Most of the experiments performed so far to understand the heat transfer and quench front progression during the reflood phase of a LOCA were carried out using bundles of "solid type" heater rods. These rods have a close thermal contact between the stainless steel claddings and the electric insulation filler material. For such heater rod design, the effects of a Zircaloy cladding and a gas filled gap between the cladding and the pellets, features that exist in an actual fuel rod design, could not be simulated adequately. To address the uncertainties related to the rod design, the SEFLEX (Fuel Rod Simulator Effects in Flooding Experiments) program is being carried out to quantify the influence of different fuel rod simulators on the thermal-hydraulic behavior during forced feed bottom reflooding of unblocked and blocked arrays. The 5 × 5 FEBA rod bundle tests performed with gapless heater rods provided reference data for the comparison with data from identical tests with 5 × 5 fuel rod simulators with a gas filled gap between the Zircaloy cladding and alumina pellets. The decoupling effect of the gap lowers the maximum cladding temperatures in the unwetted portion of the bundle and shortens the quench times significantly compared to the conservative results from tests with conventional heater rods. © 1987.
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