Comparison of phosphor screen autoradiography and micro-pattern gas detector based autoradiography for the porosity of altered rocks

This study aims to further develop the C-14-PMMA porosity calculation method with a novel autoradiography technique, the Micro-pattern gas detector autoradiography (MPGDA). In this study, the MPGDA is compared with phosphor screen autoradiography (SPA). A set of rock samples from Martinique Island exhibiting a large range of connected porosities was used to validate the MPGDA method. Calculated porosities were found to be in agreement with ones from the SPA and the triple-weight method (TW). The filmless nature of MPGDA as well as straightforward determination of C-14 radioactivity from the source rock makes the porosity calculation less uncertain. The real-time visualization of radioactivity from C-14 beta emissions by MPGDA is a noticeable improvement in comparison to SPA.Peer reviewe

Comparison of the chemo-mechanical behavior of low-pH cement exposed to calcareous water and to argillite pore water

International audienceThis paper focuses on the impact of ground water on the chemo-mechanical behavior of low-pH cementitious materials. Two kinds of ground water are considered, calcareous water and argillite pore water. A sample of low-pH cement paste was immersed for two months in each solution. Chemo-mechanical characterizations (SEM, C-14 PMMA autoradiography, nano-indentation) were performed to estimate the degradation state of the samples. Then, geochemical modelling was performed to validate experimental observations. The decalcification of C–S–H, the dissolution of ettringite and a magnesium enrichment are the main phenomena in both cases. However, we observe a larger and deeper increase of porosity when the calcareous water is in contact with the cementitious material. The chemical modifications linked to the increase of porosity tend to locally decrease the mechanical strength of the cementitious matrix

Comparison of the chemo-mechanical behavior of low-pH cement exposed to calcareous water and to argillite pore water

This paper focuses on the impact of ground water on the chemo-mechanical behavior of low-pH cementitious materials. Two kinds of ground water are considered, calcareous water and argillite pore water. A sample of low-pH cement paste was immersed for two months in each solution. Chemo-mechanical characterizations (SEM, C-14 PMMA autoradiography, nano-indentation) were performed to estimate the degradation state of the samples. Then, geochemical modelling was performed to validate experimental observations. The decalcification of C–S–H, the dissolution of ettringite and a magnesium enrichment are the main phenomena in both cases. However, we observe a larger and deeper increase of porosity when the calcareous water is in contact with the cementitious material. The chemical modifications linked to the increase of porosity tend to locally decrease the mechanical strength of the cementitious matrix.Peer reviewe

DIFFUSION COEFFICIENT OF RADIONUCLIDES IN CEMENT-BASED MATERIALS: INVESTIGATING THE EFFECT OF THE CHARGED SURFACES

International audienceThis work aims to provide new insights into the effect of charged surfaces in cement-based materials through a significant sum of diffusion and electro-diffusion experiments on various radionuclides acquired in the framework of EURAD/CORI and different Andra projects. All the measurements have been analysed with the reactive transfer model PhreeqC setting apart the diffusion and retention mechanisms. HTO appeared to be the most mobile species compared with anions and cations; in particular, no anionic or cationic exclusion/acceleration effects have been found in these experiments. A modelling exercise focusing on the behavior of anionic species, which are the main contributors to the dose, improves the understanding of their diffusion within pristine and degraded cement-based materials. The outcomes of this work will specifically reduce the uncertainties regarding the fate of 36Cl

DIFFUSION COEFFICIENT OF RADIONUCLIDES IN CEMENT-BASED MATERIALS: INVESTIGATING THE EFFECT OF THE CHARGED SURFACES

International audienceThis work aims to provide new insights into the effect of charged surfaces in cement-based materials through a significant sum of diffusion and electro-diffusion experiments on various radionuclides acquired in the framework of EURAD/CORI and different Andra projects. All the measurements have been analysed with the reactive transfer model PhreeqC setting apart the diffusion and retention mechanisms. HTO appeared to be the most mobile species compared with anions and cations; in particular, no anionic or cationic exclusion/acceleration effects have been found in these experiments. A modelling exercise focusing on the behavior of anionic species, which are the main contributors to the dose, improves the understanding of their diffusion within pristine and degraded cement-based materials. The outcomes of this work will specifically reduce the uncertainties regarding the fate of 36Cl

