Intermediate Level Radioactive Waste Research Articles

Degradation of the polyacrylonitrile-based UP2W material under cementitious conditions

UP2W is a polyacrylonitrile-based filter aid material used in nuclear power plants, which is disposed in repositories for low and intermediate level radioactive waste. The degradation of UP2W was investigated in a series of batch experiments in the presence and absence of portlandite, Fe(0) and NaOH, which simulate the hyperalkaline, reducing conditions expected in repositories with cementitious engineered barriers. Degradation experiments were performed under Ar atmosphere at T = 22 or 80 (±2) °C. Aliquots of the supernatant solutions and retrieved solid phases were systematically characterized for ca. 5 years using a multi-method approach.The evolution of dissolved organic carbon shows a strong contrast between NaOH- and Ca(OH)2-buffered systems. In the absence of Ca, an early, sharp increase in the dissolved organic carbon is observed, and its magnitude correlates with the initial NaOH concentration.In Ca(OH)2-buffered systems, dissolved organic carbon remained very low (<10 ppm) up to 600 days, but steadily increased afterwards reaching ∼ 50 ppm at t ≈ 1800 days. Size-exclusion/liquid chromatography coupled with organic carbon, ultra-violet and organic nitrogen detection (LC-OCD-UVD-OND) confirmed the presence of significantly smaller fragments in solution compared to NaOH-systems, which however were found to increase with time.Overall observations underline that hydrolysis of the nitrile functional groups, chain scission of the polymer backbone and cross-linking of polymer fragments in the degradation leachates play a key role in the progress of the degradation reaction. Chain scission is hindered in the presence of Ca, possibly due to the complexation of Ca with amide and carboxylate intermediates and the consequent decrease in electron density on the β-carbon atoms. This study improves the mechanistic understanding and quantitative description of UP2W degradation in cementitious environments relevant for L/ILW disposal.

Curing temperature’s effect on argillite-metakaolin based geopolymer grouts

This study concerns the project of high and intermediate level radioactive waste disposal (Cigéo project for Centre Industriel de stockage GEOlogique). Millions of tons of argillite will be excavated during the construction of the site. Therefore, the aim of this work is to reuse this argillite in the formulation of an injectable geopolymer grout and study its evolution in different storage conditions (T = 20 and 80 °C). To achieve this challenge, a metakaolin and calcined Callovo-Oxfordian (COx) argillite were used with potassium silicate solutions. The grouts were successfully obtained from both metakaolin and COx argillite in both temperatures 20 and 80 °C. The range of viscosity of several mixtures respected the required specifications (viscosity ≤5 Pa s and setting time superior to 6 h). The metakaolin-based geopolymers didn’t display any major changes no matter the storage temperature, except for the pore size, a coalescence phenomenon was observed with the increase of temperature. In contrast, for argillite or argillite-metakaolin based samples, a decrease of pore size as well as in pH values were observed with the increase of temperature. This behaviour was attributed to the dissolution of argillite’s impurities such as illite, providing more oligomers to allow the formation of different geopolymer networks.

Effect of supplementary cementitious materials on the degradation of cement-based barriers in radioactive waste repository: A case study in Korea

This study focuses on investigating the chemical degradation characteristics of cementitious barriers used in low- and intermediate-level radioactive waste repository by reactive transport modeling. The impact of the blending with supplementary cementitious materials (SCMs) in the barriers on the chemical degradation was evaluated to find the optimum barrier design. A number of different barrier designs were examined by replacing ordinary Portland cement (OPC) by SCMs (i.e., fly ash, silica fume, and blast-furnace slag). The simulation results showed that silica fume blended barrier has better durability against chemical degradation by rainwater compared to fly ash or blast-furnace slag blended barriers. In addition, the chemical durability of silica fume-based barrier increased with increasing replacement level up to about 20 %. It seems that the amount of formed calcium silicate hydrate (CSH) in the initial cement-based barrier highly affects the overall chemical durability. The newly developed reactive transport model demonstrated its capability for understanding the barrier performance and investigating the optimal design of the barrier system.

