Christophe Poinssot | Commissariat à l'Energie Atomique (CEA) (original) (raw)

Papers by Christophe Poinssot

International audienceIn the frame of the successive 1991 and 2006 Waste Management Acts, French ... more International audienceIn the frame of the successive 1991 and 2006 Waste Management Acts, French government supported a very significant RetD program on partitioning and transmutation of minor actinides (MA) in fast reactors. This program aimed to study potential solutions for still minimizing the quantity and the hazardousness of final waste, by MA recycling. Indeed, MA recycling can reduce the heat load and the radiotoxicity of most of the waste to be buried to a couple of hundred years, overcoming the concerns of the public related to the long-life of the waste. Over the 20 years of development, different types of strategies were studied, from the early multi-stage DIAMEX-SANEX processes to the most recent innovative SANEX, from the grouped extraction of MA thanks to the GANEX process to the most recent sole Americium recycling thanks to the EXAm process. These developments were supported by a robust and long-standing approach allowing successively to screen the potential extract...

ATALANTE 2016 - Nuclear Chemistry for Sustainable Fuel Cycles, Jun 6, 2016

International audienceA better understanding of the formation of actinide (IV) silicates from the... more International audienceA better understanding of the formation of actinide (IV) silicates from the alteration of oxides in spent nuclear fuel under geologic repository conditions, and above all their stability in contact with groundwater are crucial to evaluate the possible release of radionuclides into the environment. To this aim, the formation of thorite, ThSiO4, and coffinite, USiO4 and their thermodynamic properties have already been extensively studied [1;2]. If hydrothermal syntheses appear to be an efficient way to obtain most of silicates, the aim of this study is to adapt and determine optimal synthesis conditions in the case of the plutonium system. In fact, little is known about PuSiO4 (zircon type structure); Keller's work being the only one to describe its synthesis [3]. Furthermore, no hydrothermal synthesis of orthosilicates has been performed using reactants in nitric acid medium which is usually used to stabilize PuIV.Therefore, the first step of this study was to perform and optimize the syntheses of ThSiO4 and CeSiO4, starting from ThIV and CeIV in nitric acid media, in order to determine suitable conditions for PuSiO4 synthesis. On the one hand, CeIV is usually used as a surrogate for PuIV, because of the really close ionic radii of Ce4+ and Pu4+ and the possible stabilization of their III/IV oxidation states under close conditions. CeSiO4 hydrothermal synthesis is reported [4] but the protocol is different from Keller's PuSiO4 synthesis. On the other hand, ThIV speciation in solution would be more representative of PuIV than CeIV. The first ThSiO4 hydrothermal synthesis was performed by Frondel and Collette [5], since improvements of the protocol was achieved by Costin et al. [6;7].Based on these last developments, experiments were thus performed to determine the influence of the nature of the medium, its acidity and the duration of the hydrothermal treatment on the yield of CeSiO4 and ThSiO4 formation prior to the transposition to PuSiO4.[1]A. Mesbah et al., Inorg. Chem., vol. 14, no. 54, pp. 66876696, 2015.[2]S. Szenknect et al., Inorg. Chem., vol. 52, no. 12, pp. 69576968, 2013.[3]C. Keller, Nukleonik, vol. 5, no. 2, pp. 4148, 1963.[4]J. M. S. Skakle et al., Powder Diffr., vol. 15, no. 4, pp. 234238, 2000.[5]C. Frondel and R. L. Collette, Am. Mineral., vol. 42, no. 378, pp. 759765, 1957.[6]D. T. Costin et al., Inorg Chem, vol. 50, no. 21, pp. 1111711126, 2011.[7]D. T. Costin et al., Prog. Nucl. Energy, vol. 57, pp. 155160, 2012

<jats:title>Abstract</jats:title> <jats:p>Under the geological disposal conditi... more <jats:title>Abstract</jats:title> <jats:p>Under the geological disposal conditions, spent fuel (SF) is expected to evolve during the 10,000 years while being maintained isolated from the biosphere before water comes in. Under those circumstances, several driving forces would lead to the progressive intrinsic transformations within the rod which would modify the subsequent release of radionuclides: the production of a significant volume of He, the accumulation of irradiation defects, the slow migration of radionuclides (RN) within the pellet. However, the current RN source terms for SF never accounted for these evolutions and was based on the existing knowledge on the fresh SF. Two major mechanisms were considered, the leaching of the readily available fraction (one which was supposed to be instantly accessible to water), and the release of RN through alteration of the UO2 grains. We are now proposing a new RN source term model based on a microscopic description of the system in order to also account for the early evolution of the closed system, the amplitude of which increases with the burnup and is greater for MOX fuels.</jats:p>

