Elsevier

Applied Radiation and Isotopes

Volume 139, September 2018, Pages 98-106
Applied Radiation and Isotopes

How mobile is tritiated water through unsaturated cement-based materials? New insights from two complementary approaches

https://doi.org/10.1016/j.apradiso.2018.04.019Get rights and content

Highlights

  • Osmosis allows HTO diffusion experiments to be performed up to 9 MPa of suction.

  • Diffusive vapor exchange method enables the application of suction up to 170 MPa.

  • HTO mainly diffuses as dissolved species in solution in hardened cement paste.

  • Extent of irreversible HTO uptake on by cement-based materials increases with desaturation.

  • HTO diffuses 10,000 times slower through unsaturated cement-based materials than H2 gas.

Abstract

This work presents two complementary approaches (for low and high desaturation) to study tritiated water (HTO) diffusion through unsaturated cement-based materials. The first approach was based on through-diffusion experiments where suction was controlled by osmosis. In the second approach, diffusion experiments were performed in humidity chambers controlled by under-saturated saline solutions. Results revealed a decrease of effective diffusion coefficient by a factor of 10 from 100% to 23% of saturation degree. Comparison with gaseous H2 suggests that HTO diffuses through unsaturated cement-based materials at rates 4 orders of magnitude lower.

Introduction

Cement-based materials are widely used in radioactive and hazardous wastes (Jantzen et al., 2010). In some nuclear waste disposal facilities, they constitute the primary barriers against radionuclide or toxic species release, due to their high containment properties. Indeed, their low permeability limits the radionuclide transfer to the slow diffusive process (Andra, 2005). Diffusion is a transport mechanism driven by concentration gradient. Diffusion rate of a species in a porous medium is characterized by effective diffusion coefficient as:De=ϕD0δτ2Where De is the effective diffusion coefficient (m2 s–1), D0 is the diffusion coefficient of the species in bulk water (m2 s–1), ϕ is the porosity of the porous medium (m3 m–3 or –), δ is the constrictivity factor (−) and τ is the tortuosity factor (−) (Shackelford, 1991). In cement-based materials, diffusion can occur in several types of pores: (i) the gel pores of nanometric size lying into the calcium-silicate-hydrate (C-S-H) phase which is the main solid phase in a cement paste, (ii) the capillary pores of submicrometric size corresponding to the space between mineral phases besides C-S-H such as portlandite, aluminate phases, such as ettringite (Ca6(Al2(SO4)3.26H2O), calcium monosulphate hydrate (3CaO.Al2O3.CaSO4.12H2O) and hydrogarnet (Ca3Al2(OH)12), (iii) some cracks of micrometric size that could be formed during hardening phase (Baroghel-Bouny, 2007).

Moreover, there are many situations in waste disposal facilities where cement-based materials can be partially water-saturated, leading to potential changes of radionuclide transport properties. For example, during the operational period of on-surface disposal facilities of short-lived nuclear waste, cement-based materials can be partially desaturated due to significant interactions with the atmosphere. Similar is the case in deep geological facilities for the disposal of long-lived intermediate-and high-level nuclear waste. Here, the degree of saturation of cement-based materials should be controlled by the surrounding relative humidity that is imposed by ventilation of the underground drifts and shafts during the operational period, and by gas generation (anoxic corrosion, radiolysis …) during the post-closure period. Under these various conditions, the saturation degree of cement-based material could go down to a few tens % (Andra, 2005).

The evaluation of the saturation effect on radionuclide migration, i.e. mainly by diffusion, in cement-based materials therefore requires investigation at a large range of saturation degree. An extensive review was recently provided by Zhang and Zhang (2014) on diffusion in unsaturated cement-based materials. However, they reported only few studies dealing with the determination of diffusion coefficients through unsaturated cement-based materials because performing such measurements still constitutes a knotty task. Most of diffusion testing used the saturated saline solution method for imposing suction and de-saturate samples. However, the saturated saline solution method is known to be mainly adapted to suctions higher than 8.5 MPa, because below this value the relative uncertainty on the imposed suction is significant, higher than 15% (Cuisinier and Masrouri, 2005). A first approach is an indirect one where diffusivity can be measured from the relationship between resistivity and diffusion, using Nernst-Einstein equation (Olsson et al., 2012, Mercado-Mendoza et al., 2014, Zhang and Ye, 2017). This gives an approximate effective diffusion coefficient of the unsaturated sample, not for specific species (Patel et al., 2016). A second approach consists in making the tracer of interest diffuse into unsaturated materials over a certain period and then acquiring tracer profile in sample. Dridi and Lacour (2014) used such an approach to study the diffusion of lithium as Li+, through hardened cement pastes by means of the transient half-cell technique under relative humidity conditions varying from 100% to 33% imposed by saline solutions. Their work showed a decrease by almost two orders of magnitude of the lithium effective diffusion coefficient from the fully-saturated sample to the sample saturated at 0.25. The diffusion of tritiated water (HTO) was indirectly studied by Numata et al. (1990) using HTO adsorption experiments onto mortar samples nearly-saturated with water at relative humidity of 93%. In this work, very rough estimations of HTO effective diffusion coefficient were performed using tritium profiles within samples. Sercombe et al. (2007) investigated the diffusion of gases, especially, dihydrogen, through unsaturated hardened cement pastes. Here, they found a sharp increase of dihydrogen gas effective diffusion coefficient for hardened cement pastes by more than 3 orders of magnitude when degrees of saturation were lower than 0.8 compared to full-saturated sample case. We recently presented a new setup allowing the effective diffusion coefficient of tritiated water and various solutes to be determined through natural materials (claystones and compacted clay minerals) under partially-saturated conditions using the osmotic approach (Savoye et al., 2010, Savoye et al., 2012, Savoye et al., 2014, Savoye et al., 2017). As already mentioned by Cuisinier and Masrouri (2005), the osmotic approach enables to impose suction up to 9 MPa with very low uncertainties associated to its estimation. It means that this method is complementary with respect to the saline solution method for investigating degree of saturation close to full-saturation.