DIFFUSION COEFFICIENT OF RADIONUCLIDES IN CEMENT-BASED MATERIALS: INVESTIGATING THE EFFECT OF THE CHARGED SURFACES

International audienceThis work aims to provide new insights into the effect of charged surfaces in cement-based materials through a significant sum of diffusion and electro-diffusion experiments on various radionuclides acquired in the framework of EURAD/CORI and different Andra projects. All the measurements have been analysed with the reactive transfer model PhreeqC setting apart the diffusion and retention mechanisms. HTO appeared to be the most mobile species compared with anions and cations; in particular, no anionic or cationic exclusion/acceleration effects have been found in these experiments. A modelling exercise focusing on the behavior of anionic species, which are the main contributors to the dose, improves the understanding of their diffusion within pristine and degraded cement-based materials. The outcomes of this work will specifically reduce the uncertainties regarding the fate of 36Cl

DIFFUSION COEFFICIENT OF RADIONUCLIDES IN CEMENT-BASED MATERIALS: INVESTIGATING THE EFFECT OF THE CHARGED SURFACES

International audienceThis work aims to provide new insights into the effect of charged surfaces in cement-based materials through a significant sum of diffusion and electro-diffusion experiments on various radionuclides acquired in the framework of EURAD/CORI and different Andra projects. All the measurements have been analysed with the reactive transfer model PhreeqC setting apart the diffusion and retention mechanisms. HTO appeared to be the most mobile species compared with anions and cations; in particular, no anionic or cationic exclusion/acceleration effects have been found in these experiments. A modelling exercise focusing on the behavior of anionic species, which are the main contributors to the dose, improves the understanding of their diffusion within pristine and degraded cement-based materials. The outcomes of this work will specifically reduce the uncertainties regarding the fate of 36Cl

Transport properties evolution of cement model system under degradation - Incorporation of a pore-scale approach into reactive transport modelling

International audienceThis paper describes a multi-scale approach for the modelling of the degradation of model cement pastesusing reactive transport. It specifically aims at incorporating chemistry-transport feedback results from apore-scale approach into a continuum description. Starting from a numerical representative elementaryvolume of the model cement paste, which was built according to extensive experimental dedicatedchacarterizations, this paper provides three separate descriptions of two different degradations: leachingand carbonation. First, 2D pore-scale simulations are performed and predict degradation depths in verygood agreement with experiments. Second, 3D pore scale descriptions of how the microstructre evolvesprovides accurate description of the evolution of transport properties through degradation. Finally, thoselatter results are incorporated as a feedback law between porosity and effective diffusion coefficient intoa 1D continuum approach of reactive transport. This paper provides pore-scale explanations of whyreactive transport modelling has encountered mitigated success when applied to cementitious materials,especially during carbonation or degradations consisting of precipitation reactions. In addition to that,different degradation modellings are in very good agreement with experimental observations

Imaging connected porosity of crystalline rock by contrast agent-aided X-ray microtomography and scanning electron microscopy

We set out to study connected porosity of crystalline rock using X‐ray microtomography and scanning electron microscopy with energy dispersive X‐ray spectroscopy (SEM‐EDS) with caesium chloride as a contrast agent. Caesium is an important radionuclide regarding the final deposition of nuclear waste and also forms dense phases that can be readily distinguished by X‐ray microtomography and SEM‐EDS. Six samples from two sites, Olkiluoto (Finland) and Grimsel (Switzerland), where transport properties of crystalline rock are being studied in situ, were investigated using X‐ray microtomography and SEM‐EDS. The samples were imaged with X‐ray microtomography, immersed in a saturated caesium chloride (CsCl) solution for 141, 249 and 365 days and imaged again with X‐ray microtomography. CsCl inside the samples was successfully detected with X‐ray microtomography and it had completely penetrated all six samples. SEM‐EDS elemental mapping was used to study the location of caesium in the samples in detail with quantitative mineral information. Precipitated CsCl was found in the connected pore space in Olkiluoto veined gneiss and in lesser amounts in Grimsel granodiorite. Only a very small amount of precipitated CsCl was observed in the Grimsel granodiorite samples. In Olkiluoto veined gneiss caesium was found in pinitised areas of cordierite grains. In the pinitised areas caesium was found in notable excess compared to chloride, possibly due to the combination of small pore size and negatively charged surfaces. In addition, elevated concentrations of caesium were found in kaolinite and sphalerite phases. The findings concerning the location of CsCl were congruent with X‐ray microtomography.peerReviewe