A new equation of state for helium nanobubbles embedded in UO2 matrix calculated via molecular dynamics simulations

International audienceMolecular dynamics simulations have been carried out to determine the equation of state of helium inside nanobubbles embedded into UO2 matrix.The parameters of this equation of state are fitted with the Brearley and MacInnes hard-sphere model based on the formalism of Carnahan-Starling used in fuel performance codes.This new equation of state takes into account the interactions between the surrounding UO2 matrix and the helium atoms. Four nanobubble sizes of diameters 1, 2, 5, and 10 nm have been investigated over four temperatures 300, 500, 700, and 900 K and for initial helium concentration inside the bubble ranging from 0.33x10^5 to 3.9x10^5 mol.m^-3 (corresponding to helium-to-vacancy ratio of 0.3 to 3.3, respectively).We observe that helium atoms are inhomogeneously distributed inside the bubble.A boundary layer of 1 nm thickness appears at the bubble surface in which helium atoms are more concentrated and diffuse into the UO2 matrix. We also observe a saturation of the helium atoms that can be incorporated into the bubble.This concentration limit is equal to 1.6 helium atom per vacancy in UO2.It corresponds to an atomic volume of 7.8x10^-30 m^3, which is almost half of the value proposed with the original Brearley and MacInnes model (13x10^-30 m^3).For this threshold concentration and for bubble of diameter higher than 5~nm, micro-cracks and dislocations appear at the bubble surface.We calculated the critical pressures inside the bubble that yields to this onset of crack in UO2.These critical pressures are in good agreement with those calculated with the Griffith criterion for brittle fracture

Crack fronts and damage in glass at the nanometer scale

We have studied the low speed fracture regime for different glassy materials with variable but controlled length scales of heterogeneity in a carefully mastered surrounding atmosphere. By using optical and atomic force microscopy (AFM) techniques we tracked in real-time the crack tip propagation at the nanometer scale on a wide velocity range (mm/s - pm/s and below). The influence of the heterogeneities on this velocity is presented and discussed. Our experiments reveal also -for the first time- that the crack progresses through nucleation, growth and coalescence of nanometric damage cavities within the amorphous phase. This may explain the large fluctuations observed in the crack tip velocities for the smallest values. This behaviour is very similar to what is involved, at the micrometric scale, in ductile fracture. The only difference is very likely due to the related length scales (nanometric instead of micrometric). Consequences of such a nano-ductile fracture mode observed at a temperature far below the glass transition temperature in glass is finally discussed.Comment: 12 pages, 8 figures, submitted to Journal of Physics: Condensed Matter; Invited talk at Glass and Optical Materials Division Fall 2002 Meeting, Pittsburgh, Pa, US

Scaling exponents for fracture surfaces in homogenous glass and glassy ceramics

We investigate the scaling properties of post-mortem fracture surfaces in silica glass and glassy ceramics. In both cases, the 2D height-height correlation function is found to obey Family-Viseck scaling properties, but with two sets of critical exponents, in particular a roughness exponent $\zeta\simeq 0.75$ in homogeneous glass and $\zeta\simeq 0.4$ in glassy ceramics. The ranges of length-scales over which these two scalings are observed are shown to be below and above the size of process zone respectively. A model derived from Linear Elastic Fracture Mechanics (LEFM) in the quasistatic approximation succeeds to reproduce the scaling exponents observed in glassy ceramics. The critical exponents observed in homogeneous glass are conjectured to reflect damage screening occurring for length-scales below the size of the process zone

Glass breaks like metals, but at the nanometer scale

We report in situ Atomic Force Microscopy experiments which reveal the presence of nanoscale damage cavities ahead of a stress-corrosion crack tip in glass. Their presence might explain the departure from linear elasticity observed in the vicinity of a crack tip in glass. Such a ductile fracture mechanism, widely observed in the case of metallic materials at the micrometer scale, might be also at the origin of the striking similarity of the morphologies of fracture surfaces of glass and metallic alloys at different length scales.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Lett, few minor corrections, Fig. 1b change

Contribution of Energetically Reactive Surface Features to the Dissolution of CeO2 and ThO2 Analogues for Spent Nuclear Fuel Microstructures

