's nuclear safety agency, test performed earlier on water in one section near the Fukushima Daiichi plant's reactor 1 indicated the iodine level at 1,850 times the legal limit which is now found at 3,355 times the legal limit. Japan
However, an official said the iodine would have deteriorated considerably by the time it reached people. Radioactive materials are measured by scientists in half-lives, or the time it takes to halve the radiation through natural decay.
"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
's nuclear safety agency told a news conference. Japan
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
nuclear disaster in 1986. Chernobyl
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.
's quake-hit nuclear plant are trying to prevent radioactive water from seeping into the sea. Japan
Highly radioactive liquid has been found inside and outside several reactor buildings.
Small amounts of plutonium have also been detected in soil at the plant - the latest indication that one of the reactors suffered a partial meltdown.
Plant operator Tepco and the safety agency say the exact source of the radioactive leak is unknown. But, like the discovery of plutonium, the high levels of radiation found inside and outside reactor buildings are likely to have come from melted fuel rods.
Theories for the leak centre on two possibilities: steam is flowing from the core into the reactor housing and escaping through cracks, or the contaminated material is leaking from the damaged walls of the water-filled pressure control pool beneath the No 2 reactor.
The plutonium - used in the fuel mix in the No 3 reactor - is not at levels that threaten human health, officials said.
Engineers are battling to restore power and restart the cooling systems at the stricken nuclear plant.