Radioactive releases to the Pacific Ocean from the Fukushima Daiichi nuclear plant, 21 March-8 April 2011. Levels of caesium-137 and iodine-131 in the sea close to the damaged Fukushima reactors have shattered legal limits (40 becquerels per liter for iodine-131 and 90 becquerels per liter for caesium-137. nature.com

By Quirin Schiermeier
12 April 2011

As radioisotopes pour into the sea from the crippled Fukushima Daiichi nuclear plant, one reassuring message has been heard over and over again: the Pacific Ocean is a big place.

That the isotopes will be vastly diluted is not in question. Nevertheless, scientists are calling for a marine survey to begin as soon as possible to assess any damage to ecosystems in the area around Fukushima. Although the contamination is unlikely to cause immediate harm to marine organisms, long-lived isotopes are expected to accumulate in the food chain and may cause problems such as increased mortality in fish and marine-mammal populations.

"Just because you can measure it, doesn't mean it's dangerous," says Ken Buesseler, a marine geochemist at the Woods Hole Oceanographic Institution in Massachusetts. "Even so, this is the biggest man-made release ever of radioactive material into the oceans. We haven't yet seen enough data to assess what's going on, so anything that can be done in terms of further monitoring would be very welcome."

The past two weeks have seen extremely high concentrations of radioactive iodine-131 (with a half life of 8 days) and caesium-137 (which has a half life of 30 years) in samples of sea water collected near the Fukushima reactors, and even as far as 30 kilometres offshore. By late March, levels were tens of thousands of times higher than before the accident (see 'Radioisotope contamination'). Many other radioisotopes, both long- and short-lived, are also likely to have been released.

But the total amount of radioactivity that has entered the ocean is unknown, and discharges — both accidental and deliberate — are continuing and may even be substantial if any further problems occur at the Fukushima plant (see page 146). …

Radiation release will hit marine life

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