Abstract
A method is described for calculating fuel rod cladding temperatures in a blockage formed by a group of ballooned fuel rods in a larger rod array, for heat transfer conditions appropriate to the reflooding phase of a postulated PWR LOCA. The model is suitable for describing the extreme case of co-planar axially extended balloons, where steam superheating and skin friction effects are believed to have an important effect on blockage heat removal. Attention is restricted to the constricted zone within the blockage. Reasonable agreement is shown with available heat transfer data from partially ballooned rod arrays, for conditions of steam cooling, steam-and-droplet cooling and reflood cooling. The model is also able to describe flow velocity distribution data from partially blocked rod bundles with reasonable accuracy. Parametric calculations for typical PWR LOCA heat transfer conditions suggest that blockage length has a strong effect on fuel coolability, mainly as a result of extra superheating of the steam within the blockage. However calculations also indicate that the presence of entrained water droplets has a powerful effect in reducing the clad temperatures attained.
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