Dalton Transactions, 2020

In-situ SWAXS measurements attested that thorite synthesis was possible at quite low temperature,... more In-situ SWAXS measurements attested that thorite synthesis was possible at quite low temperature, moving from colloidal state at low reaction time to crystalline phase at infinite reaction time.

Dalton Transactions, 2020

This study reports an innovative way of synthesis of PuSiO4by hydrothermalin situoxidation of sol... more This study reports an innovative way of synthesis of PuSiO4by hydrothermalin situoxidation of solid Pu(iii) silicate precursors. It also identifies how representative Th-, U- and Ce-surrogates are of Pu chemistry in silicate ions rich media.

Crystal Growth & Design, 2020

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Dalton Transactions, 2019

Optimized conditions to form CeSiO4 (zircon type) under hydrothermal conditons were determined an... more Optimized conditions to form CeSiO4 (zircon type) under hydrothermal conditons were determined and allowed to propose a potential chemical mechanism.

Inorganic Chemistry, 2018

A multiparametric study of the hydrothermal synthesis of ThSiO 4 , thorite, was performed with th... more A multiparametric study of the hydrothermal synthesis of ThSiO 4 , thorite, was performed with the aim of determining the most efficient conditions to form single-phase samples. Among the experimental parameters examined, significant effects were found for the concentration of reactants in the starting mixture, pH of the reactive media, and temperature of the hydrothermal process. Such parameters affected both the rate of formation of thorite and the morphology of the final products synthesized. Precipitation of pure ThSiO 4 was obtained over a wide range of pH on going from C HNO3 = 0.3 mol L −1 to pH 9.1 with a yield of over 95%. Temperatures higher than 160°C favor the formation of thorite. Finally, thorium and silicon concentrations above 2.1 × 10 −3 mol L −1 are required to obtain pure thorium silicate.

European Journal of Mineralogy, 1996

Hydrothermal alteration of a simulated nuclear waste glass efTects of a thermal gradient and of a... more Hydrothermal alteration of a simulated nuclear waste glass efTects of a thermal gradient and of a chemie al barrier CHRISTOPHE POINSSOT (!)(2l , BRUNO GOFF E(!) , MARlE-CLAUDE MAGONTHIER (2) and PIERRE TOULHOAT (2)

Energies, 2017

In this early 21st century, our societies have to face a tremendous and increasing energy need wh... more In this early 21st century, our societies have to face a tremendous and increasing energy need while mitigating the global climate change and preserving the environment. Addressing this challenge requires an energy transition from the current fossil energy-based system to a carbon-free energy production system, based on a relevant energy mix combining renewables and nuclear energy. However, such an energy transition will only occur if it is accepted by the population. Powerful and reliable tools, such as life cycle assessments (LCA), aiming at assessing the respective merits of the different energy mix for most of the environmental impact indicators are therefore mandatory for supporting a risk-informed decision-process at the societal level. Before studying the deployment of a given energy mix, a prerequisite is to perform LCAs on each of the components of the mix. This paper addresses two potential nuclear energy components: a nuclear fuel cycle based on the Generation III European Pressurized Reactors (EPR) and a nuclear fuel cycle based on the Generation IV Sodium Fast Reactors (SFR). The basis of this study relies on the previous work done on the current French nuclear fuel cycle using the bespoke NELCAS tool specifically developed for studying nuclear fuel cycle environmental impacts. Our study highlights that the EPR already brings a limited improvement to the current fuel cycle thanks to a higher efficiency of the energy transformation and a higher burn-up of the nuclear fuel (−20% on most of the chosen indicators) whereas the introduction of the GEN IV fast reactors will bring a significant breakthrough by suppressing the current front-end of the fuel cycle thanks to the use of depleted uranium instead of natural enriched uranium (this leads to a decrease of the impact from 17% on water consumption and withdrawal and up to 96% on SO x emissions). The specific case of the radioactive waste is also studied, showing that only the partitioning and transmutation of the americium in the blanket fuel of the SFR can reduce the footprint of the geological disposal (saving up to a factor of 7 on the total repository volume). Having now at disposition five models (open fuel cycle, current French twice through fuel cycle, EPR twice through fuel cycle, multi-recycling SFR fuel cycle and at a longer term, multi-recycling SFR fuel cycle including americium transmutation), it is possible to model the environmental impact of any fuel cycle combining these technologies. In the next step, these models will be combined with those of other carbon-free energies (wind, solar, biomass. . .) in order to estimate the environmental impact of future energy mixes and also to analyze the impact on the way these scenarios are deployed (transition pathways).

