[en] Iodine has been used effectively to maintain a large volume of water (5.7 x 106L) essentially free of bacteria (<10 microorganisms/mL) at the Idaho Chemical Processing Plant. The water, contained in a large concrete basin, is used for storing nuclear fuels, and a low bacteria population is essential to provide good water clarity for handling the fuel. Test results are presented to show the concentration of residual iodine in the water vs time, the effect of pH, and the measured bacteria population