Tokyo Electric Power Co (Tepco) has admitted that radioactive ground water from the Fukushima Daiichi nuclear power plant has probably been leaking into the Pacific Ocean. It is the first time Tepco has officially acknowledged that contaminated water from the plant may have reached the sea, despite several studies and findings from the Nuclear Regulation Authority (NRA) which confirmed leakages.

“Now we believe that contaminated water has flown out to the sea," Masayuki Ono, Tepco’s general manager, told a news conference in comments broadcast on Japan’s public NHK television. "We would like to offer our deep apology for causing grave worries for many people, especially for people in Fukushima."

Tepco’s admission has underlined concerns raised by NRA, which earlier in November said its experts had found high levels of caesium in samples taken from coastal seawater and the pit water near the facility. NRA had ordered Tepco to investigate the possibility of a leak, but Tepco said there was insufficient evidence to link the high levels of caesium to a leakage from the plant.

While Tepco acknowledged that contaminated water from the reactors is seeping through ground water channels before flowing into the sea, it said water sample tests showed that the impact of the leakage appeared to be controlled by silt fences built around the reactors, as there was no significant rise in the levels of radioactivity in the sea water.

However, Tepco admitted in April, that around 120t of radioactive water may have leaked into the surrounding ground from a storage tank, and earlier this month, tests on ground water samples showed that levels of caesium-134 had increased more than 110 times in a few days. To prevent further seepage of ground water to the ocean, Tepco is injecting chemical sodium silicate into part of the seawall separating the sea and nuclear plant, which will solidify a larger part of the seawall with the chemical, Reuters reported.

Tepco is also struggling to contain radioactivity at the plant. Workers on 24 November reported steam from inside the unit 3 reactor building for the second time in a week. Tepco is investigating the cause after initially suggesting rainwater could have been the source.

“We think it’s possible that rain made its way through the reactor building and, having fallen on the primary containment vessel, which is hot, evaporated [and created] steam," Tepco spokeswoman Maymi Yoshida said, according to Reuters.

The steam rising from unit 3 was noticed by repair crews who were removing contaminated debris from the facility. "All work to remove debris in and around unit 3 was stopped," a spokesperson for Tepco told The Daily Telegraph. "We have confirmed that radiation levels around the pressure chamber have not changed, and we were able to confirm that the reactor has not reached criticality."

The incident underscores the concerns and challenges involved in decommissioning the Fukushima plant, including how to dispose of the water used to cool its melting reactors. Tepco has poured thousands of gallons of water over the reactors since the 2011 incident, and disposing of the water with radioactive content is a major problem.