Chemistry of Materials, 2016

High-temperature oxide melt solution calorimetric measurements were completed to determine the en... more High-temperature oxide melt solution calorimetric measurements were completed to determine the enthalpies of formation of the uranothorite, (USiO 4) x −(ThSiO 4) 1−x , solid solution. Phase-pure samples with x values of 0, 0.11, 0.21, 0.35, 0.71, and 0.84 were prepared, purified, and characterized by powder X-ray diffraction, electron probe microanalysis, thermogravimetric analysis and differential scanning calorimetry coupled with in situ mass spectrometry, and hightemperature oxide melt solution calorimetry. This work confirms the energetic metastability of coffinite, USiO 4 , and U-rich intermediate silicate phases with respect to a mixture of binary oxides. However, variations in unit cell parameters and negative excess volumes of mixing, coupled with strongly exothermic enthalpies of mixing in the solid solution, suggest short-range cation ordering that can stabilize intermediate compositions, especially near x = 0.5.

Data Revues 16310713 03340014 02018497, Apr 4, 2008

Geochimica et Cosmochimica Acta, 2016

Dissolution experiments have been performed in order to determine the solubility constant of coff... more Dissolution experiments have been performed in order to determine the solubility constant of coffinite, USiO 4. Several assemblages of phases were used in under-saturated experiments performed in 0.1 mol L-1 HCl under Ar atmosphere, as well as in air. These samples were fully-characterized and were composed of either USiO 4 , solely, or USiO 4 and additional oxide byproducts that resulted from the synthesis procedure. The solubility constant of coffinite was determined at 25°C and 1 bar (log *K S°(USiO 4 , cr) =-5.25 ± 0.05), as well as the standard free energy of formation of coffinite (Δ f G°(298 K) =-1867.6 ± 3.2 kJ mol-1), which enables one to infer the relative stability of coffinite and uraninite as a function of groundwater composition. Geochemical simulations using PHREEQC 2 software and the Thermochimie data base indicate that coffinite precipitates at 25°C under reducing conditions, at pH = 6, for H 4 SiO 4 (aq) concentration of 7 10-5 mol L-1 and U(OH) 4 (aq) concentration of 10-11 mol L-1. The Δ f G°

MRS Proceedings, 1997

ABSTRACTThe initial stage of some HLW disposal systems will be characterised by a large thermal p... more ABSTRACTThe initial stage of some HLW disposal systems will be characterised by a large thermal pulse in the near-field environment, due to the heat of the radioactivity decay. This will lead to the development of a transient spatial thermal gradient between the hot canister and the cold geological medium, which could significantly affect the composition and the elemental distribution within the near-field environment. A coupled experimental and modelling work is presented in order to determine the influence of a thermal gradient on water-rock interaction processes. First experiments with a simulated nuclear glass evidenced mass transfer processes leading to chemical differentiation in the solid phases between the hot and the cold end of the system. The relevance of these experimental results to the case of a HLW disposal is strongly supported by in-situ experiments at Stripa, in which a realistic EBS under thermal gradient developed exactly the same mass transfers.In order to under...

Journal of Nuclear Science and Technology, 2002

This paper aims to give a brief overview of the wide research undertaken in France in order to el... more This paper aims to give a brief overview of the wide research undertaken in France in order to elucidate the potential long term evolution of spent nuclear fuels in long term storage or geological disposal. Scientific key issues related to the potential long term evolution in closed system, in presence of and oxidative phase and in presence of water are presented as well as the anticipated trends. A particular emphasis is put on the major outcomes of this research which is a new definition of the radionuclides source term for the geological disposal: we estimate that we have now to allocate a higher fraction of the radionuclides inventory to the so-called instant release fraction which is instantaneously released in presence of water.