Mobility of water and ions in partially water-saturated compacted MX-80 bentonite in relation with swelling pressure

Many countries are planning to host long-lived high- and intermediate-level radioactive waste in disposal facilities sited in deep argillaceous formations. Bentonite, as a swelling material, is often chosen as engineered buffers due to its low permeability and high swelling capacity. However, its properties can be influenced by the hydraulic/gas conditions prevailing in-situ that are expected to change throughout the duration of the repository. The objective of this work is to estimate the effect of partially water-saturated conditions on the mobility of water and ions in compacted MX-80 bentonite in relation to the evolution of its swelling pressure. For that purpose, (i) diffusion of water and ions (I− and Na+) and (ii) the swelling pressure of compacted bentonite samples partially water-saturated were studied by means of the osmotic technique that allows controlling the suction from 9 to 0 MPa.Results showed that (i) water diffusivity remained unchanged independently of the degrees of saturation (70, 84 and 100%) and (ii) iodide diffusivity decreased by a factor of 12 (from 100 to 70%), while (iii) Na+ diffusion was reduced by a factor of more than 70 (from 100 to 70%), associated to a drop of the distribution factor by a factor of 16. Comparison of these results with diffusion data already acquired in clayey materials whose porous structure evolves little with suction (i.e. compacted kaolinite and intact Collovo-oxfordian clay-rocks) underlined how the bentonite porous structure whose organization evolved with the degree of saturation is a key parameter. This statement is supported by mechanical tests, which indicated a distinct evolution of the swelling pressure curves depending on the degree of saturation. At 70% saturation, mechanical tests indicate (i) a preservation of the large inter-aggregate porosity where water can diffuse in both vapor and liquid phases, and (ii) a final swelling pressure 1.5 to 2 times lower than at full water saturation, meaning that a part of swelling clay particles was not accessible to water ingress and so to 22Na+. Conversely, at 84 % and 100 % saturation, change in swelling pressure curves was explained by a collapse of bentonite inter-aggregate porosity, in favor of expanding the inter-particle and interlayer porosities where 22Na + diffusion is enhanced by surface diffusion phenomenon.

Effect of PFDS on the immobilization of Cs+ by metakaolin-based geopolymers in complex environments

Metakaolin-based geopolymers are very promising materials for improving the safety of low and intermediate level radioactive waste disposal, with respect to ordinary Portland cement, due to their excellent immobilization performance for Cs+ and superior chemical stability. However, their application is limited by the fact that the leaching behavior of Cs+ is susceptible to the presence of other ions in the environment. Here, we propose a way to modify a geopolymer using perfluorodecyltriethoxysilane (PDFS), successfully reducing the leaching rate of Cs+ in the presence of multiple competitive cations due to blocking the diffusion of water. The leachability index of the modified samples in deionized water and highly concentrated saline water reached 11.0 and 8.0, respectively. The reaction mechanism between PDFS and geopolymers was systematically investigated by characterizing the microstructure and chemical bonding of the material. This work provides a facile and successful approach to improve the immobilization of Cs ions by geopolymers in real complex environments, and it could be extended to further improve the reliability of geopolymers used in a range of applications.

Examining the Contributions of Various Source Layers and Transport Pathways to Dose Rates Exterior to a Low- and Intermediate-Level Radioactive Waste Storage Facility Using ADVANTG/MCNP

An in-depth shielding analysis of a large indoor facility designed for storing low- and intermediate-level radioactive waste generated from nuclear power plant decommissioning was performed. The facility has a storage capacity of 1260 box-type containers, which are stacked in five layers. The characteristics of the radiation field around the facility were investigated comprehensively through efficient ADVANTG/MCNP simulations. The contribution of each source layer to the dose rates exterior to the facility was determined. Furthermore, dose components from various transport pathways (e.g., direct, streaming, and skyshine) were isolated using algebraic manipulation of the outputs obtained from additional simulations. The combined results provided valuable information for shielding optimization during the design phase and could facilitate the future operation of the storage facility.

