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Determination of the radiative conductivity tensor of a damaged core from the RDFI METHOD

Congress title :NURETH-12 (Nuclear Reactor Thermal Hydraulics) 
Congress town :Pittsburg
Congress date :30/09/2007


The objective of this paper is to characterize an effective radiative conductivity tensor for a damaged Pressurized Water Reactor (PWR) core during a hypothetical severe accident sequence, from a small scale model of this reactor core. The core, made of bundles of parallel fuel rods, is considered as a porous medium. The solid phase is considered as opaque and the fluid phase as transparent. Images of damaged rod bundles have been obtained in the international PHEBUS-FP experimental programs carried out by the French Institut de Radioprotection et de S\^uret'e Nucl'eaire (IRSN) in collaboration with international partners. The images show various typical states of degradation which may occur during a severe accident. A complete set of bidimensional $\gamma$ ray tomographies allow us to obtain the damaged core geometry. The directional extinction coefficient of an equivalent continuous semi-transparent porous medium is obtained by the Radiation Distribution Function Identification (RDFI) method (Tancrez and Taine, 2004), for several core states of degradation. A real reactor core can be considered as an optically thick medium at a centimetric scale. A perturbation approach leads then to a radiative conductivity tensor deduced from the previous extinction coefficient under the assumption of an isotropic phase function. Deviations from the solutions obtained for non damaged bundles are also analyzed.


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