MRS Proceedings, 2006

ABSTRACTEu migration in a Callovo-Oxfordian argilite sample was studied using the micro LIBS tech... more ABSTRACTEu migration in a Callovo-Oxfordian argilite sample was studied using the micro LIBS technique. Quantitative elemental mapping were made by micro LIBS that showed the actual distribution of the various micro areas observed on the Callovo-Oxfordian sample's surface. Calcite, dolomite, alumino-silicates, quartz, pyrite and iron oxides were identified and their statistical distribution was determined. Experimental Eu profiles observed are consistent with diffusion process accompanied by heterogeneous sorption on alumino-silicate surface.

Nuclear Engineering International

The global energy context argues in favour of the sustainable development of nuclear energy, sinc... more The global energy context argues in favour of the sustainable development of nuclear energy, since the demand for energy will significantly increase, while resources will tend to get scarcer. Reprocessing and recycling nuclear fuel, together with fast reactors, can help nuclear power to conserve existing uranium resources and reduce the nuclear waste burden for future generations.

MRS Proceedings, 2000

ABSTRACTUnder the geological disposal conditions, spent fuel (SF) is expected to evolve during th... more ABSTRACTUnder the geological disposal conditions, spent fuel (SF) is expected to evolve during the 10,000 years while being maintained isolated from the biosphere before coming in contact with water. Under these circumstances, several driving forces would lead to the progressive intrinsic transformations within the rod which would modify the subsequent release of radionuclides. The major mechanisms are the production of a significant volume of He within the UO2 lattice, the accumulation of irradiation defects due to the low temperature which avoids any annealing, the slow migration of radionuclides (RN) within the system by (i) the α self-irradiation-induced athermal diffusion and (ii) locally the building-up of internal mechanical stresses which could turn the pellets into powder. However, the current RN source terms for SF have never accounted for this evolution as they are based on existing knowledge of the fresh SF. Two major mechanisms were considered, the leaching of the readi...

International audienceIn the frame of the successive 1991 and 2006 Waste Management Acts, French ... more International audienceIn the frame of the successive 1991 and 2006 Waste Management Acts, French government supported a very significant RetD program on partitioning and transmutation of minor actinides (MA) in fast reactors. This program aimed to study potential solutions for still minimizing the quantity and the hazardousness of final waste, by MA recycling. Indeed, MA recycling can reduce the heat load and the radiotoxicity of most of the waste to be buried to a couple of hundred years, overcoming the concerns of the public related to the long-life of the waste. Over the 20 years of development, different types of strategies were studied, from the early multi-stage DIAMEX-SANEX processes to the most recent innovative SANEX, from the grouped extraction of MA thanks to the GANEX process to the most recent sole Americium recycling thanks to the EXAm process. These developments were supported by a robust and long-standing approach allowing successively to screen the potential extract...

