from the hydrosphere and thus from the biosphere over 10,000 years. In order to ensure that the waste would be successfully trapped, the multiple barriers are typically designed to comprise the engineered barrier system (EBS) and repository host rock (e.g., Callovo-Oxfordian clay (Grambow, 2016), Boom clay (Deng et al., 2011), or Opalinus clay (Amann et al., 2017), granite (Krishnamoorthy et al., 1992), welded volcanic tuff rocks (Price and Bauer, 1985), and layered salt strata or domes). Generally, the EBS, from inner to outer levels, includes canisters/casks made of metal (e.g., iron or copper) or concrete containing the radioactive waste (being vitrified or not), cement or clay (e.g., bentonite) buffers and/or backfill materials. The choice of waste container materials and design, as well as the buffer/backfill material depends on the type of waste to be contained and on the nature of the surrounding host rock-type.

Generally, two fundamental prerequisites for deep geological disposal are (1) stable geological formations and (2) stable human institutions in the long timescales. In addition, disposal sites with minimum possibility of deep groundwater intrusion are preferred, in order to minimize the possibility of chemical mobilization of waste. Except for limited exceptions, such as the case of the Nordic countries, after a relatively short initial oxic period by oxygen occluded in closed repositories, a long-lasting reducing environment in the near field should be maintained by the limited oxygen underground and the reduced minerals (e.g., sulfides and ferrous minerals) and engineered barriers (e.g., iron/copper canisters and steel reinforcements). The reducing conditions are crucial to help to retain the mobile RNs via reductive precipitations.

A number of countries have already started planning/building their own HLW repository. China has announced a national long-term plan for the management of HLW, and Beishan granitic area in Gansu province was pre-selected as a key research area for the waste repository. France, as the world's largest nuclear power exporter, has 58 nuclear power reactors, and the country energy portfolio consists on about 80% of nuclear electricity. The French national radioactive waste disposal agency, Andra, is designing a deep geological repository, named Cigéo, in a clay-rock formation at Bure, in eastern France. The targeted wastes that will be disposed are vitrified HLW and ILW-LL encapsulated in inert materials (concrete). With the 150-million-year-old Callovo-Oxfordian argillaceous rock as the host rock, the disposal concept in France is to use glass (for HLW), concrete (for ILW-LL), and stainless steel casks as the EBS. Huge amount of cementitious materials are used for ILW-LL disposal in France. The specially formulated cement backfill material would provide a long-lasting alkaline environment that contributes to containment of the waste by preventing many radionuclides from dissolving in the groundwater. Similar cement-based schemes for ILW-LL disposal have been proposed in Switzerland, Czech Republic, Finland, Sweden, Germany, UK, and the USA (IAEA, 2017; IAEA, 2018). In the repositories, reinforced cementitious materials are used for tunnel foundations and backfill, waste containers and waste matrices, which are considered as barriers that inhibit the mobility of RNs in case of eventual leakage.

After a relatively short initial oxic period (shorter than 5 years in the case of ILW disposal (Duro et al., 2014)) controlled by oxygen trapped during repository building, the redox potential (Eh) will become reducing. This will be imposed by the anoxic corrosion of steel, hydrogen production (maximum pressure range in the waste canister: 70–100 bars (Truche et al., 2009)), and dissolution of reduced minerals (e.g, ferrous-bearing minerals and sulfide/disulfide minerals) (Duro et al., 2014). H₂ is potentially an electron donor for oxidized RNs species present at the site (Truche et al., 2010), although its reduction efficiency is still not clearly known due to the chemical inertness without specific catalyzers. In addition, the complex compositions of steel corrosion interfaces and the contribution of reduced mineral species are still unclear, which makes that the actual Eh value prevailing in the alveoli remains undetermined.

The mobility of radionuclides is a key issue regarding the safety assessment of nuclear waste repositories. RN mobility is governed by the geochemical interaction with each of the barriers present in the system. To a large extent, the extremely low mobility of the most radiotoxic RNs, the actinides, increases the reliability of long-term safety of deep nuclear waste repositories. However, according to the leakage scenario, a few RNs, mainly anionic species like ¹²⁹I, ³⁶Cl, ⁷⁹Se, and ⁹⁹Tc, would diffuse fast and contribute to the ultimate radioactive exposure risks to the biosphere (Chen et al., 1999; Grambow, 2008). Besides, in the spent fuel, 96% of the mass is formed by remaining Uranium (U), i.e., most of the original ²³⁸U and less than 0.83 wt% ²³⁵U. Thus, uranium migration behavior in multi-barrier system is also critical when evaluating nuclear waste disposal options.

Among the widely concerned RNs, U isotopes, ⁷⁹Se, and ⁹⁹Tc are redox sensitive. Under Eh-pH conditions typical of oxidative alteration of spent nuclear fuel, oxidized RN species, such as U(VI), Se(IV)/Se(VI), and Tc(VII), are the dominant aqueous species (Chen et al., 1999, 2000). Generally, these aqueous RN species are highly mobile under alkaline conditions, due to their higher solubility and relatively lower adsorption affinity to the barrier materials, compared to their reduced species. Thus, reductive immobilization can be considered as a very important pathway to significantly reduce the mobility of redox-sensitive RNs, which can be reduced into insoluble species with lower oxidation states.

In this review paper, we aim to give an overview of proposed sorption mechanisms of redox-sensitive RNs on potential barriers existing in geological nuclear waste repositories, and to give a detailed scientific understanding of the chemical form of the solid phases associated to the RNs.