The formation of a colloidal suspension of hydrated aluminum oxide, Gibbsite or α-Al2O3 · 3D2O, in the heavy water of the NRU reactor is described, and compared with turbidity formation in other aluminum-water reactor systems. The observed corrosion rate of aluminum in NRU is consistent with a mass transfer mechanism involving the continuous dissolution of the corrosion product film. Two primary mechanisms for removing the dissolved aluminum from solution are postulated. These are direct crystallization onto deposits in the heat exchangers and direct crystallization onto Gibbsite particles in the water. The former effectively removes alumina from the system while the latter produces turbidity in the water. The rate of appearance of turbidity depends on its rate of formation and its rate of removal by the purification system. Turbidity is removed by filtration and adsorption in the ion-exchange columns and by evaporation. It is desirable to reduce the rate of formation of turbidity by choosing water conditions which minimize the solubility of the corrosion product film, rather than controlling the turbidity level by an adequate purification capacity.