ATALANTE 2016 - Nuclear Chemistry for Sustainable Fuel Cycles, Jun 6, 2016

International audienceA better understanding of the formation of actinide (IV) silicates from the... more International audienceA better understanding of the formation of actinide (IV) silicates from the alteration of oxides in spent nuclear fuel under geologic repository conditions, and above all their stability in contact with groundwater are crucial to evaluate the possible release of radionuclides into the environment. To this aim, the formation of thorite, ThSiO4, and coffinite, USiO4 and their thermodynamic properties have already been extensively studied [1;2]. If hydrothermal syntheses appear to be an efficient way to obtain most of silicates, the aim of this study is to adapt and determine optimal synthesis conditions in the case of the plutonium system. In fact, little is known about PuSiO4 (zircon type structure); Keller's work being the only one to describe its synthesis [3]. Furthermore, no hydrothermal synthesis of orthosilicates has been performed using reactants in nitric acid medium which is usually used to stabilize PuIV.Therefore, the first step of this study was to perform and optimize the syntheses of ThSiO4 and CeSiO4, starting from ThIV and CeIV in nitric acid media, in order to determine suitable conditions for PuSiO4 synthesis. On the one hand, CeIV is usually used as a surrogate for PuIV, because of the really close ionic radii of Ce4+ and Pu4+ and the possible stabilization of their III/IV oxidation states under close conditions. CeSiO4 hydrothermal synthesis is reported [4] but the protocol is different from Keller's PuSiO4 synthesis. On the other hand, ThIV speciation in solution would be more representative of PuIV than CeIV. The first ThSiO4 hydrothermal synthesis was performed by Frondel and Collette [5], since improvements of the protocol was achieved by Costin et al. [6;7].Based on these last developments, experiments were thus performed to determine the influence of the nature of the medium, its acidity and the duration of the hydrothermal treatment on the yield of CeSiO4 and ThSiO4 formation prior to the transposition to PuSiO4.[1]A. Mesbah et al., Inorg. Chem., vol. 14, no. 54, pp. 66876696, 2015.[2]S. Szenknect et al., Inorg. Chem., vol. 52, no. 12, pp. 69576968, 2013.[3]C. Keller, Nukleonik, vol. 5, no. 2, pp. 4148, 1963.[4]J. M. S. Skakle et al., Powder Diffr., vol. 15, no. 4, pp. 234238, 2000.[5]C. Frondel and R. L. Collette, Am. Mineral., vol. 42, no. 378, pp. 759765, 1957.[6]D. T. Costin et al., Inorg Chem, vol. 50, no. 21, pp. 1111711126, 2011.[7]D. T. Costin et al., Prog. Nucl. Energy, vol. 57, pp. 155160, 2012

<jats:title>Abstract</jats:title> <jats:p>Under the geological disposal conditi... more <jats:title>Abstract</jats:title> <jats:p>Under the geological disposal conditions, spent fuel (SF) is expected to evolve during the 10,000 years while being maintained isolated from the biosphere before water comes in. Under those circumstances, several driving forces would lead to the progressive intrinsic transformations within the rod which would modify the subsequent release of radionuclides: the production of a significant volume of He, the accumulation of irradiation defects, the slow migration of radionuclides (RN) within the pellet. However, the current RN source terms for SF never accounted for these evolutions and was based on the existing knowledge on the fresh SF. Two major mechanisms were considered, the leaching of the readily available fraction (one which was supposed to be instantly accessible to water), and the release of RN through alteration of the UO2 grains. We are now proposing a new RN source term model based on a microscopic description of the system in order to also account for the early evolution of the closed system, the amplitude of which increases with the burnup and is greater for MOX fuels.</jats:p>

Dalton Transactions, 2020

In-situ SWAXS measurements attested that thorite synthesis was possible at quite low temperature,... more In-situ SWAXS measurements attested that thorite synthesis was possible at quite low temperature, moving from colloidal state at low reaction time to crystalline phase at infinite reaction time.

Dalton Transactions, 2020

This study reports an innovative way of synthesis of PuSiO4by hydrothermalin situoxidation of sol... more This study reports an innovative way of synthesis of PuSiO4by hydrothermalin situoxidation of solid Pu(iii) silicate precursors. It also identifies how representative Th-, U- and Ce-surrogates are of Pu chemistry in silicate ions rich media.

Crystal Growth & Design, 2020

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Dalton Transactions, 2019

Optimized conditions to form CeSiO4 (zircon type) under hydrothermal conditons were determined an... more Optimized conditions to form CeSiO4 (zircon type) under hydrothermal conditons were determined and allowed to propose a potential chemical mechanism.

Inorganic Chemistry, 2018

A multiparametric study of the hydrothermal synthesis of ThSiO 4 , thorite, was performed with th... more A multiparametric study of the hydrothermal synthesis of ThSiO 4 , thorite, was performed with the aim of determining the most efficient conditions to form single-phase samples. Among the experimental parameters examined, significant effects were found for the concentration of reactants in the starting mixture, pH of the reactive media, and temperature of the hydrothermal process. Such parameters affected both the rate of formation of thorite and the morphology of the final products synthesized. Precipitation of pure ThSiO 4 was obtained over a wide range of pH on going from C HNO3 = 0.3 mol L −1 to pH 9.1 with a yield of over 95%. Temperatures higher than 160°C favor the formation of thorite. Finally, thorium and silicon concentrations above 2.1 × 10 −3 mol L −1 are required to obtain pure thorium silicate.

