ABSTRACT
The state-of-the-art experimental and analytical work on post critical heat flux (CHF) effects of spacer grids and flow blockage in rod bundles is reviewed and summarized from the point of view of basic heat transfer effects and mechanisms. Modeling of post CHF grid heat transfer effects is relatively new and only a few attempts, have been made. Whether complete grid rewetting occurs will depend on the amount of heat being supplied to the grid from radiation and convection and the number of liquid droplets which might be deposited on the grids during reflood transient. In the refilling and reflooding phases of a calculated loss-of-coolant accident in a pressurized water reactor Zircaloy fuel rod claddings may reach temperatures which cause them to balloon and to burst due to internal overpressure. The data does show that the most pronounced effect of the grid is at early times when the grid is furthest from the quench front.
