In this paper, results of experimental studies of the coolant flow mixing at different boron concentration and different temperature in the VVER-1000 reactor downcomer and lower plenum are presented. Studies concerning boron dilution were carried out on the reactor model in scale 1:5 for cases of reactor coolant pump (RCP) startup and resumption of natural circulation during a small-break loss-of-coolant accident. It has been shown that for RCP startup, the effect of Reynolds number on mixing is negligible. Studies of flow mixing in the reactor downcomer were carried out applied to emergency core cooling system water injection into the cold loop leg and reactor downcomer. Results of the tests on model and reactor have been analyzed, and recommendations for calculations have been provided.