In the quench front and froth region the thermal-hydraulic parameters experience a sharp axial variation. The heat transfer regime changes from single-phase liquid, to nucleate boiling, to transition boiling and finally to film boiling in a small axial distance. One of the major limitations of all the current best-estimate codes is that a relatively coarse is used to solve the complex fluid flow and heat transfer problem in proximity of the quench front of a reactor core during reflood. The use of a fine axial mesh for the entire core becomes prohibitive because of the large computational costs involved. Moreover as the mesh size decreases, the standard numerical methods based on a semi-implicit scheme, tend to become unstable. A sub-grid model was developed to resolve the complex thermal-hydraulic problem at the quench front and froth region. The model is a Fine Hydraulic Moving Grid that overlies a coarse Eulerian mesh in the proximity of the quench front and froth region. The fine mesh moves in the core and follows the quench front as it advances in the core while the rods cool and quench. The Fine Hydraulic Moving Grid (FHMG) software package was developed and implemented into the COBRA-TF computer code. This paper presents the model and discusses preliminary results obtained with the COBRA-TF/FHMG computer code.

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