Comparative Studies between Frequency Domain Analysis and Time Domain Analysis on Free-Field One-Dimensional Shear Wave Propagation

In Korea, the underground silo structure for low- and intermediate-level radioactive waste disposal facilities has been constructed and operated since 2014. Large-scale earthquakes occurred in 2016 and 2017, respectively, in Gyeongju and Pohang areas near the underground silo structures, and interest in the stability of the underground silo increased significantly. In this paper, one-dimensional free-field analyses have been carried out before the three-dimensional silo dynamic analyses subjected to earthquake loadings. As an additional study, a new form of the finite element equilibrium equation is derived in terms of relative motions, which is essentially the same equation expressed in terms of total motions where the base shear force is applied to the earthquake load. The accuracy of conventional finite element solutions is evaluated by directly comparing them with closed-form solutions by frequency domain analysis such as SHAKE91.

Modelling geomechanical stability of a large deep borehole in shale for radioactive waste disposal

Large diameter boreholes drilled to a depth of 1000–2000 m are currently investigated in Australia as a potential solution for waste emplacement and long-term containment for long-lived intermediate-level radioactive waste (ILW). Different potentially suitable types of host rocks are considered, including granite, shale and salt. This study is aimed at assessing the risk of borehole instability for a large diameter deep borehole in shale. The investigation is conducted with numerical simulations using the CSIRO-developed modelling tools FRACOD and FRCAOD3D, and the commercial code Irazu. Previous laboratory experiments on boreholes in deep shale reported in literatures have been used as validation tests of the numerical models. Under hydrostatic stresses, the FRACOD model showed extensive shear-dominated “log-spiral” fractures around a borehole in shale which is similar to the laboratory testings reported in literatures. Subsequently, 2D and 3D numerical models representing a 0.7-m diameter vertical borehole drilled to the depths of 1000 m and 2000 m have been used. “Strong” and “weak” shales are simulated. The strong and weak shales represent the higher and lower ends of the shale strength reported in literature, respectively. Results showed that a “strong” shale exhibited only small borehole breakouts at the depth of 2000 m. The “weak” shale showed large breakouts (>1.5 m) at depths of 1000 m and 2000 m caused by extensive shear fracturing. In the scenario where a pre-existing 3D fracture crosscuts the borehole, the existing fracture may propagate deeper into the rock formation depending on its orientation and on the in situ stress magnitudes. Predictions based on analytical solutions indicate a 0.02-m and 0.04-m thick casing could prevent severe failure of rock mass at 1000 and 2000 m depths, respectively. Insights from model-based investigations of borehole breakouts are critical during the site selection of rocks with suitable geomechanical properties for waste disposal and for optimising the design of large-diameter boreholes for drilling, waste emplacement and long-term safety.

Hydrogeochemistry and Statistical Analysis for Low and Intermediate Level Radioactive Waste Disposal Site in Gyeongju

Hydrogeochemistry and Statistical Analysis for Low and Intermediate Level Radioactive Waste Disposal Site in Gyeongju

Sorption Behavior of Radionuclides on Engineered and Natural Barriers and Prediction of Sorption Distribution Coefficients Using Support Vector Regression

A low- and intermediate-level radioactive waste repository contains various types of radionuclides and organic complexing agents. Their chemical interaction within the repository can lead to the formation of radionuclide-ligand complexes, influencing the limited retention behaviors of radionuclides. This study focuses on the sorption behavior of radionuclides on both engineered (concrete) and natural barriers (sedimentary rock and granite), as well as the prediction of sorption distribution coefficients (Kd) using support vector regression. Batch studies were conducted to determine the K d values for three radionuclides (99Tc, 137Cs, and 238U) under different conditions, including pH, temperature, and the presence of organic ligands (such as ethylenediaminetetraacetic acid, nitrilotriacetic acid, and isosaccharinic acid). The K d values for 238U exhibited a sharp decrease with increasing concentrations of organic ligands. In contrast, the K d values for 99Tc showed only a slight reduction at higher organic ligand concentrations. Meanwhile, the K d values for 137Cs remained relatively unchanged, regardless of the type and initial concentration of organic ligands. This suggests a high level of retention for 137Cs in the rock samples. The support vector regression model with a radial basis kernel function proved effective in predicting the K d values under different experimental conditions. This enhancement in predicting accuracy contributes valuable insights into understanding the sorption processes involved in radionuclide behavior. Overall, this study advances our knowledge of radionuclide behavior on both engineered and natural barriers while providing a reliable prediction tool for estimating sorption distribution coefficients.

