CEA Valduc Center for Nuclear Studies

Valduc, France

CEA Valduc Center for Nuclear Studies

Valduc, France
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Soulard L.,CEA DAM Ile-de-France | Bontaz-Carion J.,CEA DAM Ile-de-France | Cuq-Lelandais J.P.,CEA Valduc Center for Nuclear Studies | Cuq-Lelandais J.P.,CNRS Pprime Institute
European Physical Journal B | Year: 2012

Using X-microtomography and non equilibrium classical molecular dynamics, we present a study of the elementary processes of spallation of single crystal tantalum. The single crystal is illuminated by a laser pulse which induces the propagation of a strong unsustained shock. The analysed data mainly are number and shape of pores resulting from the tensile inside the material when the incident shock reflects on the opposite face. Experimental pores size distribution exhibits two power laws attributed to the growth and the coalescence stages. The average pore shape is ellipsoid with main axis along the shock axis propagation. This first part is completed by a large scale molecular dynamics simulation mimics at reduced scale the real experiment. After preliminary calculations validating the chosen potential function the formation and shock propagation is detailed. Then we extract from the simulation similar data than in experiment. The pores size distribution shows three power laws identified as the nucleation, the growth and the coalescence stages. The slopes of the two last stages are very similar to the experimental one, confirming the scale invariance of this data as suggested by their analytical form. The general pore shape also is close to the experiment shape but with a different orientation (perpendicular to the shock propagation axis). © 2012 EDP Sciences, Società Italiana di Fisica, Springer-Verlag.

Lalire F.,CEA Valduc Center for Nuclear Studies | Appolaire B.,ONERA | Aeby-Gautier E.,CNRS Jean Lamour Institute
Materials Today: Proceedings | Year: 2015

A step-by-step modeling approach, from the Phenomenological Theory of Martensite (PTM) to full field elasto-viscoplastic calculations, has been used to investigate the isothermal δ → α' martensitic transformation in Pu-1at.%Ga. Based on the different calculations, an explanation is proposed for the peculiar morphological arrangements observed in this alloy at transformation temperatures near -20. °C. © 2015 The Authors.

Joubert J.-M.,CNRS East Paris Institute of Chemistry and Materials Science | Thiebaut S.,CEA Valduc Center for Nuclear Studies
Acta Materialia | Year: 2011

The quinary system D-H-Pd-Rh-T has been described thermodynamically by the CALPHAD approach. Previous descriptions of the binary subsystems have been used. To model the high pressure data an equation of state for the gases D 2 and T2 compatible with the CALPHAD approach has been obtained similar to that previously used for H2. A complete literature search has been undertaken for the three ternary systems H-Pd-Rh, D-Pd-Rh and Pd-Rh-T and the most significant experimental data have been selected for a thermodynamic assessment of these systems. In order to complement the available data, pressure-composition curves have been measured at different temperatures for the two last systems in the present work. Calculations and optimization of the system under para-equilibrium conditions, i.e. in pseudo-binary systems (Pd,Rh)-H, (Pd,Rh)-D or (Pd,Rh)-T, have been achieved using a pseudo-atom describing the Pd-Rh solid solution. This special method allows the presence of a miscibility gap in the binary metallic system to be dealt with. We show that a simple combination of the binary systems alone is unable to properly describe these ternary systems and that ternary interaction parameters have to be introduced. The binary and ternary systems may then be combined to perform calculations in the quinary D-H-Pd-Rh-T system. It is believed that extrapolation in systems containing different isotopes are fairly accurate provided that the so-called Toop model is used. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Ollagnier A.,Laboratory Interdisciplinaire Carnot de Bourgogne | Fabre A.,CEA Valduc Center for Nuclear Studies | Thundat T.,University of Alberta | Finot E.,Laboratory Interdisciplinaire Carnot de Bourgogne
Sensors and Actuators, B: Chemical | Year: 2013

Microcantilever-based thin film palladium hydrogen sensors have high selectivity and sensitivity. Reproducibility and accuracy of the sensor performance depend on the activation process of the polycrystalline palladium film deposited on the cantilever. When the hydrogen is in solid solution (α-phase), the cantilever bending is mostly governed by the residual film stress induced by the swelling at the grain boundaries in the film. When the palladium hydride (β-phase) starts to be formed, the cantilever undergoes a large deflection due to hydrogen absorption-induced film swelling (10% change in volume). Differences in the phase diagrams of the palladium hydride for two film thicknesses show that the cantilever bending is governed by hydrogen uptake as well as the release of the residual stress of the film through cyclic exposure and cycling number. © 2013 Elsevier.

Bissey-Breton S.,CEA Valduc Center for Nuclear Studies | Vignal V.,Laboratory Interdisciplinaire Carnot de Bourgogne
Procedia CIRP | Year: 2014

In this paper, the influence of a post-machining thermal treatment (PMTT) on the corrosion behaviour of copper was investigated in a salt fog atmosphere. The corrosion behaviour was affected by the presence of a high density of grain boundaries generated during machining or dislocations formed during PMTT under certain conditions. The obtained results showed that it is possible to find PMTT conditions to cancel changes induced by machining and that the critical factor leading to a sharp increase of the percentage of oxidized surface seems to be the density of dislocations near the machined surface. © 2014 The Authors. Published by Elsevier B.V.

