[en] Spent fuel from nuclear reactors is stored underwater for cooling and shielding prior to reprocessing at the Idaho Chemical Processing Plant (ICPP) located near Idaho Falls, Idaho. Transfer and identification of fuel elements underwater requires that the water be almost totally free of suspended solids and microorganisms. This paper describes laboratory and plant operating experience using iodine with an algaecide to control the microbiological population in the water. The residual effect of iodine as a bacteriacide in water is dependent on the concentration of iodine and the concentration of reducing chemicals in the water, but overall, the effectiveness of iodine regenerated by chlorine is superior to that of chlorine alone. Iodine concentrations above 0.75 ppM inhibit the growth of microorganisms, and unlike chlorine, iodine is relatively noncorrosive to metals. The iodide formed by the reduction of iodine is readily reoxidized to iodine with a small amount of chlorine or other oxidant. The iodine injection equipment is simple and requires little maintenance