Rod-to-rod thermal radiation is explicitly modeled mainly in Boiling Water Reactor (BWR) applications because of the presence of the assembly housing. In Pressurized Water Reactor (PWR) applications, realistic calculations of core reflood include the thermal radiation component from the hot surfaces to the steam and the droplet in the Dispersed Flow Film Boiling (DFFB) regime, whereas the effect of the rod-to-rod thermal radiation contribution is not explicitly modeled and is either neglected or bounded for simplicity. On the other hand reflood heat transfer packages are assessed against data obtained with rod bundle tests such as FLECHT, FLECHT-SEASET, RBHT, etc.. The presence of the housing in the reflood tests creates a temperature radial distribution across the bundle which promotes thermal radiation heat transfer. Moreover the thermal radiation contribution can be a significant fraction of the total heat transfer when hot rods faces thimbles or other unheated structures. Since only the total heat transfer can be measured from the test, the thermal radiation effect needs to be evaluated to assess the applicability of reflood heat transfer models to different fuel lattices. A detailed rod-to-rod radiation model was developed to solve the thermal radiation problem in a generic square lattice. Housing and thimbles are explicitly modeled as separate surfaces. The temperature in each rod and colder surfaces is obtained solving the coupled thermal radiation and convection heat transfer problem for a given fluid temperature and convective heat transfer coefficient. The effect of the thermal radiation is estimated for different lattices. The analysis includes the FLECHT-SEASET test bundle, the FLECHT-Skewed test bundle and two different Westinghouse fuel assembly designs. The effect of the location of the hot rod within the assembly is considered.
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