[en] Boiling water reactors rely on the injection of soluble neutron absorbers to control power in case of failure in the control rod (scram) system. Typically this liquid ''poison'' is injected from eight small holes on a standpipe positioned vertically near the outer edge of the core shroud in the lower plenum. The achievement of control is predicated on good mixing of this injected liquid with the coolant which is recirculating around the core upper plenum and downcomer. However, because the flows are rather low (∼20% of rated with pumps tripped as expected under such conditions) and the injected solution density is much higher than that of the primary fluid, there have been concerns raised about the efficiency and completeness of this mixing. This work provides the first openly available data addressing such concerns. To avoid potentially important scaling compromises, the data were obtained from full-scale simulations. From the experiments performed so far, we can conclude that complete boron mixing (entrainment) will occur for recirculation flow rates down to 8.2% of rated. 3 refs., 60 figs., 2 tabs