The aim of the present work is to investigate the diffusion of tritiated water through cement-based materials under a large suction range (0–170 MPa), using the two complementary approaches. Both enable the study of a large range of water saturation degree. Motivation to focus on HTO diffusive behavior is twofold. First of all, under fully-saturated conditions, tritiated water is classically used as a reference tracer in diffusion experiments for qualifying the containment properties of porous materials (claystones, cement-based materials, etc…) (Melkior et al., 2007, Savoye et al., 2015, Larbi et al., 2016). Therefore, one difficulty is to know how tritiated water effective diffusion coefficient evolves with saturation. Resolving this issue would thus demonstrate as whether assuming tritiated water as reference is still valid when dehydrating. Secondly, tritiated water can occur under both liquid and vapor forms in unsaturated cement-based materials used in several types of nuclear facilities, e.g. on-surface disposal facilities, fusion reactor or tritium handling facility (Takata et al., 2005, Furuichi et al., 2007, Gudelis et al., 2010, Twining et al., 2011, Kim et al., 2012). Under these conditions, further investigations are thus necessary to determine if HTO diffusion rates are as high as gas diffusive rates (Sercombe et al., 2007) or as low as the dissolved ionic solutes ones (Dridi and Lacour, 2014, Mercado-Mendoza et al., 2014) when dehydrating.

For this reason, the first approach allowing through-diffusion experiments to be conducted under suction values imposed by osmosis up to 9 MPa has been adapted to alkaline conditions because this approach was initially developed for natural materials, such as claystones (Savoye et al., 2010). The second approach based upon the vapor exchange method has been considered to determine the diffusion behavior of HTO under higher suction values, i.e. up to 170 MPa. This approach was initially developed by Rübel et al. (2002), improved by Savoye et al. (2006) and Altinier et al. (2007) to determine HDO content in pore-water of claystones close to the full saturation. In these studies, a HDO-reference saline solution was equilibrated with a clayey pore-water solution via vapor exchange in a tight container. Then, the HDO content measurement in saline solution leads to an indirect estimation of the pore-water HDO content. Here, we have proposed to adapt this experimental protocol considering two main changes. Firstly, in our case, the under-saturated saline solution, initially spiked with HTO was regularly sampled for monitoring its HTO activity evolution until equilibrium via vapor exchange. This monitoring enabled us to determine the diffusive rate at which HTO penetrated into cement-based materials. Afterwards, by replacing the saline solution containing the residual amount of HTO by a freshly prepared saline solution with no HTO addition, the diffusive rate at which HTO goes out of cement-based material was estimated. These two phases correspond to two successive in-and out-diffusion experiments. Secondly, in order to impose broader suction range to cement-based materials, the under-saturated-NaCl-saline solution usually used in literature (Rübel et al., 2002, Savoye et al., 2006, Altinier et al., 2007) was replaced by under-saturated solutions of various LiCl concentrations, capable of imposing a larger suction range, up to 300 MPa (Kitic et al., 1986).

Section snippets

Preparation of the samples and initial hydric conditioning with saturated saline solutions

Hardened cement pastes, HCP, based on a CEM V/A composite cement (designation CEM V/A (S-V) 42.5N PM-ES-CP1 NF “PMF3”, Rombas, Calcia) were used for this study. Cement paste samples were prepared with an initial water to cement ratio of 0.4. They were poured into closed cylindrical polyethylene plastic molds (51.5 mm in diameter and 53.0 mm in height) and initially cured in a 100% relative humidity chamber for 28 days. After the curing period, some samples were submitted to a high-pressure

Petrophysical data

Fig. 2 shows the evolution as a function of imposed relative humidity of each degree of saturation measured on HCP. The degree of saturation measured on the Osm 3 HCP sample for which water-saturation was imposed by osmosis is equal to (0.85 ± 0.01). This value is in good agreement with those measured on HCPs that have undergone re-saturation at RH fixed by saturated saline solutions (Osm 4, 5 and 6). Moreover, the impact of relative humidity values used for the initial de-saturation is clearly

Conclusion

The diffusion of tritiated water through unsaturated cement-based materials was studied using two innovative and complementary approaches. Both of these approaches enabled us to investigate degree of saturation (Sw) from 0.23 to 1.00, i.e. full-saturation. Although the first approach was initially developed for studying diffusion through clay-rich materials that are partially saturated using the osmosis process, our present work shows that it can be successfully adapted to cement-based

Acknowledgements

This work received financial support from Andra. The manuscript was much improved by the constructive comments of Dr Pierre De Cannière and one anonymous reviewer.

References (38)

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