Nathalie Favretto-Cristini

Compaction of a Bed of Fragmentable

The nuclear fuel of light water power reactors is manufactured by powder metallurgy. This method is also used for the production of fuels containing minor actinides which have high activity and long life. Given their radiotoxicity, maximum simplification in the manufacturing process is necessary in order to limit dissemination and retention of matter. In addition to this, the fuel must have a mostly open porosity. Implementation of particles of a few hundred micrometers and controlled cohesion could achieve this dual objective. However, the mechanical strength of compacts before sintering must be sufficient without adding binder. The phenomena which occur during the manufacturing of compacts are thus analyzed and quantified. We show that only a part of the particles breaks upon application of a stress of up to 600 MPa and it is possible to detect this fragmentation by acoustic emission (AE).

{\rm UO}_2

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.

Particles and Associated Acoustic Emission

The nuclear fuel of light water power reactors are manufactured by powder metallurgy. This is also the method that is used for the production of fuels containing minor actinides that have high activity and long life. Given their radiotoxicity, it is necessary to simplify the manufacturing process to the maximum, limiting dissemination and retention of matter. In addition, the fuel must have a mostly open porosity. Implementation of particles of a few hundred micrometers and controlled cohesion could meet this dual objective. However, it should be ensured that the mechanical strength of compacts before sintering is sufficient without adding binder. The phenomena that occur during the manufacture of compact are thus analyzed and quantified. It is shown that only a portion of the particles breaks upon application of a stress up to 600 MPa and it is possible to detect this fragmentation by acoustic emission (AE).

Identification of the fragmentation of brittle particles during compaction process by the acoustic emission technique

The Discretized Kirchhoff Integral method has been recently tested against laboratory experiments using a model with surface curvatures and sharp edges generating wave diffraction effects. Comparisons between numerical and laboratory data have exhibited a good quantitative fit in terms of time arrivals and amplitudes, except in the vicinity of secondary shadow boundaries created by the interaction of the edges of some topographical structures. Following this work, the effect of multiple scattering and the surface curvatures on the wavefield is studied here, using the so-called diffraction attenuation coefficient, in order to define the cases where these effects may be neglected in the numerical modeling without loss of accuracy.

Compaction of a bed of fragmentable particles and associated acoustic emission

Accurate simulations of seismic wave propagation in complex geological structures with great and rapid variations of topography are of primary interest for environmental and industrial applications. Unfortunately, difficulties arise for such complex environments, due essentially to the existence of shadow zones, head waves, diffractions and edge effects. An original approach for seismics is to compare synthetic seismic data to controlled laboratory data for a well-described configuration, in order to analyze the respective limitations of each method/code. In this presentation we will present some preliminary results provided by both laboratory experiments conducted in a water-tank and numerical simulations of wave propagation obtained by two methods: the Tip-Wave Superposition Method and the Spectral Element Method.

Analysis of Wave Scattering from a Viscoelastic Layer with Complex Shape

In a context of nuclear Reactivity Initiated Accident, we describe acoustic emission signals, for which a problem of classification is open. As classical approaches with a reduced number of variables do not give satisfactory discrimination, we propose to use the envelopes of the received signals. We perform a k-means clustering and discuss the first results of this approach.

Comparison of Numerical Seismic Modeling Results with Acoustic Water-tank Data

O. I. Traore1), N. Favretto-Cristini1), L. Pantera2), P. Cristini1), S. Viguier-Pla3,4), P. Vieu3) 1) Aix-Marseille Univ., CNRS, Centrale Marseille, LMA, France. favretto@lma.cnrs-mrs.fr 2) CEA, DEN, DER/SRES, Cadarache, 13108 Saint-Paul-Lez-Durance, France 3) Equipe de Stat. et Proba., Institut de Mathématiques, UMR5219, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse Cedex 9, France 4) Université de Perpignan via Domitia, LAMPS, 52 av. Paul Alduy, 66860 Perpignan Cedex 9, France

Contribution of Functional Approach to the Classification and the Identification of Acoustic Emission Source Mechanisms

In acoustical and seismic fields, wavefield extraction has always been a crucial issue to solve inverse problem. Depending on the experimental configuration, conventional methods of wavefield decomposition might no longer likely to hold. In this paper, an original approach is proposed based on a multichannel decomposition of the signal into a weighted sum of elementary functions known as chirplets. Each chirplet is described by physical parameters and the collection of chirplets makes up a large adaptable dictionary, so that a chirplet corresponds unambiguously to one wave component.

Which Methods and Strategies to Cope with Noise Complexity for an Effective Interpretation of Acoustic Emission Signals in Noisy Nuclear Environment

Resolution is the ability to distinguish separate features. Improving resolution is the key problem to see thinner stratigraphic units, for instance. Horizontal resolution is connected to the size of the Interface Fresnel Zone. Vertical resolution is usually considered as being enhanced by both high frequencies and broadband signals. This common belief is true for zero-phase signals, such as Ricker wavelets, but is incorrect for mixed-phase signals (i.e., non zero-phase signals). For this type of signals, the complex trace, and more specifically the envelope, is a fundamental tool for the analysis of seismic resolution. We propose a criterion based on the separation of two envelopes as a new key element. This criterion is connected to the bandwidth that is consequently the key parameter for enhancing vertical resolution.

Wavefield extraction using multi-channel chirplet decomposition.

The spatial region in the vicinity of the interface which actually affects the interface response, and hence the reflected wavefield, is of particular interest for the characterization of reflectors. This region represents a volume of integration of properties above and beyond the interface whose maximum lateral extent corresponds to the lateral extent of the Interface Fresnel Zone (IFZ), and whose maximum vertical extent is equal to a thickness we evaluate approximately for subcritical incidence angles and for the case of a plane homogeneous interface. The maximum vertical extent may be greater than the seismic wavelengths for subcritical incidence angles close to the critical angle and for strong impedance contrast at the interface. The whole part of reflector which actually affects the reflected wavefield is then larger than described by previous estimates which considered only the spatial region beyond the interface. In addition to the case of a flat interface, we also discuss the change in the charac...