(Photo: Kyodo/VNA)

The plant's operator, Tokyo Electric Power Company Holdings (TEPCO), told news agencies that its workers activated a pump to dilute the treated water with large amounts of seawater, slowly sending the mixture, through an underground tunnel, into the sea.

The first discharge of wastewater from the plant  — which was struck by a devastating tsunami in 2011 — began on August 24 and ended on September 11. A total of  7,800 tons of treated water from 10 tanks was discharged during that release, TEPCO said.

The plant plans to release another 7,800 tons of treated water into the Pacific Ocean over 17 days in the second phase.