Police and other workers practise a search on the Fukushima coast earlier this month. Exhaust towers of the Fukushima No. 1 nuclear power plant are visible in the background (Image: Masamine Kawaguchi/The Yomiuri Shimbun/AP/PA)

Levels of radiation in the sea off the Fukushima-Daiichi nuclear plant remain stubbornly high six months after the earthquake and tsunami struck Japan on 11 March.

After levels peaked at around 100,000 becquerels per cubic metre of seawater in early April, much of the radioactive iodine, caesium and plutonium from Fukushima was expected to rapidly disperse in the Pacific Ocean.

Instead, it seems that the levels remain high. That could be because contaminated water is still leaking into the sea from the nuclear plant, because currents are trapping the material that’s already there, or both.


Ken Buesseler of Woods Hole Oceanographic Institution, Massachusetts, has told The New York Times that he has received samples of seawater taken in July from near the plant that contained 10,000 becquerels per cubic metre. The corresponding level last year, only months before the disaster, was just 1.5 becquerels, he says.

Simon Boxall, an oceanographer at the University of Southampton, UK, says that much of the radioactive material will still be sinking down to the seabed and being absorbed by marine life.

Current trap

Boxall says that a strong ocean current called the kuroshio – the Japanese equivalent of the Atlantic Gulf Stream – may be responsible for the persistence of the radiation.

The kuroshio skirts the Japanese seaboard, sweeping material into the deep ocean. But closer to shore, it creates huge eddies 80 to 100 kilometres across, which may send the material back towards the shore instead of dispersing it.

In June, Buesseler took his own samples off the coast of Japan. He wants to have his findings analysed before publishing them, but at present they suggest Boxall may be right: further out to sea, from 30 to 600 kilometres offshore, the radiation threat drops off.

It’s not surprising that sea life hasn’t yet swept up the radioactive material, says Boxall. Seaweed tends to accumulate radioactive iodine-131, which rapidly decays, he says – but caesium-134 and caesium 137, with half-lives of two and 30 years respectively, accumulate in shellfish and could persist for decades. “There’s no reason to think it would drop so soon after the disaster,” he says.

Peak leaks

Official estimates from the Japanese government and TEPCO, the company that owns Fukushima-Daiichi, suggest that 3500 terabecquerels of caesium-137 from the plant entered the ocean between 11 March and late May. The pollution was exacerbated in April by problems locating a persistent leak of contaminated water and a decision by TEPCO to dump contaminated water at sea. A further 10,000 terabecquerels of caesium-137 is thought to have found its way into the ocean after escaping as steam from the facility. And TEPCO said last week that Fukushima-Daiichi may still be leaking as much as 500 tonnes of contaminated water into the sea every day.

“It wouldn’t surprise me if there is still caesium entering the sea off Fukushima, but it can’t be as bad as in March and April,” says Richard Wakeford of the University of Manchester, UK.

“The important thing is to keep monitoring to understand what’s happening, and particularly to keep an eye on levels in seafood,” he says. “The reports I’ve seen suggest that there isn’t an immediate problem with seafood contamination, but it is important to maintain a comprehensive monitoring programme.”

Dirty soil

There have also been significant developments this week in Japan’s plans to cope with land and soil contaminated by airborne pollution from the reactors, mainly released in explosions and fires in March and April.

On Tuesday, the Japanese environment ministry said that about 30 million cubic metres of contaminated soil and vegetation from around Fukushima prefecture may need to be disposed of – 23 times the volume of the iconic Tokyo Dome baseball stadium in the capital. The volume is so high because an expert panel recommended that 5 centimetres of topsoil should be shaved off contaminated areas, mainly farmland, but also including forested and residential areas.

A day later, the ministry unveiled a plan to build temporary storage facilities for the soil in eight prefectures in different parts of Japan.

Meanwhile there was good news this week from Fukushima-Daiichi itself. TEPCO reported on Wednesday that for the first time since the disaster on 11 March, the temperatures of all three of the most severely damaged reactor units had fallen below 100 °C – a key step towards the goal of cold shutdown, which will effectively mothball the reactors for good.

The final reactor to fall below 100 °C was unit 2, the source of much of the leaked water in April.