The redox behavior of uranium on Beishan granite: Effect of Fe²⁺ and Fe³⁺ content

Highlights

• Beishan granite contains Fe²⁺-bearing fluorannite.

• U(VI) is partially reduced by the Fe²⁺ contained in granite.

• UO₂ is partially oxidized by the minor Fe³⁺ contained in granite.

• Aliovalent oxides (U₃O₈, U₄O₉, etc.) are the most thermodynamically stable phase in granite.

Abstract

The Beishan granitic area in Gansu Province is a site with the greatest potential for a repository of high-level radioactive waste (HLW) in China. In this study, the redox behavior of uranium on Beishan granite was investigated at pH values from ~4.4 to ~9.2. Due to the presence of Fe²⁺-containing fluorannite, results showed that U(VI) was partially reduced by the granites from boreholes 2 (486 m) and 28 (670 m) at a relatively low initial pH whether Na₂CO₃/NaCl or native groundwater was used as a background electrolyte. Partial oxidation of UO₂ was observed when UO₂ contacted Beishan granite directly. Therefore, this incomplete reduction of U(VI) was mainly attributed to minor Fe³⁺ that was either originally contained in the granite or generated during U(VI) reduction. Consequently, aliovalent oxides (e.g., U₃O₈, U₃O₇, U₄O₉, etc.) should be the thermodynamically stable phase in Beishan granite. A mechanism involving the dissolution of Fe²⁺ from the granite structure followed by interfacial adsorption/reaction was proposed for the U(VI) reduction. This study demonstrates that Beishan granite has a good reducing capacity, which is suitable for the immobilization of redox-sensitive radionuclides. However, potential oxidation of spent fuel by Fe³⁺ in the granite should also been taken into account.

Introduction

Currently, deep geological disposal based on a “multibarrier system” concept is a widely accepted way to permanently isolate the high-level radioactive waste (HLW). The Beishan granitic area located in Gansu Province in northwestern China was preselected as a key research site for a HLW repository (China Atomic Energy Authority (CAEA) et al., 2006). The construction of an underground HLW research laboratory (URL) was further proposed in the 13th Five-year Plan (2016–2020) by the Chinese government (The Ministry of Environmental Protection (NNSA) et al., 2017).

The prevalent reactor type in China is the light water reactor. Nevertheless, the Qinshan Nuclear Power Plant (NPP) has adopted the Canadian Deuterium Uranium (CANDU) type, the spent fuel from which, containing only 0.3% ²³⁵U at the end of the fuel cycle, is normally required to be disposed directly as HLW without reprocessing. Because of its ingrown daughter nuclides from the decay series, uranium shows a high metal toxicity and radiation hazard. In addition to the unburned uranium, there are also a great variety of high-level radioactive nuclides (e.g., plutonium, minor actinides and fission products). Oxidative damage of the spent fuel will lead to the leaching of these radionuclides and consequently severe environmental problems. Therefore, understanding the geochemical behavior of uranium under Beishan granitic conditions is essential for the performance assessment of the repository.

Uranium is redox-sensitive, and its solubility and mobility are largely controlled by its oxidation state, which in turn depends on the redox conditions of the surrounding environment. While tetravalent UO₂ or hyperstoichiometric oxides, e.g., U₄O₉, U₃O₇, or U₃O₈, are nearly nonsoluble from weakly acidic to alkaline conditions, hexavalent uranium (UO₂²⁺) can form strong complexes with many inorganic ligands, in particular carbonate, leading to a high mobility in the groundwater-rock system (Dong and Brooks, 2006). Fe(II)-bearing minerals are ubiquitous in the geologic environment, and thus, the abiotic reduction of U(VI) by Fe(II) may be significant in sediments, groundwater, or other geological media associated with nuclear waste disposal. Numerous studies have been performed to investigate the reductive immobilizations of U(VI) by Fe(II)-bearing minerals, such as pyrite (Bruggeman and Maes, 2010; Descostes et al., 2010; Liu et al., 2016; Scott et al., 2007; Yang et al., 2014), mackinawite (Gallegos et al., 2013; Hua and Deng, 2008; Hyun et al., 2012; Troyer et al., 2014; Veeramani et al., 2013), and magnetite (Latta et al., 2013; Rovira et al., 2007; Scott et al., 2005; Skomurski et al., 2011; Veeramani et al., 2011; Yuan et al., 2015). In contrast to the thermodynamic predictions, incomplete reduction has commonly been observed (Bruggeman and Maes, 2010; Descostes et al., 2010; Rovira et al., 2007; Scott et al., 2005, 2007; Yang et al., 2014). Nevertheless, possible reasons remain poorly known.

The reducing properties of the host rock play a profound role in the long-term safety of HLW repositories. Waste repositories require alkaline and reducing conditions, but in the case of groundwater invasion, the pH may change due to exothermal radwaste or the corrosion of engineering barriers. In the case of the repository failure in the far future, faults and fractured zones in the granite will become the main pathway for radionuclide transportation. The corresponding chemical environment can be largely influenced by the granite matrix. Previous studies have investigated the adsorption behavior of U(VI) on Beishan granite. Fan et al. (2014) suggested that the adsorption of U(VI) by Beishan granite was mainly controlled by ion exchange and outer-sphere complexation at low pH, whereas inner-sphere complexes were prevalent in the pH range of 4.0–9.0. Jin et al. (2016) investigated U(VI) adsorption on granite at both ambient and elevated temperatures and proposed a generalized composite model with three surface complexes, ≡SOUO₂⁺, ≡SO(UO₂)²(OH)₂⁺ and ≡SO(UO₂)₃(OH)₅⁺, to explain and further predict the adsorption behavior. Although coupled sorption and reduction of aqueous U(VI) by ferrous micas (Ilton et al., 2004, 2005) or by intact granite drill cores (Berichte, 2005) has long been recognized, the corresponding reducing and oxidizing capacities of Beishan granite toward U(VI) and UO₂ (the matrix material of spent fuel) remain unknown. Studies on the redox processes of other radionuclides (e.g., ⁷⁵Se) have also been reported (He et al., 2017; Yang et al., 2017). In reality, the mobility of many redox-sensitive radionuclides (⁹⁹Tc, ⁷⁹Se, etc.) is much attenuated under reducing conditions (Huber et al., 2017; Kang et al., 2013). However, the redox behavior of redox-sensitive radionuclides on Beishan granite remains to be well addressed.

For the Beishan URL, a 50-m-deep exploration tunnel has been constructed so far at the Jiujing site to validate and develop construction technologies as well as to perform some in situ tests for the Beishan URL. Following this investigation, the Xinchang site in Beishan has been selected for China's first URL built in granite (Wang et al., 2018). In this context, representative granite drill cores were collected from these two sites at different depths of various boreholes, and batch experiments were performed to investigate the redox behavior of uranium on the granites at various pH values. Our results will provide valuable data for the site selection and safety assessment of China's HLW repository.