Japan started dumping 11,500 tons of low-level radioactive water at sea on Monday to free up storage space at its crippled Fukushima nuclear plant for more highly contaminated water.
The release into the Pacific Ocean started shortly after 7.00 pm (1000 GMT) local time, public broadcaster NHK and news agencies reported.
An official from plant operator Tokyo Electric Power Co. (Tepco) fought back tears during a televised appearance announcing the decision to start the water dump, which was expected to take several days.
"We have already caused such pain and nuisance to local residents," he said. "We cannot express how sorry we are to have to impose another burden."
Another Tepco official said the water was only of low-level radioactivity and had to be cleared to make room for more radioactive water.
"Highly radioactive waste water has been accumulated at turbine buildings at Fukushima Daiichi, especially at the reactor unit two," he said.
"There is a need to release already stored water in order to accept the additional waste water."
The water to be poured into the sea includes 10,000 tons of the stored water and 1,500 tons of water from the pits below reactors five and six that are hindering access to crucial cooling equipment.
Government spokesman Yukio Edano said it was the only available option.
"We have no choice but to release water tainted with radioactive materials into the ocean as a safety measure," Edano said.
A nuclear safety agency spokesman said that under normal circumstances the water would be treated to reduce radioactive traces below legal limits.
"Usually, an operator transfers radioactive water, generated by operating a nuclear plant, to a facility to intensively reprocess it and reduce such isotopes as iodine," the official said.
"It is only after workers confirm the water has a radioactivity lower than the limit that they release it to the outside."