In the safety case for the geological disposal of nuclear waste, the release of radioactivity from the repository is controlled by the dissolution of the spent fuel in groundwater. There remain several uncertainties associated with understanding spent fuel dissolution, including the contribution of energetically reactive surface sites to the dissolution rate. In this study, we investigate how surface features influence the dissolution rate of synthetic CeO2 and ThO2, spent nuclear fuel analogues that approximate as closely as possible the microstructure characteristics of fuel-grade UO2 but are not sensitive to changes in oxidation state of the cation. The morphology of grain boundaries (natural features) and surface facets (specimen preparation-induced features) was investigated during dissolution. The effects of surface polishing on dissolution rate were also investigated. We show that preferential dissolution occurs at grain boundaries, resulting in grain boundary decohesion and enhanced dissolution rates. A strong crystallographic control was exerted, with high misorientation angle grain boundaries retreating more rapidly than those with low misorientation angles, which may be due to the accommodation of defects in the grain boundary structure. The data from these simplified analogue systems support the hypothesis that grain boundaries play a role in the so-called “instant release fraction” of spent fuel, and should be carefully considered, in conjunction with other chemical effects, in safety performance assessements for the geological disposal of spent fuel. Surface facets formed during the sample annealing process also exhibited a strong crystallographic control and were found to dissolve rapidly on initial contact with dissolution medium. Defects and strain induced during sample polishing caused an overestimation of the dissolution rate, by up to 3 orders of magnitude

New helium equation of state for pressurized nanobubbles in UO2 matrix calculated via molecular dynamics simulations

International audienc

Evolution of graphite structure under irradiation a new scenario from molecular dynamics simulations

International audienceGraphite response to irradiations has been widely studied in the past because of its importance for nuclear engineering. Although its behavior under irradiation has been widely investigated, the very details of the underlying mechanisms are still under debate and several scenarios are available [1,2,3]. With molecular dynamics simulations using empirical potentials we investigate in this paper the creation of damages by single irradiation event and the effect of the irradiation dose until complete amorphisation of the graphite structure.Two methodologies are used displacement cascades to investigate the primary damage and Frenkel pair accumulations to study the dose effect. We show that graphite is not amorphised by a direct impact mechanism for initial energies of the primary knocked-on atoms (carbon and chlorine) up to 40 keV. Instead, various defects stabilize and remain after a single irradiation event. However, amorphisation is reached by accumulation of defects. Before amorphisation, we show that graphite follows a three stages evolution characterized by (1) an increase of point defects (2) a pinning and wrinkling of the graphene planes at small amorphous pockets and (3) an amorphisation by percolation of the small amorphous pockets. These three stages all together constitute an alternative scenario to those already existing [4]. We also show the mechanisms of each stage drive the change of lattice dimension of graphite. Interstitials contribute as expected to the swelling of the c-axis, while vacancies link the shrinkage of the basal plane in the first stage. The point defects contribution of the first stage is replaced by the dimension change induced by the rippling of the graphene planes. This topological change is depicted by a power law relation between the c-axis swelling and the basal-plane shrinkage as a function of the irradiation dose. [1] K. Niwase, Phil. Mag. Lett. 82 (2002) 401.[2] M.I. Heggie, I. Suarez-Martinez, C . Davidson, and G. Haffenden, J. Nucl. Mater. 413 (2011) 150.[3] B.J. Marsden, and G.N. Hall, in Comprehensive Nuclear Materials, chap. 4.11 (2012) 325.[4] A. Chartier, L. Van Brutzel, B. Pannier, and P. Baranek, submitted in Carbon (2014

Modeling of point defects and rare gas incorporation in uranium mono-carbide

International audienceAn embedded atom method (EAM) potential has been established for uranium mono-carbide. This EAM potential was fitted on structural properties of metallic uranium and uranium mono-carbide. The formation energies of point defects, as well as activation energies for self migration, have been evaluated in order to cross-check the suitability of the potential. Assuming that the carbon vacancies are the main defects in uranium mono-carbide compounds, the migration paths and energies are consistent with experimental data selected by Catlow[C.R.A. Catlow, J. Nucl. Mater. 60 (1976) 151]. The insertion and migration energies for He, Kr and Xe have also been evaluated with available inter-atomic potentials [H.H. Andersen, P. Sigmund, Nucl. Instr. and Meth. B 38 (1965) 238]. Results show that the most stable defect configuration for rare gases is within uranium vacancies. The migration energy of an interstitial Xe is 0.5 eV, in agreement with the experimental value of 0.5 eV [Hj. Matzke, Science of advanced LMFBR fuels, Solid State Physics, Chemistry and Technology of Carbides, Nitrides and Carbonitrides of Uranium and Plutonium, North-Holland, 1986]

Evolution de la structure du graphite sous irradiation émergence d'un nouveau scénario par simulation de dynamique moléculaire

National audienceEn tant que matériau utilisé comme modérateur dans les premières centrales nucléaires,le graphite et son évolution structurale sous irradiation ont été largement étudiés par lepassé. Malgré ces nombreuses études, le détail des mécanismes qui conduisent à sonamorphisation font encore débat et plusieurs scénarii sont disponibles. A l’aidede simulations par dynamique moléculaire utilisant des potentiels empiriques, nousavons étudié la création de dommages pour un seul événement d’irradiation et les effetsde dose jusqu’à l’amorphisation complète de la structure graphite. Pour se faire, deuxméthodologies ont été utilisées : les cascades de déplacements pour l’étude del’endommagement primaire et l’accumulation de défauts ponctuels pour l’évolution enfonction de la dose d’irradiation