Light Scattering on Random Dielectric Layers

Scattering of light by a random stack of dielectric layers represents a one-dimensional scattering problem, where the scattered field is a three-dimensional vector field. We investigate the dependence of the scattering properties (band gaps and Anderson localization) on the wavelength, strength of randomness and relative angle of the incident wave. There is a characteristic angular dependence of Anderson localization for wavelengths close to the thickness of the layers. In particular, the localization length varies non-monotonously with the angle. In contrast to Anderson localization, absorptive layers do not have this characteristic angular dependence.Comment: 14 pages, 11 figure

Anisotropic multiple scattering in diffuse media

The multiple scattering of scalar waves in diffusive media is investigated by means of the radiative transfer equation. This approach amounts to a resummation of the ladder diagrams of the Born series; it does not rely on the diffusion approximation. Quantitative predictions are obtained, concerning various observables pertaining to optically thick slabs, such as the mean angle-resolved reflected and transmitted intensities, and the shape of the enhanced backscattering cone. Special emphasis is put on the dependence of these quantities on the anisotropy of the cross-section of the individual scatterers, and on the internal reflections due to the optical index mismatch at the boundaries of the sample. The regime of very anisotropic scattering, where the transport mean free path $\ell^*$ is much larger than the scattering mean free path $\ell$, is studied in full detail. For the first time the relevant Schwarzschild-Milne equation is solved exactly in the absence of internal reflections, and asymptotically in the regime of a large index mismatch. An unexpected outcome concerns the angular width of the enhanced backscattering cone, which is predicted to scale as $\Delta\theta\sim\lambda/\sqrt{\ell\ell^*}$, in contrast with the generally accepted $\lambda/\ell^*$ law, derived within the diffusion approximation.Comment: 53 pages TEX, including 2 tables. The 4 figures are sent at reques

Analyse de tests de séquences d’accidents graves et interprétation du relâchement des produits de fission : interdépendance des relâchements du Cs, du Mo et du Ba

The behaviour of fission products during fuel life in a nuclear reactor is of particular importance as it plays a significant role in the release kinetics during hypothetical reactor severe accident transients.A description of (Cs, Mo, Ba) chemical states after some UO2 fuel irradiation stages is drawn. Molybdenum is supposed to be localised in a MoO2 precipitate (volatile over 1273K) and chemical reactions with condensed (Cs, Ba) oxides and/or uranates are possible.According to this description, a coherent interpretation is presented for some accidental sequence tests providing new ideas for release mechanisms of (Cs, Mo, Ba) and an attempt is done to calculate their release with MFPR code, which is being developed by IBRAE in collaboration with IRSN

Mechanistic modelling of urania fuel evolution and fission product migration during irradiation and heating

The models of the mechanistic code MFPR (Module for Fission Product Release) developed by IBRAE in collaboration with IRSN are described briefly in the first part of the paper. The influence of microscopic defects in the UO2 crystal structure on fission-gas transport out of grains and release from fuel pellets is described. These defects include point defects such as vacancies, interstitials and fission atoms, and extended defects such as bubbles, pores and dislocations. The mechanistic description of chemically active elements behaviour (fission-induced) is based on complex association of diffusion-vaporisation mechanism involving multi-phase and multi-component thermo-chemical equilibrium at the grain boundary with accurate calculation of fuel oxidation. In the second part, results of the code applications are given to different situations: normal LWR reactor operation, high temperature annealing, loss of coolant accident (LOCA) and severe accidents conditions. © 2007 Elsevier B.V. All rights reserved

Progress in understanding fission-product behaviour in coated uranium-dioxide fuel particles

Supported by results of calculations performed with two analytical tools (MFPR, which takes account of physical and chemical mechanisms in calculating the chemical forms and physical locations of fission products in UO2, and MEPHISTA, a thermodynamic database), this paper presents an investigation of some important aspects of the fuel microstructure and chemical evolutions of irradiated TRISO particles. The following main conclusions can be identified with respect to irradiated TRISO fuel: first, the relatively low oxygen potential within the fuel particles with respect to PWR fuel leads to chemical speciation that is not typical of PWR fuels, e.g., the relatively volatile behaviour of barium; secondly, the safety-critical fission-product caesium is released from the urania kernel but the buffer and pyrolytic-carbon coatings could form an important chemical barrier to further migration (i.e., formation of carbides). Finally, significant releases of fission gases from the urania kernel are expected even in nominal conditions. © 2008 Elsevier B.V. All rights reserved

Development of the mechanistic code MFPR for modelling fission-product release from irradiated UO2 fuel

The models of the mechanistic code MFPR developed in collaboration between IBRAE (Moscow, Russia) and IRSN (Cadarache, France) are described. Exhaustive description of fission-gas behaviour in grain and out of grain is given in relation with individual validation results on analytical experiments under various conditions (steady irradiation, transient, post-irradiation annealing). It is shown that microscopic defects in the UO2 crystal structure can strongly influence fission-gas transport out of grains and release from fuel pellets. These defects include point defects such as vacancies, interstitials and fission atoms, and extended defects such as bubbles, pores and dislocations. A model for the dislocation generation and evolution in irradiated UO2 fuel has been developed and implemented in the mechanistic code MFPR along with a fuel densification model. Being combined with the set of equations describing evolution of point defects (vacancies and interstitials) and their interactions with fission-gas bubbles, a completely self-consistent consideration of the whole system of point and extended defects in irradiated UO2 fuel has been obtained. The mechanistic description of chemically active-elements behaviour is also presented. It is based on complex association of diffusion-vaporisation mechanism involving multi-phase and multi-component thermo-chemical equilibrium at the grain boundary with accurate calculation of fuel oxidation. Examples of application to the VERCORS 4 and 5 experiments show the possibilities of the code in the frame of severe-accident interpretation. © 2005 Elsevier B.V. All rights reserved