[en] Long-term dry storage and direct geological disposal of spent nuclear fuel are two options for nuclear waste management studied in the framework of the 1991 French law. This report briefly describes the source term models developed for dry storage and geological disposal of spent fuel. These models have been developed in the framework of the research program on the spent fuel long term evolution (PRECCI project). These models are called operational because they are based on a simplification of evolution processes by using a pessimistic approach when uncertainties subsist. Potential improvements have been identified. (authors)

[en] The 'ADAGIO'(French acronym for Discriminating Analysis of Accumulation of Inter granular and Occluded Gas) technique, which allows the inter versus intra granular gas proportion to be determined thanks to a partial oxidation of an irradiated fuel at low temperature, has been applied to high burn up UO₂ sample. During these types of experiment, the 85Kr gas release is measured as a function of time. In addition to the results normally obtained from these experiments, the comparison of 85Kr release kinetic with the fuel oxidation kinetic brought into prominence two different gas release kinetics: the first one is proportional to the oxidation kinetic, the second one has a sigmoid shape and could be linked to the gas release of the so called 'High Burn up Structure'.

[en] The aim of the study is to describe the evolution of the microstructure during the setting process of the geo-polymer using an original rheological method named Optimal Fourier Rheology (OFR). The alkali activation of meta-kaolin enables physicochemical transformation from a fresh paste to a hard meso-porous matrix. Classically, oscillatory rheology technique provides viscoelastic moduli spectrum and enables to determine rheological comportment of the material under investigation. However the duration to perform a complete spectrum (more than 2.5 h) makes useless this technique in the case of changing material. The OFR technique decreases the measurement duration under 10 minutes and enables to perform several snapshots of the evolving rheological behaviour. Contrary to monochromatic iterations, here the applied stress takes the form of a chirp function which contains the full usable bandwidth. Interpretations of spectrums provide efficient access to structural evolution along the setting. Results show that the number of oligomers increases into the solution due to the dissolution of the meta-kaolin leading to a constant increase of the viscoelastic parameters until the gradual appearance of the percolating networks. The gelling time was rigorously assessed by using the Winter and Chambon criterion. A fractal percolating network is formed inside the material after a reaction time depending on the formulation parameters; corresponding fractal dimensions were established. After the gel point, the viscoelastic moduli grow rapidly until geo-polymers reach a classic viscoelastic state. Structural unit size were determined using moduli curves crossover and equalled to 2.1 nm in the case of Na geo-polymer; this value fits extremely well with value previously obtained by SAXS. Finally, the elasticity becomes constant in a large frequency range and the viscous parameter strongly decreases which means that the solid porous network is under formation. In conclusion, this rheological approach gives an efficient tool to accurate the mechanisms occurring during the geo-polymerization. (authors)

[en] Highlights: • Formulation with 20 vol.% of oil in a geopolymer have been successful tested. • Oil waste is encapsulated as oil droplets in metakaolin-based geopolymer. • Oil/geopolymer composite present good mechanical performance. • Carbon lixiviation of oil/geopolymer composite is very low. - Abstract: The solidification/stabilisation of liquid oil waste in metakaolin based geopolymer was studied in the present work. The process consists of obtaining a stabilised emulsion of oil in a water-glass solution and then adding metakaolin to engage the setting of a geopolymer block with an oil emulsion stabilised in the material. Geopolymer/oil composites have been made with various oil fraction (7, 14 and 20 vol.%). The rigidity and the good mechanical properties have been demonstrated with compressive strength tests. Leaching tests evidenced the release of oil from the composite material is very limited whereas the constitutive components of the geopolymer (Na, Si and OH"−) are involved into diffusion process

[en] In dry storage, the breach of the container and cladding will expose the spent fuel to oxygen. This incidental scenario could be detrimental for the integrity of the spent fuel rod. Indeed, UO₂ is not thermodynamically stable in air. The oxidation of UO₂ up to U₃O₈ leads to crystalline swelling and sample bulking that may increase the initial defect in the cladding and enhance the release of fission products. Oxidation experiments with in-situ characterisation of structural phases have been performed on unirradiated UO₂ powders and led to propose an oxidation model for the first step of oxidation, i.e. from UO₂ to U₃O₇ (Poulesquen et al., 2007). This model has been up-graded in order to describe the oxidation of spent fuel fragments. It takes into account the stoichiometric phases, which have observed during spent fuel oxidation, and the role of grain boundaries. The oxidation model is able to reproduce the different weight gain curves provided in literature and obtained as part of the French PRECCI program. Moreover, the diffusion coefficients of oxygen in the intermediate structural phases, which have been derived from the fitting of the model to experimental data, are consistent with literature data. Furthermore in-situ SEM investigations during oxidation of UO₂ grain have evidenced sample bulking, due to grain cracking, as soon as a critical thickness of U₃O₇ is achieved in the grain. It has led to propose a new criterion for spent fuel degradation. The time to achieve this criterion is calculated by using the oxidation model. A duration of safety, or time before consequent fracturing of spent fuel in air, is thus deduced. Sensitivity and uncertainty analyses allow us to express the duration of safety as a simplified analytical formula of the input parameters of the oxidation model, such as temperature of storage and spent fuel characteristics. The duration of safety can be qualitatively linked to irradiation parameters such as the burn up, the linear power or else the Fission Gas Release in reactor (FGR). This operational model will be presented. This work is performed in the framework of the CEA research program PRECCI, with the financial support of EDF. Poulesquen et al. (2007). Journal of Nuclear Material, 362, 402-410 (authors)