European Journal of Mineralogy, 1996

Hydrothermal alteration of a simulated nuclear waste glass efTects of a thermal gradient and of a... more Hydrothermal alteration of a simulated nuclear waste glass efTects of a thermal gradient and of a chemie al barrier CHRISTOPHE POINSSOT (!)(2l , BRUNO GOFF E(!) , MARlE-CLAUDE MAGONTHIER (2) and PIERRE TOULHOAT (2)

Energies, 2017

In this early 21st century, our societies have to face a tremendous and increasing energy need wh... more In this early 21st century, our societies have to face a tremendous and increasing energy need while mitigating the global climate change and preserving the environment. Addressing this challenge requires an energy transition from the current fossil energy-based system to a carbon-free energy production system, based on a relevant energy mix combining renewables and nuclear energy. However, such an energy transition will only occur if it is accepted by the population. Powerful and reliable tools, such as life cycle assessments (LCA), aiming at assessing the respective merits of the different energy mix for most of the environmental impact indicators are therefore mandatory for supporting a risk-informed decision-process at the societal level. Before studying the deployment of a given energy mix, a prerequisite is to perform LCAs on each of the components of the mix. This paper addresses two potential nuclear energy components: a nuclear fuel cycle based on the Generation III European Pressurized Reactors (EPR) and a nuclear fuel cycle based on the Generation IV Sodium Fast Reactors (SFR). The basis of this study relies on the previous work done on the current French nuclear fuel cycle using the bespoke NELCAS tool specifically developed for studying nuclear fuel cycle environmental impacts. Our study highlights that the EPR already brings a limited improvement to the current fuel cycle thanks to a higher efficiency of the energy transformation and a higher burn-up of the nuclear fuel (−20% on most of the chosen indicators) whereas the introduction of the GEN IV fast reactors will bring a significant breakthrough by suppressing the current front-end of the fuel cycle thanks to the use of depleted uranium instead of natural enriched uranium (this leads to a decrease of the impact from 17% on water consumption and withdrawal and up to 96% on SO x emissions). The specific case of the radioactive waste is also studied, showing that only the partitioning and transmutation of the americium in the blanket fuel of the SFR can reduce the footprint of the geological disposal (saving up to a factor of 7 on the total repository volume). Having now at disposition five models (open fuel cycle, current French twice through fuel cycle, EPR twice through fuel cycle, multi-recycling SFR fuel cycle and at a longer term, multi-recycling SFR fuel cycle including americium transmutation), it is possible to model the environmental impact of any fuel cycle combining these technologies. In the next step, these models will be combined with those of other carbon-free energies (wind, solar, biomass. . .) in order to estimate the environmental impact of future energy mixes and also to analyze the impact on the way these scenarios are deployed (transition pathways).

Chemistry of Materials, 2016

High-temperature oxide melt solution calorimetric measurements were completed to determine the en... more High-temperature oxide melt solution calorimetric measurements were completed to determine the enthalpies of formation of the uranothorite, (USiO 4) x −(ThSiO 4) 1−x , solid solution. Phase-pure samples with x values of 0, 0.11, 0.21, 0.35, 0.71, and 0.84 were prepared, purified, and characterized by powder X-ray diffraction, electron probe microanalysis, thermogravimetric analysis and differential scanning calorimetry coupled with in situ mass spectrometry, and hightemperature oxide melt solution calorimetry. This work confirms the energetic metastability of coffinite, USiO 4 , and U-rich intermediate silicate phases with respect to a mixture of binary oxides. However, variations in unit cell parameters and negative excess volumes of mixing, coupled with strongly exothermic enthalpies of mixing in the solid solution, suggest short-range cation ordering that can stabilize intermediate compositions, especially near x = 0.5.

