Sendai: Radiation levels have jumped 10-fold in days in seawater near Japan`s tsunami-hit nuclear plant, officials said, as workers battled to stabilise the crippled power station.
Drinking a half-litre (20-ounce) bottle of similarly contaminated fresh water would expose a person to their annual safe dose, said an official who however ruled out an immediate threat to aquatic life and seafood safety.
The iodine-131 level in the Pacific Ocean waters just off the Fukushima plant was 1,250 times above the legal limit -- compared with readings of 126 times higher taken on Tuesday, and 145 higher on Thursday.
"This is a relatively high level," nuclear safety agency official Hidehiko Nishiyama said in a televised press conference on the test results from Friday released by plant operator the Tokyo Electric Power Company (TEPCO).
Assessing the likely impact on aquatic life, Nishiyama added: "Generally speaking, radioactive material released into the sea will spread due to tides, so you need much more for seaweed and sea life to absorb it."
He added that because iodine-131 has a half-life -- the time in which half of it decays -- of eight days, "by the time people eat the sea products, its amount is likely to have diminished significantly."
However, TEPCO in a statement also said that levels of caesium-137, which has a half life of about 30 years, was 79.6 times the legal maximum.
The assurances did little to lift the gloom that has hung over Japan since a massive 9.0-magnitude earthquake struck on March 11 and sent a monster tsunami crashing into the northeast coast in the country`s worst post-war disaster.
The wave easily overwhelmed the world`s biggest sea defences and swallowed entire communities. The confirmed death toll rose to 10,151 on Saturday, with little hope seen for most of the 17,053 listed as missing.
The wave knocked out the cooling systems for the six reactors of the Fukushima plant, leading to suspected partial meltdowns in three of them and hydrogen explosions and fires that have ripped through the facility.
Fire engines have hosed thousands of tons of seawater onto the facility in a bid to keep the fuel rods inside reactor cores and pools from being exposed to the air, where they could reach critical stage and go into full meltdown.
Radioactive iodine, caesium and cobalt levels in water in the turbine buildings next to reactors one and three were 10,000 times the normal level -- raising fears that the reactor vessels or their valves and pipes are leaking.
The worst-case scenario at reactor three would be that the fuel inside the reactor core -- a volatile uranium-plutonium mix -- has already started to burn its way through its steel pressure vessel.
Worried about the salt buildup in the crippled plant, engineers have started pumping in fresh water instead. More is being shipped in, including on two US military water barges headed for the plant from a naval base near Tokyo.
Radioactive vapour from the plant has contaminated farm produce and dairy products in the region, leading to shipment halts in Japan as well as the United States, European Union, China and a host of other nations.
Higher than normal radiation has also been detected in tap water in and around Tokyo, some 250 kilometres (155 miles) from the plant, leading authorities at one stage to warn against using it for baby milk formula.
Japan widened the zone around the plant where it suggests people evacuate, to 30 kilometres (20 miles) -- still below the 80 kilometres advised by the United States, and larger areas including Tokyo in other nations` alerts.
Environmental watchdog Greenpeace started its own monitoring near the plant, charging that "authorities have consistently appeared to underestimate both the risks and extent of radioactive contamination".
"We have come to Fukushima to bear witness to the impacts of this crisis and to provide some independent insight into the resulting radioactive contamination," said the group`s radioactivity safety advisor Jan van de Putte.
The campaign group said it would provide "an alternative to the often contradictory information released by nuclear regulators".