A series of numerical calculations was performed to study the effect of apparatus scale size on the magnitude and duration of emergency core coolant (ECC) bypass and the time delay for refill of the lower plenum of a pressurized water reactor during a hypothetical loss-of-coolant accident. These results indicate that for certain idealized flow and thermal conditions, flow similarity can be obtained at all scale sizes, but that for more realistic conditions, the effects of apparatus scale size and lower plenum pressure on ECC bypass and lower plenum refill can be large. In particular, the duration of ECC bypass and the time delay for refill appear to be more sensitive to momentum exchange at full scale and high lower plenum pressure than they are at 2/15 scale and low pressure. The sensitivity to mass exchange, ECC subcooling, and wall heat transfer decreases with increasing scale and lower plenum pressure. The effect of introducing steam, rather than air, into the downcomer through the broken ECC injection port when the pressure in the downcomer falls below that in the containment vessel is to decrease the rate of lower plenum refill.