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News Extra: Report says Pacific Ocean radiation levels decreasing rapidly after Fukushima

08 August 2016

Radiation levels across the Pacific Ocean are rapidly returning to normal five years after the Fukushima-Daiichi accident in Japan led to extensive releases of radioactive gases, volatiles, and liquids, particularly to the coastal ocean, a report by the Scientific Committee on Oceanic Research has concluded. 

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The committee, which brings together ocean experts from across the world, said radioactive material had been carried as far as the US. But after analysing data from 20 studies of radioactivity associated with Fukushima, it found radiation levels in the Pacific were rapidly returning to normal after being “tens of millions” of times higher than usual following the accident.

The study, concentrating mainly on the distribution of radioactive caesium, found the one remaining area of concern to be the seafloor and harbour near Fukushima, which are still highly contaminated.

The main points of the report are:

*  The incident caused extensive releases of radioactive gases, volatiles and liquids, in particularly to the coastal ocean. The radioactive fall-out on land was well-documented, but the distribution of radioactivity in the seas and onto the wider oceans was much more difficult to quantify, due to variability in the ocean currents and greater difficulty in sampling.

*  Fukushima was one of the largest nuclear accidents and unprecedented for ocean releases but the magnitude of the total release of caesium 137 was approximately 50 times smaller than that of global fallout from atmospheric nuclear weapons testing (which peaked in the early 1960s) and approximately five times smaller than that of the Chernobyl incident, and was similar to that resulting from intentional discharges from the Sellafield nuclear fuel reprocessing plant in the UK.

*  In terms of initial fallout, the main release of radioactive material was the initial venting to the atmosphere. Models suggest that around 80% of the fallout fell on the ocean, the majority close to the plant. There was some runoff from the land, peaking around 6 April 2011.

*  Caesium is very soluble and it was rapidly dispersed in the ocean. Prevailing sea currents meant that some areas received more fall-out than others due to ocean mixing processes. At its peak in 2011, the Caesium 137 signal near Fukushima was tens of millions of times higher than prior to the accident. Over time, and with distance from Japan, levels decreased significantly. By 2014 the 137 signal 2000km North of Hawaii was equivalent to around six times that remaining from fallout from atmospheric nuclear tests from the 1960s, and about two to three times higher than prior fallout levels along the west coast of N. America. Most of the fallout is concentrated in the top few hundred metres of the sea. It is likely that maximum radiation levels will be attained off the North American coast in the 2015-16 period, before declining to the level associated with background nuclear weapon testing by 2020.

*  In 2011, around half the fish samples in coastal waters off Fukushima had caesium levels above the Japanese 100Bq/kg limit, but by 2015 this had dropped to less than 1% above the limit. High levels are still found in fish near the plant. Generally, with the exception of species close to the plant, there seem to be little long-term measurable effects on marine life.

*  The report concluded the radiation risk to human life is comparatively modest in comparison to the 15,000 lives that were lost in the 2011 Tohoku earthquake and tsunami. So far, there have been no direct radiation deaths. The most exposed Fukushima evacuees received a total dose of 70 mSv, which (if they are representative of the general population) would increase their lifetime fatal cancer risk from 24% to 24.4%. However, there are still over 100,000 evacuees from the Fukushima area, and many industries such as fishing and tourism have been badly hit.

The report authors emphasised there were still gaps in data and understanding of the complex processes involved in the oceanic distribution of radionucleides, and they called for continued monitoring of radioactivity levels and sea life around Japan and across the Pacific.

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