Claux B.,CEA Valduc Center for Nuclear Studies | Claux B.,Joseph Fourier University | Serp J.,CEA Valduc Center for Nuclear Studies | Fouletier J.,Joseph Fourier University
Electrochimica Acta | Year: 2011

The Fray Farthing and Chen (FFC) and Ono and Suzuki (OS) processes were developed for the reduction of titanium oxide to titanium metal by electrolysis in high temperature molten alkali chloride salts. The possible transposition to CeO2 reduction is considered in this study. Present work clarifies, by electro-analytical techniques, the reduction pathway leading to the metal. The reduction of CeO2 into metal was feasible via an indirect mechanism. Electrolyses on 10 g of CeO2 were carried out to evaluate the electrochemical process efficiency. Ca metal is electrodeposited at the cathode from CaCl2-KCl solvent and reacts chemically with ceria to form not only metallic cerium, but also cerium oxychloride. © 2011 Elsevier Ltd. All rights reserved.

Faure P.,CEA Valduc Center for Nuclear Studies | Genestier C.,CEA Valduc Center for Nuclear Studies
Journal of Nuclear Materials | Year: 2010

Previous extensive studies of the δ → α′-phase transformation induced by temperature and/or by pressure in δ-stabilized plutonium alloys indicate strong dependence on parameters such as solute type, solute distribution, chemical impurities, kinetics, thermodynamic path....The present paper reports results obtained on two Pu-2.3at.%Ga binary alloys differing by solute homogenization treatment and studied under pressure by in situ by X-ray diffraction in diamond anvil cells. The γ′-phase appears as an intermediate phase during the δ → α′-phase transformation. In cored samples, unexpanded α′-phase is formed at the beginning of the transformation, from domains with low solute content, and expanded α′-phase subsequently forms (from domains with higher solute content) as the transformation progresses with the pressure increase. © 2010 Elsevier B.V. All rights reserved.

Perron A.,CEA Valduc Center for Nuclear Studies | Ravat B.,CEA Valduc Center for Nuclear Studies | Oudot B.,CEA Valduc Center for Nuclear Studies | Lalire F.,CEA Valduc Center for Nuclear Studies | And 2 more authors.
Acta Materialia | Year: 2013

Reverse transformation in plutonium alloyed with a small amount of gallium (Pu-1 at.% Ga) was investigated in detail by coupling CALPHAD-based simulations with experiments. As observed previously, reverse transformation in a two-phase δ + α′ alloy exhibits the existence of competition between direct reversion (α1at.%′→δ1at.%) and indirect reversion (α1at.%′→βpure Pu(+ δe)→γpure Pu(+δe) →δ1at.%). The latter leads to Ga enrichment of the δ phase (δe) as well as the emergence of several phases of pure Pu. The present paper focuses on the thermodynamic and kinetic aspects of the competition between these two reversion modes, especially between direct reversion and the first step of indirect reversion, whenever competition occurs. Thermodynamic simulations indicated that both reversion modes were possible and that indirect reversion was more favorable. Specific isothermal experiments combined with simulations revealed the existence of two potential Ga diffusion paths during indirect reversion: (i) from the α′ phase to the δ phase and (ii) from a new transient β′ phase, preceding the appearance of β phase of pure Pu, to the δ phase. These newly discovered mechanisms have enabled the present authors to fully understand the competition between the reversion modes. Finally, the ratio of direct to indirect reversion was found to depend on three factors, namely heating rate, initial α′ phase fraction and Ga content. All simulations were confirmed experimentally and highlighted key aspects of reversion mechanisms. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nadal M.-H.,CEA Valduc Center for Nuclear Studies | Bourgeois L.,CEA Valduc Center for Nuclear Studies
Journal of Applied Physics | Year: 2010

The evolution of the elastic moduli (Young modulus-E; adiabatic bulk modulus-BS and shear modulus-G and Poisson's ratio-ν) in the temperature range T ∈ [300;1000] K or T/Tm ∈ [0.2;0.65] of isotropic polycrystalline beryllium samples was investigated using an ultrasonic method. The key point is to perform the relevant and consistent analysis of the complete set of elastic data taking account of the specific low values of ν (at ambient and at high temperature). The evolution of the elastic moduli is shown to be governed by the density evolution through the thermal expansion coefficient. Then, the elastic moduli versus temperature are accurately discussed and are at variance with the only known experimental data from 1972 [W. D. Rowland, J. Phys. F: Met. Phys. 2, 231 (1972)]. The analysis is completed by a comparison with computations performed by first-principle calculations [G. Robert (unpublished)]. © 2010 American Institute of Physics.

Ravat B.,CEA Valduc Center for Nuclear Studies | Oudot B.,CEA Valduc Center for Nuclear Studies | Perron A.,CEA Valduc Center for Nuclear Studies | Lalire F.,CEA Valduc Center for Nuclear Studies | Delaunay F.,CEA Valduc Center for Nuclear Studies
Journal of Alloys and Compounds | Year: 2013

The purpose of this work was to study PuGa 1 at.% alloy behavior from martensitic transformation to the whole reversion process. The relevant phase transformations were investigated as a function of temperature via in situ X-ray diffraction and dilatometry analyses carried out at low and high temperatures. The results show that the δ-to-α′ martensitic transformation occurring at low temperatures is direct and does not involve any intermediate c0 phase. They also reveal that the reverse transformation of the two-phase δ + α′ alloy during heating involves two competing modes, namely direct and indirect reversion. The latter is associated with a Ga diffusion process that governs the ratio between these reversion modes. More precisely, our study demonstrates that the indirect reversion process consists of a Ga-enrichment of the remaining δ phase as well as the emergence of the β and γ phases of pure plutonium. Every stage of indirect reversion has been discussed in relation to the equilibrium phase diagram of the Pu-Ga binary system. © 2013 Elsevier B.V. All rights reserved.

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