Adsorption/Desorption Performances of Simulated Radioactive Nuclide Cs+ on the Zeolite-Rich Geopolymer from the Hydrothermal Synthesis of Fly Ash

The operation of nuclear power plants generates a large amount of low- and intermediate-level radioactive waste liquid. Zeolite-rich geopolymers, which are synthesized under hydrothermal conditions from industrial waste fly ash, can effectively immobilize radioactive nuclides. In this study, the synthesis law of zeolite-rich geopolymers and the adsorption/desorption performances of radioactive nuclide Cs+ were researched using XRD, SEM and ICP. The results show that the increase in curing temperatures and NaOH concentrations leads to the transformation of Y-type zeolite to chabazite and cancrinite at low NaNO3 concentrations. However, at high NaNO3 concentrations, NaOH above 2 M has no obvious effect on the phase transformation of the main zeolite of chabazite and cancrinite. In the adsorption and desorption experiment of Cs+ on the chabazite/garronite-rich geopolymer, it was found that the adsorption of Cs+ in the low initial concentration range is more suitable for the Freundlich equation, while the Langmuir equation fits in the adsorption process at the high initial concentration range. Moreover, the desorption kinetics of Cs+ are in good agreement with the pseudo-second-order rate equation. Thus, the adsorption of Cs+ on chabazite/garronite-rich geopolymers is controlled by both physical and chemical reactions, while desorption is a chemical process.

Effects of EDTA on the leaching behaviors of Cs(I) and Co(II) from cement waste forms

Cementation is extensively employed for immobilizing radionuclides in low- and intermediate-level radioactive wastes generated during the decommissioning of nuclear power plants. Ethylenediaminetetraacetic acid (EDTA), used as a decontamination agent during the decommissioning process, can be introduced into the cement waste form containing radionuclides. This study investigated the effects of the EDTA present in simulated radioactive decommissioning wastes on the leaching behaviors of Co and Cs immobilized in the cement waste form. Co leaching was facilitated by the formation of highly mobile Co–EDTA complexes. However, Cs leaching was impeded by the competition for leaching with other metal–EDTA complexes. Moreover, the EDTA-induced carbonated layer with a dense pore structure played a crucial role as a retardation barrier for the Cs leaching. The calcite contents of the samples with 8 wt% EDTA were approximately three times higher than those of the samples without EDTA. The introduced EDTA affected the leaching behaviors of both Co and Cs, as well as the microstructure evolution of the cement waste form. Nevertheless, the addition of EDTA had a relatively low positive effect on the efficiency of Cs immobilization, but also an obvious negative effect on the efficiency of Co immobilization, regardless of the concentration of EDTA. Finally, an EDTA dosage of 1 wt% in the cement waste forms containing Co or Cs is suggested as a potential waste acceptance criterion for solidified low- and intermediate-level radioactive waste.

Bimodal structured chromium-tungsten composite as plasma-facing materials: Sinterability, mechanical properties, and deuterium retention assessment

Even though tungsten (W) is considered to be the most favored plasma-facing material (PFM) for nuclear fusion reactors thanks to its beneficial physical properties, its advantage for those plasma-facing components to be subjected to medium heat flux and high neutron irradiation dose is compromised by unfavorable nuclear behavior such as intermediate-level radioactive waste activation and embrittlement caused by lattice damage and transmutation. Chromium (Cr) or Cr-based materials may have the potential to mitigate these shortcomings while maintaining acceptable thermal and physical performances required for PFM. They could offer attractive design options, particularly for those parts where the particle fluxes are relatively low and the heat flux is not too high (≤ 5 MW/m²) while nuclear loads are high (≥ 5 dpa). In this article, we report the first results of a preliminary technology feasibility study for an engineered Cr-based material fabricated based on a powder-metallurgical route. To explore an optimal metallurgical condition ensuring reasonable mechanical properties and low deuterium retention, a novel two-phase Cr-W composite was developed via current-assisted rapid sintering at low sintering temperature. The composite showed a cell-like bimodal microstructure consisting of a coarse-grained Cr phase surrounded by a 3D interconnected network of an ultrafine-grained W phase. The composite exhibited substantial improvements in ductility as well as high-temperature strength compared to pure W and Cr. After undergoing W heavy ion irradiation, the damaged Cr-W composite with a two-phase bimodal microstructure showed a comparable deuterium retention capacity to pure W. Finally, the potential applicability as PFM is discussed.

