Analysis was performed to determine the thermal-hydraulic behavior in the electrically heated core simulator of the Semiscale Mod-1 system during the early stage of a simulated LOCA initiated by a large cold leg break. The calculated incore hydraulic behavior was used to obtain a better understanding of early CHF (480 to 700 ms after rupture) and the occurrence of rewet in some locations after CHF. Analysis indicated that shortly after rupture the flow in the upper core stagnated for 600 ms, and the core rapidly voided of coolant. In the center and lower regions of the core, the calculated fluid qualities were between 30 and 70 percent at the time of measured CHF. The high fluid qualities in the flow channels about the heater rods indicated that the mechanism of CHF was dryout of the heater rod surfaces. Critical heat flux did not happen at the location of instantaneous flow stagnation associated with the flow reversal; nor did CHF occur in the region of the prolonged flow stagnation. At about 700 ms after rupture the core flow completely reversed direction, and the influx of coolant from above the heated core was responsible for the measured rewets.

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