[en] In this paper, the behavior of a dense UO₂ (porosity less than 2%) was studied experimentally on a range of temperatures (1100-1700 C) and strain rates (10^-4-10^-1/s) representative of RIA loading conditions. The yield stress was found to increase with strain rate and to decrease with temperature. Macroscopic cracking of the samples was apparent after the tests at 1100 C. Scanning Electron Microscopy (SEM) image analyses revealed a pronounced grain boundary cracking in the core of the samples tested at 10^-1/s and at 1550-1700 C. A hyperbolic sine model for the viscoplastic strain rate with a clear dependency on porosity was first developed. It was completed by a Drucker-Prager yield criterion with associated plastic flow to account for the porosity increase induced by grain boundary cracking. Finite Elements simulations of the compression tests on the dense UO₂ were then successfully compared to the stress-strain curves, post-test diameter profiles and porosities at the pellets center, periphery and top extremity. The response of the grain boundary cracking model was then studied in biaxial compression, this condition being closer to that of the pellet during a RIA power transient. (authors)

[en] We investigate the dynamic fracture of a single particle impacting a flat surface using 3D DEM simulations based on a fragmentation model involving both a stress threshold and a fracture energy. The particle is assumed to be perfectly rigid and discretized into polyhedral voronoi cells with cohesive interfaces. A cell-cell interface loses its cohesion when it is at a normal or tangential stress threshold and an amount of work equal to the fracture energy is absorbed as a result of the relative cell-cell displacements. Upon impact, the kinetic energy of the particle is partially consumed to fracture cell-cell contacts but also restituted to the fragments or dissipated by inelastic collisions. We analyze the damage and fragmentation efficiency as a function of the impact energy and stress thresholds and their scaling with fracture energy and impact force. In particular, we find that the fragmentation efficiency, defined as the ratio of the consumed fracture energy to the impact energy, is un-monotonic as a function of the impact energy, the highest efficiency occurring for a specific value of the impact energy. (authors)

[en] The standard powder metallurgy preparation of SFR (Sodium Fast Reactor) oxide fuel involves UO₂ and PuO₂ co-milling. An alternative route, using a solid-solution of mixed oxide obtained by oxalic co-conversion as the starting material, is presented. It was used to manufacture nuclear fuels for the “COPIX” irradiation conducted in the Phenix SFR. Two processes using co-converted powders were tested to elaborate fuel pellets: (1) the Direct Process that consists in pressing and sintering the mixed oxide with the final Pu content and (2) the Dilution Process, which involves the dilution of a high Pu content mixed oxide with UO₂. After studying the structural and microstructural evolution with temperature of these innovative raw materials, the elaboration parameters were adjusted to obtain final pellets in accordance with the Phenix fuel specifications. This study demonstrates the feasibility of such new fabrication route at laboratory scale and, from a more fundamental prospect, allows a better understanding of the underlying phenomena involved during sintering

[en] We present a detailed analysis of the morphology of granular systems composed of frictionless pentagonal particles by varying systematically both the size span and particle shape irregularity, which represent two poly-dispersity parameters of the system. The microstructure is characterized in terms of various statistical descriptors such as global and local packing fractions, radial distribution functions, coordination number, and fraction of floating particles. We find that the packing fraction increases with the two parameters of poly-dispersity, but the effect of shape poly-dispersity for all the investigated structural properties is significant only at low size poly-dispersity where the positional and/or orientational ordering of the particles prevail. We focus in more detail on the class of side/side contacts, which is the interesting feature of our system as compared to a packing of disks. We show that the proportion of such contacts has weak dependence on the poly-dispersity parameters. The side-side contacts do not percolate but they define clusters of increasing size as a function of size poly-dispersity and decreasing size as a function of shape poly-dispersity. The clusters have anisotropic shapes but with a decreasing aspect ratio as poly-dispersity increases. This feature is argued to be a consequence of strong force chains (forces above the mean), which are mainly captured by side-side contacts. Finally, the force transmission is intrinsically multi-scale, with a mean force increasing linearly with particle size. (authors)

[en] By means of extensive contact dynamics simulations, we analyze the combined effects of polydispersity both in particle size and in particle shape, defined as the degree of shape irregularity, on the shear strength and microstructure of sheared granular materials composed of pentagonal particles. We find that the shear strength is independent of the size span, but unexpectedly, it declines with increasing shape polydispersity. At the same time, the solid fraction is an increasing function of both the size span and the shape polydispersity. Hence, the densest and loosest packings have the same shear strength. At the scale of the particles and their contacts, we analyze the connectivity of particles, force transmission, and friction mobilization as well as their anisotropies. We show that stronger forces are carried by larger particles and propped by an increasing number of small particles. The independence of shear strength with regard to size span is shown to be a consequence of contact network self-organization, with the falloff of contact anisotropy compensated by increasing force anisotropy. (authors)