[en] The dynamic rheological behavior of geo-polymers, inorganic materials synthesized by activation of an aluminosilicate source by an alkaline solution, is described. The pastes studied were mixtures of an activation solution (alkali + silica) and metakaolin. The influence of the activation solution (NaOH vs. KOH), the silica (Aerosil vs. Tixosil), and the temperature on the evolution of the elastic modulus (G') and viscous modulus (G'') over time were studied in the linear viscoelastic range. The results show that the nature of the silica has little influence on the viscous and elastic moduli when the geo-polymer is activated by KOH, and that the setting time is faster with sodium hydroxide and at higher temperatures regardless of the geo-polymer. In addition, during geo-polymerization the stepwise variation of the modulus values indicates that the formation of the 3-dimensional network occurs in several steps. Moreover, geo-polymers activated by potassium hydroxide exhibit slower kinetics but the interactions between constituents are stronger, as the loss tangent (tan(δ)=G''/G') is lower. Finally, the maximum loss tangent, tan(δ), was also used as a criterion to determine the temperature dependence of the geo-polymers synthesized. This criterion is a precursor of the transition to the glassy state. The activation energies could thus be determined for the geo-polymers synthesized with potassium hydroxide or sodium hydroxide. (authors)

[en] It is shown that coupling nuclear magnetic resonance (NMR) 1D-imaging with the measure of NMR relaxation times and self-diffusion coefficients can be a very powerful approach to investigate fluid infiltration into porous media. Such an experimental design was used to study the very slow seeping of pure water into hydrophobic materials. We consider here three model samples of nuclear waste conditioning matrices which consist in a dispersion of NaNO₃ (highly soluble) and/or BaSO₄ (poorly soluble) salt grains embedded in a bitumen matrix. Beyond studying the moisture progression according to the sample depth, we analyze the water NMR relaxation times and self-diffusion coefficients along its 1D-concentration profile to obtain spatially resolved information on the solution properties and on the porous structure at different scales. It is also shown that, when the relaxation or self-diffusion properties are multimodal, the 1D-profile of each water population is recovered. Three main levels of information were disclosed along the depth-profiles. They concern (i) the water uptake kinetics, (ii) the salinity and the molecular dynamics of the infiltrated solutions and (iii) the microstructure of the water-filled porosities: open networks coexisting with closed pores. All these findings were fully validated and enriched by NMR cryo-poro-metry experiments and by performing environmental scanning electronic microscopy observations. Surprisingly, results clearly show that insoluble salts enhance the water progression and thereby increase the capability of the material to uptake water. (authors)

No abstract available

[en] It has long been common practice to solidify and stabilize low- and intermediate-level radioactive wastes with calcium silicate cements (ordinary Portland cement, or composite cement). However, wastes produced by nuclear activities are very diverse and some of their components may chemically react with cement phases or mixing water, reducing in some cases the quality of the product. This paper reviews the potential of three kinds of alternative inorganic binders to treat problematic wastes: -1) calcium aluminate and sulpho aluminate cements, -2) magnesium and calcium phosphate cements, and -3) alkali-activated binders (geo-polymers). Their setting and hardening process is briefly presented, and their potential for waste conditioning is discussed and illustrated through the comparison of 5 features: elaboration, heat production during fabrication, setting and hardening, durability and interaction with wastes

[en] The dynamic rheological behavior of geo-polymers, inorganic materials synthesized by activation of an aluminosilicate source by an alkaline solution, is described. The pastes studied were mixtures of an activation solution (alkali + silica) and metakaolin. The influence of the activation solution (NaOH vs. KOH), the silica (Aerosil vs. Tixosil), and the temperature on the evolution of the elastic modulus (G') and viscous modulus (G') over time were studied in the linear viscoelastic range. The results show that the nature of the silica has little influence on the viscous and elastic moduli when the geo-polymer is activated by KOH, and that the setting time is faster with sodium hydroxide and at higher temperatures regardless of the geo-polymer. In addition, during geo-polymerization the stepwise variation of the modulus values indicates that the formation of the 3D network occurs in several steps. Moreover, geo-polymers activated by potassium hydroxide exhibit slower kinetics but the interactions between constituents are stronger, as the loss tangent (tanδ = G''/G') is lower. Finally, the maximum loss tangent, tanδ, was also used as a criterion to determine the temperature dependence of the geo-polymers synthesized. This criterion is a precursor of the transition to the glassy state. The activation energies could thus be determined for the geo-polymers synthesized with potassium hydroxide or sodium hydroxide. (authors)