An experimental study on the measurement of gas diffusivities in un-saturated clay based materials

Gas migration in porous media in the context of nuclear waste disposal is dominated by diffusion. The availability of abundant data on diffusion coefficients of different gases in many clay based materials such as Boom Clay, Opalinus Clay and Callovo-Oxfordian Clays provides a good overview of the rates of gas transport through these different clay rocks [1]. Such formations are being considered as potential host rocks for the disposal of high and intermediate level radioactive wastes across Europe. Diffusion coefficients have been studied with particular interest due to the unavoidable gas generation phase in the nuclear waste repository [2]. However, all the diffusion measurements performed so far have been under fully saturated conditions, with little experimental data on gas diffusivities in partially saturated conditions, which have been hypothesized to exist during the early part of the life cycle of the repository [3]. The objective of the current study is to provide an overview of the effect of desaturation on the rate of diffusive transport of gases in partially saturated clay based materials.&#x0D; The experimental methodology is based on the double through diffusion setup used in previous studies to measure gas diffusivities in saturated clays [1]. However, the setup is modified so as to keep a fixed level of unsaturation in the clay sample. This is done by employing the vapour equilibrium technique, using oversaturated salt solutions in both upstream and downstream reservoirs to maintain a constant relative humidity in the entire setup [4].&#x0D; Preliminary pore network modelling has already shown that the gas diffusivity increases with a decrease in water saturation of the clay material. However, the predicted trend also suggests that this increase is only marginal and should not normally exceed an order of magnitude. This is shown in the figure below.&#x0D; All the diffusion measurements are carried out on synthetic clay samples of dimensions 3.8 cm (diameter) and 2 cm (height). In Table 1, the Sw stands for water saturation. The compositions of the synthetic clay samples (60% clay, 20% sand, 20% silt) have been adjusted so as to most closely mimic the mineralogical compostion of Boom Clay. Synthetic samples are used instead of natural samples like Boom Clay because of their better physical response to drying than Boom Clay.&#x0D; The selected control variable to lower the saturation of the material is suction, which is imposed by changing the relative humidity of the environment using oversaturated salt solutions [4]. Suction can be kept constant given constant ambient temperature and pressure. Thus, the clay sample is conditioned at a certain suction, which in turn leads to desaturation in the range of 70-100% of total saturation depending on the level of suction. For this study, the saturation range has been fixed in between 70-100% of total saturation. This range has been chosen because of what is expected in a real life nuclear waste repository. The first measured diffusion coefficients in two partially saturated samples are shown in Table 1. The initial measurements of gas diffusion coefficients in unsaturated synthetic clays show that the increase in diffusivity of helium is higher than that of argon. Helium is the second lightest known element and has an extremely small molecular size and hence the impact of desaturation on helium diffusivity could be more pronounced than the same for heavier gases.

Two-phase reactive transport modeling of heterogeneous gas production in a low- and intermediate-level waste repository

