[en] This paper presents results of experiments performed in the SEMISCALE MOD-1 system to investigate the effectiveness of upper plenum ECC injection during a hypothesized loss-of-coolant accident in a commercial pressurized water reactor (PWR). A four loop PWR was simulated with accumulator ECC injected into the upper plenum; a two loop PWR was simulated with the low pressure injection system injecting ECC into the upper plenum. The effectiveness of the top downward core quench that occurred for each of the two concepts investigated compares favorably with that achieved by other ECC concepts now employed in American made PWR's. (author)