The steel platform can store up to 10,000 tons of water. The container was previously used as a fishing park.
In April, it was made watertight and rust-resistant at a shipyard. A large crane was mounted and pipes were attached.
After a final inspection, the platform is scheduled to arrive off the coast at the Fukushima Dai-ichi power plant in about two weeks.
The March 11 quake and tsunami left more than 25,000 people dead or missing on the northeast coast and triggered the worst nuclear crisis since Chernobyl in 1986.