[en] Some nuclear fuels are currently manufactured by a powder metallurgy process that consists of three main steps, namely preparation of the powders, powder compaction, and sintering of the compact. An optimum between size, shape and cohesion of the particles of the nuclear fuels must be sought in order to obtain a compact with a sufficient mechanical strength, and to facilitate the release of helium and fission gases during irradiation through pores connected to the outside of the pellet after sintering. Being simple to adapt to nuclear-oriented purposes, the Acoustic Emission (AE) technique is used to control the microstructure of the compact by monitoring the compaction of brittle Uranium Dioxide (UO₂) particles of a few hundred micrometers. The objective is to identify in situ the mechanisms that occur during the UO₂ compaction, and more specifically the particle fragmentation that is linked to the open porosity of the nuclear matter. Three zones of acoustic activity, strongly related to the applied stress, can be clearly defined from analysis of the continuous signals recorded during the compaction process. They correspond to particle rearrangement and/or fragmentation. The end of the noteworthy fragmentation process is clearly defined as the end of the significant process that increases the compactness of the material. Despite the fact that the wave propagation strongly evolves during the compaction process, the acoustic signature of the fragmentation of a single UO₂ particle and a bed of UO₂ particles under compaction is well identified. The waveform, with a short rise time and an exponential-like decay of the signal envelope, is the most reliable descriptor. The impact of the particle size and cohesion on the AE activity, and then on the fragmentation domain, is analyzed through the discrete AE signals. The maximum amplitude of the burst signals, as well as the mean stress corresponding to the end of the recorded AE, increase with increasing mean diameter of the particles. Moreover, the maximum burst amplitude increases with increasing particle cohesion. (authors)

[en] We perform systematic particle dynamics simulations of granular flows composed of breakable particles in a 2D rotating drum to investigate the evolution of the mean particle size and specific surface as a function of system parameters such as drum size, rotation speed, filling degree, and particle shape and size. The specific surface increases at a nearly constant rate up to a point where particle breakage begins to slow down. The rates of particle breakage for all values of system parameters are found to collapse on a master curve when the times are scaled by the characteristic time defined in the linear regime. We determine the characteristic time as a function of all system parameters, and we show that the rate of particle breakage can be expressed as a linear function of a general scaling parameter that incorporates all our system parameters. This scaling behavior provides a general framework for the upscaling of drum grinding process from laboratory to industrial scale. (authors)

[en] Mixing, agglomerating, and milling of granular materials within a rotating drum is often performed in the cascading flow regime; however, the scaling behavior of these industrial applications remains poorly understood. It involves both centrifugal forces and an inertial surface flow with a curved profile. By means of discrete element simulations, we investigate the rheology of cascading flows in rotating drums as a function of drum size, rotation speed, and filling degree. We find that the surface profile, described by the ratio between the steepest descent slope and an average slope, is strongly correlated with flow variables such as active layer thickness, contact force variability, and wall slip. We show that the flow variables cannot be scaled by Froude number alone but are instead nicely scaled by a dimensionless parameter that combines the Froude number with other system parameters. This scaling works even for small drums and low filling degrees where finite-size effects prevail and the slippage of particles at the drum wall considerably affects the cascading flow at the free surface. The observed correlation between this parameter and contact force fluctuations suggests that it may also be a relevant upscaling parameter for milling and agglomeration in rotating drums. (authors)

[en] We investigate sheared granular materials composed of crushable particles by means of contact dynamics simulations and the bonded-cell model for particle breakage. Each particle is paved by irregular cells interacting via cohesive forces. In each simulation, the ratio of the internal cohesion of particles to the confining pressure, the relative cohesion, is kept constant and the packing is subjected to biaxial shearing. The particles can break into two or more fragments when the internal cohesive forces are overcome by the action of compressive force chains between particles. The particle size distribution evolves during shear as the particles continue to break. We find that the breakage process is highly inhomogeneous both in the fragment sizes and their locations inside the packing. In particular, a number of large particles never break whereas a large number of particles are fully shattered. As a result, the packing keeps the memory of its initial particle size distribution, whereas a power-law distribution is observed for particles of intermediate size due to consecutive fragmentation events whereby the memory of the initial state is lost. Due to growing polydispersity, dense shear bands are formed inside the packings and the usual dilatant behavior is reduced or cancelled. Hence, the stress-strain curve no longer passes through a peak stress, and a progressive monotonic evolution towards a pseudo-steady state is observed instead. We find that the crushing rate is controlled by the confining pressure. We also show that the shear strength of the packing is well expressed in terms of contact anisotropies and force anisotropies. The force anisotropy increases while the contact orientation anisotropy declines for increasing internal cohesion of the particles. These two effects compensate each other so that the shear strength is nearly independent of the internal cohesion of particles. (authors)

[en] We study the influence of particle shape on the evolution of particle breakage process taking place inside rotating cylinders. Extensive particle dynamics simulations taking into account the dynamics of the granular flow, particle breakage, and polygonal particle shapes were carried out. We find that the rate of particle breakage is faster in samples composed of initially rounder particles. The analysis of the active flowing layer thickness suggests that for samples composed of rounder particles a relatively lower dilatancy and higher connectivity lead to a less curved free surface profile. As a result, rounder particles rolling down the free surface have a higher mobility and thus higher velocities. In consequence, the faster breakage observed for rounder initial particles is due to the larger particles kinetic energy at the toe of the flow.