In the wake of the March 11 th earthquake and tsunami and the subsequent flooding of several of the Fukushima Dai-Ichi reactors, Japan and the Japanese utility TEPCO faced a crisis situation with incredible challenges: substantial amounts of radioactive mixed seawater and freshwater accumulated in the basements of four reactor and other buildings on the site. This water held varying levels of contamination due to the fact that it had been in contact with damaged fuel elements in the cores and with other contaminated components. The overall water inventory was estimated at around 110,000 tons of water with contamination levels up to the order of 1Ci/l. Time was of the essence to avoid overflow of this accumulated water into the ocean.AREVA proposed, designed and implemented a water treatment solution using a proven chemical coprecipitation process with ppFeNi reagent, which is currently in use for effluent treatment on several nuclear sites including AREVA sites. In addition to the extremely short schedule the other challenge was to adapt the chemical treatment process to the expected composition of the Fukushima water and, in particular, to evaluate the impact of salinity on process performance. It was also necessary to define operating conditions for the VEOLIA equipment that had been selected for implementation of the process in the future facility. The operation phase began on June 17 th , and by the end of July more than 30,000 tons of highly radioactive saltwater had been decontaminated -the Decontamination Factor (DF) for Cesium was ~10 4 . It allowed recycling the contaminated water to cool the reactors while protecting workers and the environment. This paper focuses on the Actiflo™-Rad water treatment unit project that was part of the TEPCO general water treatment scheme. It presents a detailed look at the principles of the Actiflo™-Rad, related on-the-fly R&D, an explanation of system implementation challenges, and a brief summary of operation results todate.