Gaseous carbonation of cementitious backfill for geological disposal of radioactive waste: Nirex Reference Vault Backfill

Highlights

• Carbonation of a potential backfill resulted in fully, partially and uncarbonated zones.

• Potassium (and to a lesser extent sodium) were concentrated in the carbonated region.

• Sulphur and aluminium were found to be enriched just ahead of the carbonation front.

• Carbonation impacted the porosity and mineral assemblage of the backfill material.

Abstract

The ability of Nirex Reference Vault Backfill (NRVB), a cement backfill material, to capture carbon dioxide from Intermediate Level Radioactive waste packages after repository backfilling, has been assessed. Large-scale trials assessed the physical and chemical reaction of carbon dioxide with the hardened backfill grout. A carbonation front, radial in nature, was observed extending into the grout and three distinct regions were identified in the hardened grouts. A carbonated region, a carbonation front, and a partially carbonated zone were discerned. Potassium, and to a lesser extent sodium, were concentrated in the carbonated region just behind of the main reaction front. The area just ahead of the carbonation front was enriched in both sulphur and aluminium, while sulphur was found to be depleted from the carbonated material behind the main reaction front. Within the main carbonated region, virtually all of the hydrated cement phases were found to be carbonated, and carbonation extended throughout the grout, even within material indicated by phenolphthalein solution to be uncarbonated. Importantly, carbonation was observed to impact both the mineral assemblage and porosity of the cement backfill; it is therefore important to understand these characteristics in terms of the long term evolution of NRVB and its groundwater buffering safety function within the geological disposal facility near-field.

Introduction

Geological disposal in an engineered facility underground is the UK Government's policy for disposal of higher activity radioactive waste. (Department of Energy and Climate Change, 2014). Such geological disposal facilities (GDFs) or repositories are based on the use of a multi barrier containment approach, which involves the application of engineered barriers, working in combination with natural geological features, to reduce the rate of radionuclide release to the biosphere. In the UK, an illustrative concept for disposal of Intermediate Level Waste (ILW) in a fractured crystalline rock (e.g. granitic), is that packages of grouted waste will be emplaced in sub-surface vaults and surrounded with a Portland cement-based backfill called Nirex Reference Vault Backfill (NRVB) (Pusch et al., 2017).

The bulk of gaseous emissions from ILW packages are expected to be hydrogen (H₂), mainly produced from the corrosion of metallic waste products and methane (CH₄) and carbon dioxide (CO₂), produced via the microbial degradation of organic waste materials under anaerobic or aerobic conditions (Radioactive Waste Management Limited, 2016a; Amec Foster Wheeler, 2017). A small proportion of the gas produced would include tritium (³H), ¹⁴C species (including ¹⁴CH₄ and ¹⁴CO₂) and radon (Rn-222). It is desired that, after backfilling, either the cementitious material in the waste packages, or in the backfill, would capture any waste CO₂ (including ¹⁴CO₂), thus retarding its egress to the geosphere (Hoch et al., 2016).

In ordinary Portland cements used for construction purposes, CO₂ from the atmosphere diffuses through gas-filled pores and dissolves into the pore solution forming aqueous HCO₃⁻. The uptake of acidic CO₂ into the alkaline pore solution reduces the internal pH of the binder, and the dissolved carbonate also reacts with calcium-rich hydration products present in the matrix, mainly with portlandite (Ca(OH)₂), calcium silicate hydrate (C–S–H¹) and the various calcium aluminate hydrates present, to form solid calcium carbonates, silica gel and hydrated aluminium and iron oxides (Johannesson and Utgenannt, 2001; Živica and Bajza, 2001; Fernández-Bertos et al., 2004).

The effect of carbonation on the mineralogy and porosity, are the two most important characteristics that influence the ability of a cementitious backfill grout to buffer groundwater to high pH (as desired to retard release of radionuclides to the geosphere), (cf. Nuclear Decommissioning Authority. 2010; Wilson et al., 2017), and the effect has not been fully elucidated. The reaction of CO₂ in the gas phase with typical waste encapsulation grouts and NRVB, in unsaturated conditions, has previously been studied in laboratory experiments e.g. Harris et al., (2003a; 2003b) and; Sun (2010) Carbonation was considered to be associated principally with the portlandite and C–S–H components, facilitated by the dissolution of CO₂ into films of water condensing on the cement phases and consequently, the reaction rate is strongly influenced by relative humidity (Bamforth et al., (2012), More recently, experimental studies simulating saturated repository environments have been undertaken to examine the effect of carbonation on gas-transport properties of NRVB (Rochelle et al., 2013; Purser et al., 2015). There has also been significant interest in studying cement carbonation in deep saline brine groundwater environments to evaluate the performance of cementitious well seals in relation to the geological sequestration of CO₂ (e.g. Kutchko et al., 2007; Rochelle et al., 2009; Wilson et al., 2011; Rochelle and Milodowski, 2013). Natural analogue studies provide further insights into the long-term effects of carbonation on cementitious materials (Milodowski et al., 2011; Pitty and Alexander, 2011), and indicate that both the processes and degree of carbonation depend upon the geological environment and the partial pressure of CO₂ in the groundwater (Bamforth et al., 2012). However further information is required to understand this degradation mechanism in the context of the post-closure performance of a GDF. We report here on detailed experimental studies investigating the reaction of gaseous CO₂ with hardened NRVB to support understanding of the backfill material following closure of the GDF.

It is important to revisit the chemistry and engineering performance of NRVB at this time, as the understanding of its role and function within a GDF have evolved since its initial formulation as the UK opens its siting process for such a facility.