Alkali-activated materials for radionuclide immobilisation and the effect of precursor composition on Cs/Sr retention

Highlights

• Stoichiometrically controlled Ca-Si-Al slags were designed as precursors for AAMs.

• Very effective immobilisation of Cs⁺ (97.6%) and Sr²⁺ (99.9%) was achieved.

• Cs⁺ immobilisation increases with decreasing precursor Si/Al and Ca/(Si + Al) ratios.

• Cs⁺ is partitioned into an easily-leachable fraction and a strongly-bound fraction.

• Sr²⁺ immobilisation increases with decreasing precursor Ca/(Si + Al) ratio.

Abstract

One of the major challenges for the nuclear industry is the safe and sustainable immobilisation of radioactive wastes (RAW). Currently, the most commonly used immobilisation matrices for low and intermediate level wastes are based on ordinary Portland cement. For the more difficult to immobilise nuclides, such as caesium (Cs⁺) and strontium (Sr²⁺), researchers have been studying alternative immobilisation matrices, of which alkali-activated materials (AAM) are a very promising option. However, the differences in precursor compositions and the use of different types of activating solutions make it difficult to fully understand the effects of precursor composition on the immobilisation of introduced nuclides. Therefore, six different compositions of laboratory-synthesised Ca-Si-Al slags were developed to serve as precursors for low-alkaline AAMs to study their immobilisation behaviour. Immobilisation capacities up to 97.6% Cs⁺ and 99.9% Sr²⁺ were achieved with 1 wt% and 0.1 wt% waste loading, respectively, when leaching for 7 days at 20 °C in Milli Q water. Cs⁺ immobilisation is higher at lower Si/Al and Ca/(Si + Al) ratios. Immobilisation of Sr²⁺ is higher at a lower Ca/(Si + Al) ratio and independent of Si/Al ratio. The results of this study offer a deeper understanding of the immobilisation behaviour of AAMs and can encourage further research and application of AAMs for RAW immobilisation.

Introduction

In the pursuit of a sustainable society, one of the main challenges for researchers and industries is the safe management and disposal of industrial wastes, with radioactive waste (RAW) being particularly important. RAW is produced by many different sources; the main ones being the energy sector (nuclear fuel cycle), the dismantling of nuclear installations, and applications in medicine, agriculture and industry. Because of the ever-increasing amounts of RAW, continuous innovation in immobilisation is becoming more and more important. As an alternative to the currently widely used cementitious immobilisation matrices, alkali-activated materials (AAM) have been increasingly studied. Depending on the type of precursor and the composition of the hydration products, AAMs (or subclasses thereof) are also known as geopolymers, inorganic polymers, soil cements, geocements, alkaline cements, zeoceramics, alkali-activated slag cement and a variety of other names [1,2]. AAMs have demonstrated promising results in immobilising radionuclides such as caesium and strontium [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]]. As an example, the superior Cs⁺ retention of AAMs was reported by Shi and Fernández-Jiménez (2006) [9], who tested leaching of Cs⁺ and Sr²⁺ from AAMs containing zeolites and/or metakaolin (MK) as additives. They concluded that wastes show much less interference with the hydration of AAMs than that of ordinary Portland cement. Blackford et al. (2007) [11] developed a geopolymer matrix derived from MK, in which Cs⁺ was introduced. They concluded that Cs⁺ was fully incorporated into the amorphous geopolymer phase, proving the potential of AAMs for RAW immobilisation. In addition to the immobilisation of Cs⁺ and Sr²⁺, other elements such as Cd²⁺ and Pb²⁺ [23], and radionuclides such as ¹⁵²Eu, ⁶⁰Co, ⁵⁹Fe and isotopes of Am and Pu have also been successfully immobilised in AAMs [[24], [25], [26]].

Concerning the use of AAMs for RAW immobilisation, most literature covers AAMs based on commercial recipes, MK, fly ash (FA), ground granulated blast furnace slag (GGBFS) or combinations thereof. Despite the large body of research and the promising immobilisation results, a systematic study of the influence of the precursor composition on the immobilisation capacities for Cs⁺ and Sr²⁺ is lacking. According to Aly et al. (2008) [12], MK-based AAMs show optimal leach resistance at Si/Al ratios higher than 2. They reported a sharp decrease in the Cs⁺ release when the Si/Al ratio increased from 1.5 to 2 followed by a gradual increase at Si/Al ratios higher than 3, reaching the lowest value at Si/Al ratio of 2. For Sr²⁺, the lowest release was obtained at a Si/Al ratio of 1.5, increasing gradually with increasing Si/Al ratio [12].

Almost all concerning literature describes AAMs made from industrial residues, making it difficult to exclude effects of trace elements in the precursor on the immobilisation capacities. Also, the variety in precursor origin and composition in most studies make it difficult to generalize the drawn conclusions, since the immobilisation performance is very dependent on the design parameters. In addition, there exists a wide variety of alkali-activators used, most often consisting of highly-alkaline activating solutions and often containing sodium silicates.

According to the IAEA [27], the lack of standards for precursors, experience of process optimisation, and demonstration of long-term stability limit the use of AAMs in RAW immobilisation, despite the reported favourable experiences using AAMs. They stated that novel materials need a better benchmarking, and emphasised that it is also important to realise that existing test methods do not always give comparable results with different classes of materials [27]. Therefore, the effect of precursor composition on the immobilisation of Cs⁺ and Sr²⁺ in AAMs is studied in this work, by developing synthetic Ca-Si-Al slag precursors with different compositions from laboratory reagents, yielding Si/Al and Ca/(Si + Al) molar ratios of 0.95–5.1 and 0.42–1.0 respectively. In this way, immobilisation is studied excluding possible effects of trace elements in the mixture. The present results can be further used as a guideline for choosing industrial residues with a proper composition or for using proper mixing ratios.