Two reports released this month, one focusing on the marine, and the other on the atmospheric impact, find that the radiation fallout from the Fukushima Daiichi Nuclear Power Plant accident following the earthquake and tsunami in March is bigger than that reported by the Japanese government and electrical power company. One researcher says in some respects, the disaster is the most significant nuclear event since Chernobyl 25 years ago.

In an update of research done since their last report in July, France’s nuclear monitor, the Institute for Radiological Protection and Nuclear Safety (IRSN), said on Wednesday that the amount of cesium 137 that leaked into the Pacific from the wrecked power plant was the “greatest single contamination by artifical radionuclides of the sea ever seen”.

Their new assessment shows that the amount of cesium 137 that flowed into the Pacific after the disaster was probably nearly 30 times the amount stated by Tokyo Electric Power Company in May, reports Mainichi Japan.

The IRSN estimate that the amount of cesium 137 that flowed into the sea from the power plant between 21 March and mid-July reached 27 peta (10 to the power of 15) becquerels. They also report that 82% of this contamination had reached the sea by 8 April. (1 Becquerel is one radioactive decay per second).

However, in a press summary, they also said that in line with previous assessments, thanks to the siting of the plant, ocean currents have massively diluted levels of the radioactive isotope and moved contaminated waters well out into the Pacific ocean, so that as from Autumn 2011, there will be little if any threat to marine life, except perhaps to near-shore species.

To put this into context, they said that for the Pacific in general, the levels will soon reach about 0.004 becquerels per litre which is about twice the concentration measured duing atmospheric nuclear tests in the 1960s.

High levels of radioactive iodine were also detected, but because this has a half life of only just over a week, it soon reached undetetable levels by the end of May.

Cesium decays much more slowly than iodine: its half-life is 30 years (radioactive half-life is the time it takes for half of the radioactive version or isotope of an element to convert into a non-radioactive isotope). And it is a health concern: it can increase the risk of cancer.

The marine species that are most sensitive to cesium pollution are deep-water fish, fish at the summit of the marine food chain, and “filtrating” species such as molluscs, said the IRSN report.

The IRSN said it was important to keep monitoring the situation, and that there could still be significant pollution of the seawater near the plant because of rainwater runoff from the land.

Research led by the Norwegian Institute for Air Research (NILU) focuses on emissions of radioactive noble gas xenon 133 and aerosol-bound cesium 137 from the Fukushima Daiichi disaster. The study is published online for open peer review in the Atmospheric Chemistry and Physics journal.

A NILU press statement says the researchers arrived at their estimates by combining a large set of data from Japan and around the world with models that calculate atmospheric transport, together with information and “reasonable approximations on radionuclide inventories and accident events” from the power plant.

Lead author Dr Andreas Stohl, a senior scientist at NILU, said:

“Our calculations are based on about 1000 measurements of activity concentrations and deposition conducted in Japan, USA and Europe. This is the most comprehensive investigation so far. There is no doubt that the Fukushima accident is, at least in terms of the isotopes xenon 133 and cesium 137, the most significant event after the catastrophe in Chernobyl 25 years ago”.

For xenon 133, the researchers estimate that the disaster emitted 16,700 peta becquerels into the atmosphere: the “largest civilian noble gas release in history”, about two and a half times bigger than Chernobyl, says the NILU press statement.

The researchers say there is strong evidence that the emissions started early in the morning of 11 March, immediately after the big earthquake.

Xenon 133 is not a health concern as such, since we don’t ingest it or inhale it, but it is a good indicator for understanding the magnitude of the event, they explain.

Cesium 137, however, is a health concern because of its physical properties and its long half-life.

The NILU statement says Stohl and colleagues found that emissions of the isotope started earlier and ended later than most other studies have assumed.

They estimate the total release to the environment as 36 peta becquerels, which is about 42% of that emitted in the Chernobyl accident.

19% of the cesium fell on land in Japan, and about 80% went into the sea, says the NILU statement.

Stohl told Nature News he thinks the large difference between their estimates and the figures put out by the Japanese government are probably due to the differences in the data sources. The Japanese government relied mainly on the monitoring stations in Japan, which never saw the large amounts of radiation that was blown out to sea and eventually reached North America and Europe.

There was also a public pressure to get figures out quickly, and Stohl said he has some sympathy for the Japanese officials.

And, he cautions that the model he and his colleagues constructed is far from perfect. There were few reliable measures available just after the disaster and some monitoring stations were too contaminated to be regarded as reliabled sources of data.

Also, nobody really knows what really happened inside the reactor, which is important to understanding exactly what they emitted.

“If you look at the estimates for Chernobyl, you still have a large uncertainty 25 years later,” Stohl told Nature News.

Written by Catharine Paddock PhD