Water near the Fukushima Daiichi plant's reactor 1 contained radioactive iodine at 3,355 times the legal limit, Japan's nuclear safety agency said.

However, an official said the iodine would have deteriorated considerably by the time it reached people.

Meanwhile, the president of Fukushima operator Tepco has been hospitalised.

Masataka Shimizu is being treated for high blood pressure and dizziness, a Tepco spokesperson said.

Mr Shimizu has barely been seen in public since the earthquake and tsunami on 11 March which damaged the Fukushima plant.

Tepco officials have announced a press conference for 1500 local time (0600 GMT).
Half-life

Earlier samples had put the iodine level at 1,850 times the legal limit.

"Iodine 131 has a half-life of eight days, and even considering its concentration in marine life, it will have deteriorated considerably by the time it reaches people," Hidehiko Nishiyama, deputy director-general of Japan's nuclear safety agency told a news conference.

Radioactive materials are measured by scientists in half-lives, or the time it takes to halve the radiation through natural decay.

Half-lives range from fractions of a second to billions of years.

Iodine 131 was blamed for the high incidence of thyroid cancer among children exposed to fallout from the Chernobyl nuclear disaster in 1986.

Alongside uranium, other elements of greater concern are those with much longer half-lives. These include caesium, which is easily taken up by plants and animals and can be inhaled through dust, ruthenium, strontium and plutonium. Read on...