Abstract. The widely proposed approach to dispose of low- and intermediate-level radioactive waste is to store it in a deep underground repository with multiple barriers. A typical gallery is filled with concrete containers accommodating cemented steel drums of waste. Inside a container, different gases may be produced by (bio)chemical reactions, which include pH-dependent anoxic corrosion of metals and the degradation of organic matter. Both reactions consume water and may lead to pressure buildup and transport of gas, both within and around the repository. In order to investigate the controlling factors of this gas production process, a coupled reactive transport model of component-based two-phase flow in the OpenGeoSys framework is adopted here. The numerical study of Huang et al. (2021) has shown that a realistic internal structure of a waste package, including the heterogeneous distribution of materials with different chemical and hydrological properties, and the exchange of mass at the boundaries are key factors that determine the evolution of the waste package. Based on the study of Huang et al. (2021), the geometric configuration of the model has been further extended to reflect the various conditions of a multi-container disposal in a gallery. In a two-dimensional setup several model scenarios have been designed and simulated to study the change in gas production rate over time in relation to water availability in various geological and waste storage setups. In this presentation, we show simulation results covering the geochemical evolution of a waste package over 500 years. It is found that the initial water content in the waste compartment only controls the gas production rate for the first 40 to 60 years. The early pressure buildup and gas release rates are largely controlled by several critical parameters, including the permeability of cement material and water availability at the boundary. The sensitivity of these parameters is currently being investigated in detail.

Radiolytic degradation of cellulosic materials in nuclear waste: Effect of oxygen and absorbed dose

Lignocellulosic materials can be found in a significant fraction of the current low- and intermediate-level radioactive waste. During storage and disposal, radiolytic degradation of such materials can be expected, under oxic or anoxic conditions. This degradation may lead to a significant gas production and changes in the physico-chemical properties of the lignocellulosic materials, which can affect the formation of the known radionuclide-complexing agent isosaccharinic acid (ISA) as well as other (possibly complexing) degradation products during disposal. Hence, in the present work the radiolytic degradation of cellulosic tissues – realistically found in radioactive waste – was investigated under various storage and disposal conditions. For this, cellulosic tissues were exposed to γ-irradiation in gas-tight containers under oxic or anoxic conditions, at an absorbed dose ranging up to 1.4 MGy and at two different dose rates. Our results show that mainly H2, CO and CO2 are produced during irradiation of tissues, though also small amounts of CH4 are formed. The presence of oxygen does not affect the generation of H2, but results in a significant increase in the yields of CO, CO2 and CH4. Furthermore, radiation-induced chain scission is observed, causing a decreasing polymerization degree with increasing absorbed dose. Amorphization of the cellulose microstructure occurs significantly at high doses of gamma rays (≥ 0.8 MGy). An increase in the concentration of reducing functional groups is observed with increasing absorbed doses as well. For irradiation under anoxic conditions, this increase is correlated with the observed chain scission. In contrast, additional oxidation processes occur when irradiating cellulosic tissues in the presence of oxygen, resulting in a partially oxidized polymer backbone without causing considerably more chain scission or amorphization. These radiolytic changes to the cellulose structure, both under anoxic and oxic conditions, may enhance its hydrolytic degradation under the hyper-alkaline conditions of long-term final disposal, resulting in a faster production of radionuclide-complexing agents.

A novel method for gamma spectrum analysis of low-level and intermediate-level radioactive waste

A novel method for gamma spectrum analysis of low-level and intermediate-level radioactive waste

Contribution of HADES URL to the development of the Cigéo project, the French industrial centre for geological disposal of high-level and long-lived intermediate-level radioactive waste in a deep clay formation

Abstract In the 1980s, HADES (High-Activity Disposal Experimental Site) was the first underground research laboratory (URL) dedicated to the study of the geological disposal of radioactive waste in a deep clay formation, the Boom Clay. It was not until the early 2000s, after a siting process, that ANDRA implemented the Meuse/Haute-Marne URL, in the Callovo-Oxfordian formation at a depth of about 500 m in order to develop the Cigéo project (French industrial centre for geological disposal). ANDRA therefore relied heavily on the work carried out in HADES, through numerous co-operation projects (participation in in situ experiments) both between ANDRA and ONDRAF/NIRAS and SCK CEN (EURIDICE) and/or with Mont Terri consortium, and within European projects (CLIPEX, RESEAL, etc.). This was driven by a dual objective: (1) to prepare its own experimental programmes in the Meuse/Haute-Marne underground laboratory (methodology, experimental devices and protocols, etc.); and (2) to acquire general knowledge on the behaviour of argillaceous rocks, in particular in terms of similarity and differences between the various argillaceous rocks. This paper illustrates the contribution of HADES to the ANDRA programme. This concerns the characterization of the claystone behaviour, host rock and swelling clay-based seals (hydromechanical, thermo-hydromechanical, excavation damaged zone, etc.), and the design and the behaviour of underground structures and seals in deep clay formation (constructability, lining/support, etc.).

