Radiation leaking into seawater from Japan's tsunami-damaged nuclear complex has reached its highest level yet, prompting officials to declare that four reactors will be scrapped.

Hidehiko Nishiyama, deputy director general of the Nuclear and Industrial Safety Agency, said seawater tested near the Fukushima Daiichi nuclear power station was found to contain iodine-131 at 3,355 times the safety standard.

"We have no choice," Tsunehisa Katsumata, chair of Tokyo Electric Power Company, known as TEPCO, said as he told a news conference Wednesday that Units 1, 2, 3 and 4 at the plant will be scrapped.

"So what that means is they're going to keep cooling them down and when they're sufficiently cool, they'll have to remove the highly toxic radioactive material and store it safely and demolish the reactors," CBC reporter Craig Dale said in an interview.

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U.S. navy barge YOGN-115, which can carry 1.04 million litres of fresh water, is helping with efforts to cool the Fukushima Daiichi nuclear power plant. (U.S. navy/Mass Communication Specialist 3rd Class Mikey Mulcare/Reuters)

France will send in two nuclear experts and the U.S. will send robots to examine the reactor rods and fuel pools. The whole process could take three decades and cost upward of $12 billion US, and then there is the question of what to do with reactors 5 and 6.

They were off when a March 11 tsunami engulfed the facility, knocking out power to the cooling system that keeps nuclear fuel rods from overheating,

"But will the public allow TEPCO to restart the reactors?" Dale said. "Right now it seems that the answer would be 'No,' but TEPCO said it's not sure what the future of Units 5 and 6 are at this point; they're focusing on reactors 1 to 4."

Radiation leaking from the plant has seeped into the soil and seawater nearby and made its way into produce, raw milk and even tap water as far away as Tokyo, 220 kilometres to the south.

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Tokyo Electric Power Co. president Masataka Shimizu, left, speaks with members of the Nuclear and Industrial Safety Agency at a nuclear plant in Kashiwazaki, Japan, May 9, 2009. (Toru Hanai/Reuters)

Japan's government has been saying since March 20 that the entire plant must be scrapped.

The amount of iodine-131 found south of the plant does not pose an immediate threat to human health but was a "concern," said Hidehiko Nishiyama, a Nuclear and Industrial Safety Agency official. He said there was no fishing in the area.

Radioactive iodine is short-lived, with a half-life of just eight days, and in any case was expected to dissipate quickly in the ocean. It does not tend to accumulate in shellfish.

Highly toxic plutonium also has been detected in the soil outside the plant, TEPCO said. Safety officials said the amounts did not pose a risk to humans, but the finding supports suspicions that dangerously radioactive water is leaking from damaged nuclear fuel rods. There have been no reports of plutonium being found in seawater.

Earlier, Chief Cabinet Secretary Yukio Edano said the government is considering a number of options to stop the spread of radiation from the crippled nuclear plant.

They include using a special cloth to cover the damaged reactor buildings to stop radioactive substances from leaking. Another option is spraying a resin on the grounds of the facility, which could happen as soon as Thursday.

The resin would stick to the ground and form a film. This would trap any radiation that could leak from the reactors. The resin would stop the radiation from reaching the Pacific Ocean or from being released in the air, however, it's not clear if the resin can stop radiation from seeping into the soil.

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Frozen fish imported from Japan are seen at a storage facility at Bangkok customs Tuesday. Thai authorities have asked importers and distributors to avoid or at least reduce imports of Japanese food products including meat, dairy products, seafood and seaweed. (Sukree Sukplang/Reuters)

Meanwhile, the stress of reining in Japan's worst crisis since the Second World War has taken its toll on TEPCO president Masataka Shimizu, who was sent to a hospital late Tuesday.

Shimizu, 66, has not been seen in public since a March 13 news conference in Tokyo, raising speculation he had suffered a breakdown. For days, officials deflected questions about Shimizu's whereabouts, saying he was "resting" at company headquarters.

Spokesman Naoki Tsunoda said Wednesday that Shimizu had been admitted to a Tokyo hospital for hypertension after suffering dizziness and high blood pressure.

The official death toll from the earthquake and tsunami stands at 11,259, with the final toll likely surpassing 18,000. The total number of missing is 16,344.

The government said damage is expected to cost $310 billion US, making it the most costly natural disaster on record.