Data Revues 16310713 03340014 02018497, Apr 4, 2008

Geochimica et Cosmochimica Acta, 2016

Dissolution experiments have been performed in order to determine the solubility constant of coff... more Dissolution experiments have been performed in order to determine the solubility constant of coffinite, USiO 4. Several assemblages of phases were used in under-saturated experiments performed in 0.1 mol L-1 HCl under Ar atmosphere, as well as in air. These samples were fully-characterized and were composed of either USiO 4 , solely, or USiO 4 and additional oxide byproducts that resulted from the synthesis procedure. The solubility constant of coffinite was determined at 25°C and 1 bar (log *K S°(USiO 4 , cr) =-5.25 ± 0.05), as well as the standard free energy of formation of coffinite (Δ f G°(298 K) =-1867.6 ± 3.2 kJ mol-1), which enables one to infer the relative stability of coffinite and uraninite as a function of groundwater composition. Geochemical simulations using PHREEQC 2 software and the Thermochimie data base indicate that coffinite precipitates at 25°C under reducing conditions, at pH = 6, for H 4 SiO 4 (aq) concentration of 7 10-5 mol L-1 and U(OH) 4 (aq) concentration of 10-11 mol L-1. The Δ f G°

MRS Proceedings, 1997

ABSTRACTThe initial stage of some HLW disposal systems will be characterised by a large thermal p... more ABSTRACTThe initial stage of some HLW disposal systems will be characterised by a large thermal pulse in the near-field environment, due to the heat of the radioactivity decay. This will lead to the development of a transient spatial thermal gradient between the hot canister and the cold geological medium, which could significantly affect the composition and the elemental distribution within the near-field environment. A coupled experimental and modelling work is presented in order to determine the influence of a thermal gradient on water-rock interaction processes. First experiments with a simulated nuclear glass evidenced mass transfer processes leading to chemical differentiation in the solid phases between the hot and the cold end of the system. The relevance of these experimental results to the case of a HLW disposal is strongly supported by in-situ experiments at Stripa, in which a realistic EBS under thermal gradient developed exactly the same mass transfers.In order to under...

Journal of Nuclear Science and Technology, 2002

This paper aims to give a brief overview of the wide research undertaken in France in order to el... more This paper aims to give a brief overview of the wide research undertaken in France in order to elucidate the potential long term evolution of spent nuclear fuels in long term storage or geological disposal. Scientific key issues related to the potential long term evolution in closed system, in presence of and oxidative phase and in presence of water are presented as well as the anticipated trends. A particular emphasis is put on the major outcomes of this research which is a new definition of the radionuclides source term for the geological disposal: we estimate that we have now to allocate a higher fraction of the radionuclides inventory to the so-called instant release fraction which is instantaneously released in presence of water.

MRS Proceedings, 2006

ABSTRACTEu migration in a Callovo-Oxfordian argilite sample was studied using the micro LIBS tech... more ABSTRACTEu migration in a Callovo-Oxfordian argilite sample was studied using the micro LIBS technique. Quantitative elemental mapping were made by micro LIBS that showed the actual distribution of the various micro areas observed on the Callovo-Oxfordian sample's surface. Calcite, dolomite, alumino-silicates, quartz, pyrite and iron oxides were identified and their statistical distribution was determined. Experimental Eu profiles observed are consistent with diffusion process accompanied by heterogeneous sorption on alumino-silicate surface.

Nuclear Engineering International

The global energy context argues in favour of the sustainable development of nuclear energy, sinc... more The global energy context argues in favour of the sustainable development of nuclear energy, since the demand for energy will significantly increase, while resources will tend to get scarcer. Reprocessing and recycling nuclear fuel, together with fast reactors, can help nuclear power to conserve existing uranium resources and reduce the nuclear waste burden for future generations.

MRS Proceedings, 2000

ABSTRACTUnder the geological disposal conditions, spent fuel (SF) is expected to evolve during th... more ABSTRACTUnder the geological disposal conditions, spent fuel (SF) is expected to evolve during the 10,000 years while being maintained isolated from the biosphere before coming in contact with water. Under these circumstances, several driving forces would lead to the progressive intrinsic transformations within the rod which would modify the subsequent release of radionuclides. The major mechanisms are the production of a significant volume of He within the UO2 lattice, the accumulation of irradiation defects due to the low temperature which avoids any annealing, the slow migration of radionuclides (RN) within the system by (i) the α self-irradiation-induced athermal diffusion and (ii) locally the building-up of internal mechanical stresses which could turn the pellets into powder. However, the current RN source terms for SF have never accounted for this evolution as they are based on existing knowledge of the fresh SF. Two major mechanisms were considered, the leaching of the readi...