Radiation levels in the sea off Japan's stricken nuclear plant hit their highest reading yet, officials said on Wednesday, amid a struggle to deal with large amounts of radioactive water at the site.
A spokesman for Tokyo Electric Power Co., operator of the Fukushima Daiichi nuclear plant crippled by a March 11 earthquake and tsunami, said levels of radioactive iodine-131 were 3,355 times the legal limit in the sea near the plant, according to a reading taken Tuesday.
Officials said they did not know what cause the radiation level to rise.
"The figures are rising further. We need to find out as quickly as possible the causes and to stop them from rising any higher," said nuclear safety agency spokesman Hidehiko Nishiyama.
The sampling location is 330 metres south of the discharge outlet for four troubled reactors. Officials have said that tidal dispersion means that there is no immediate health threat, and that the iodine degrades relatively quickly.
A 9.0 magnitude earthquake and tsunami knocked out the cooling systems of the Fukushima plant's six reactors – triggering explosions and fires, releasing radiation and sparking global fears of a widening disaster.
Radiation from the plant northeast of Tokyo has wafted into the air, contaminating farm produce and drinking water, and seeped into the Pacific Ocean.
In a stop-gap measure to contain the crisis at the plant, crews have poured thousands of tons of water onto reactors where fuel rods are thought to have partially melted, and topped up pools for spent fuel rods.
But the run-off of the operation has accumulated in the basements of turbine rooms connected to three reactors and filled up tunnels, making it too risky for workers to go near to repair cooling systems needed to stabilise the plant.
The water out of reactor two has measured 1,000 millisieverts per hour – four times the recently hiked total exposure limit for emergency staff, and a level that can cause radiation sickness with nausea and vomiting in an hour.
On Sunday, iodine-131 measuring 1,850 times the legal maximum was reported a few hundred metres (yards) from the plant, up from 1,250 times the limit Saturday.
Japan's government has evacuated hundreds of thousands of people from within 20 kilometres of the plant, and more recently encouraged those remaining within 30 kilometres to also leave.