[en] Flow oscillations between the downcomer and the electrically heated core during reflood have been observed in reflood experimental facilities such as Semiscale and FLECHT-SET (Westinghouse). The purpose of the study described was to investigate the primary mechanism responsible for the initiation and continuation of the oscillations during reflood using the RELAP4/MOD6 code. The study produced significant results concerning the accuracy of RELAP in predicting the dynamic features of density-wave and pressure drop oscillations, and was conducted on the German three-loop PRIMAR KREISLAUF (PLK) Reflood Experimental System. RELAP4/MOD6 is an analytical computer code which can be used for best estimate analysis of PWR or BWR reactor system blowdown and reflood response to a postulated LOCA