Microstructural investigation into the damage behaviour of MX80 bentonite pellet under wetting/drying path using Mercury Intrusion Porosimetry (MIP) and Micro-Computed Tomography (μCT)

MX80 bentonite pellet has been considered as a promising sealing/backfilling material in geological disposal facilities for high-level and intermediate-level radioactive wastes. As the sealing/backfilling material which is subjected to hydration by pore water or vapour from host rock can be damaged by wetting-induced cracking, it is essential to clarify the suction effect on the damage behaviour of the material. In this study, a combined Mercury Intrusion Porosimetry/Micro-Computed Tomography (MIP/μCT) method was applied to investigate the microstructural damage of the MX80 bentonite cylinder-shaped 32-mm pellets equilibrated under suctions s from 113 to 1.5 MPa for wetting and from 113 to 262 MPa for drying. The enlarged pore size distribution (PSD) curves by MIP/μCT revealed an insignificant drying-induced closure of cracks, as opposed to massively developed wetting-induced cracks, with the size of the largest cracks reaching several millimetres. This is confirmed by the μCT observation. Indeed, the μCT images showed a negligible effect of drying on cracking, contrary to the effect of wetting characterised by significant cracking from the marginal area to the central part of pellet, indicating significant damage of pellet by wetting. Additionally, only horizontal cracks were identified at s ≥ 82 MPa. Vertical cracks emerged and propagated only at s ≤ 57 MPa, together with further expanding of horizontal cracks, due to the preferentially horizontal orientation of clay particles resulting from the one-dimensional compaction in the pellet fabrication. A damage coefficient D was defined as the ratio of the volume of macro-pores to the total volume of pellet. D increased slightly upon drying from suction 113 to 262 MPa and significantly upon wetting from suction 113 to 3.2 MPa, but kept almost unchanged when wetting furthered from suction 3.2 to 1.5 MPa because the clogging of macro-pores by the exfoliation effect slowed down the cracking rate.

Strontium interaction with unblended and Low-pH cementitious wasteforms: pH-dependent leaching experiments and geochemical modelling

Cementitious materials are widely-applied for the solidification and stabilization of low and intermediate level radioactive waste streams, that often contain strontium-90. Low-pH cement pastes, based on ordinary Portland cement (OPC) supplemented with pozzolanic additives, were found to have better Sr retention capabilities than unblended OPC pastes. However, precise definition of the phases and processes that control Sr retention in unblended and low-pH OPC pastes is still lacking. The present study combines experimental work with thermodynamic modelling to investigate the retention of Sr in unblended pastes compared to low-pH pastes. The unblended and low-pH pastes were prepared each in two formulations: one based on reagent grade alite (tri-calcium silicate) and the other based on OPC. The chemical and mineralogical behavior of the paste were studied by pH-dependent leaching test. The pastes characteristics were compared in order to assess the impact of: (1) the addition of amorphous silica to the pastes and (2) the presence of minor OPC phases (which are absent in the alite-based pastes) on Sr binding. The low-pH systems showed one order of magnitude lower Sr leachability than the unblended systems at the natural pH. Moreover, the leaching curves indicated that the low-pH systems bind Sr effectively at pH > 7 while for the unblended systems binding is effective only at pH > 10. These results are attributed to the difference in Sr binding to low Ca/Si C–S–H, dominant in low-pH systems, versus the high Ca/Si C–S–H, dominant in unblended systems. Additionally, it was shown that Sr binds mainly to the C–S–H phase rather than to minor phases. The leaching results were thermodynamically modelled and correlated successfully with the experimental findings. A novel modelling approach is offered for the consideration of partial pozzolanic reaction and SiO2-gel formation. The effect of these processes on strontium retention has been successfully demonstrated.