In their paper published March 12, 2015 in the open access, peer reviewed journal Scientific Reports, Povinec and Hirose investigate how radionuclides activity concentrations varied in seawater and the marine biota from 2011 to 2013. They then calculated the expected dose exposure to people consuming seafood using two separate approaches described below.

Variation of FDNPP released radionuclides in surface seawater and fish close to Japan

Seawater contamination

The activities of 90Sr and 137Cs in seawater near the North and South water outlets of the FDNPP over time are shown in the figure below:

Temporal variations of 137Cs and 90Sr activity concentrations in surface waters near the north (1) and south (2) water outlets at FDNPP

In March 2011 137Cs and 134Cs reached maximum activities of ~100 million Bq m-3 due to direct discharges of contaminated water to the ocean and deposition of atmospheric releases. By April 2011 dramatic decreases in releases to the plants lead to much lower 137Cs activities in seawater that were ~1000's of times smaller than the month before.

Between 2012-2014 the 137Cs activity in seawater near the outlets varied between 100 and 10,000 Bq m-3 as can be seen in the figure. In coastal waters within 20 km of the FDNPP during the time period between November 2011 and March 2014 137Cs activity decreased dramatically relative to nearshore values and over time from 3 - 2000 Bq m-3 in 2012, to 2 - 600 Bq m-3 in 2013 (see following figure):

Temporal changes of 137Cs activity concentrations in surface waters at monitoring sites within 20 km from the Fukushima Dai-ichi NPP

The 90Sr activity in seawater at the plants north and south outlets generally varied between 100 - 10,000 Bq m-3. An event in March 2012 raised 90Sr activity to 1 million Bq m-3 where more intense sporadic leakage from the site occurred. The 90Sr/137Cs in the north and south outlet seawater varied between 0.005 and 500 with total releases of 137Cs being much larger when integrated over time.

Overall the trends of contamination with time support very large direct discharges to the ocean and deposition from atmospheric releases in March-April 2011 that have dramatically decreased with time at the FDNPP site and up to 20 km offshore.

Contamination of surface and bottom dwelling fish

The 137Cs + 134Cs activity concentrations in fish caught close to the FDNPP between April 2011 and April 2013 varied several orders of magnitude from 0.5 Bq kg-1 wet weight (ww) to 15,000 Bq kg-1 ww:

Temporal variations of radiocesium (134Cs + 137Cs) concentrations in surface-dwelling fish for 2011–2013 (closed circles: Japanese Sandlance, open circles: whitebait)

while activities in bottom dwelling fish varied between 0.3 and 3000 Bq kg-1 some time later between April 2012-2014. Bottom dwelling fish tend to have higher contamination as they tend to stay in the place in areas with contaminated sediments rather than migrate in and out of contaminated zones as surface fish do.

Variation of FDNPP released radionuclides in surface seawater and fish in the open Pacific Ocean

In the NW Pacific Ocean radiocesium levels measured between 2011-2013 varied between <1 to 100 Bq m-3 while levels in fish (bluefin and yellowfin tuna) caught in the open ocean April 2011 to November 2012 were between 0.3 to 41 Bq kg-1 ww. The 90Sr levels in seawater far offshore after Fukushima varied between <1 to 10 Bq m-3 and in fish 0.01 to 1.2 Bq kg-1 ww where 90% of fish analyzed had no detectable 90Sr at all.

Conclusions: Estimating doses to consumers of nearshore and offshore seafood

Povinec and Hirose use two methods to calculate doses to consumers of seafood contaminated by the FDNPP meltdowns. The first method uses seawater activities and concentration factors (CF's) of different marine organismsto determine how much radioactivity is present in food which contributes a known dose to a consumer. The second method uses the activity of radioisotopes measured or estimated in the seafood to calculate dose to the consumer.

In each case the authors assumed an average consumption of seafood based on the average Japanese diet:

Fish = 64 g day -1

Crustaceans = 5.4 g day -1

Shellfish = 3.5 g day -1

Seaweed = 10 g day-1

They determined the following:

Individual dose from consumption of radiocesium and 90Sr in seafood collected from Japanese coastal waters near Fukushima between 2011-2013 would be 0.6 +- 0.4 mSv yr-1. This is less than the maximum allowed dose to the public from non-natural sources of 1 mSv yr-1 and the world average dose from natural sources of 2.4 mSv yr-1. The dose from Fukushima contamination is similar to the dose from the naturally occurring alpha emitting isotope 210Po also present in seafood (0.7 +- 0.4 mSv yr-1). Individual dose from consumption of radiocesium and 90Sr in seafood collected from fish caught in the open NW Pacific Ocean in 2012-2013 would be an order of magnitude smaller or 0.07 +/- 0.05 mSv yr-1 being much less significant than doses from naturally occurring isotopes.

In neither case, using conservative estimates which would tend to maximize dose to consumers (see paper Discussion for details), do the estimates of dose exceed levels where health physicists expect to see any negative health impacts to consumers. Indeed, the catch and sale of fish and seafood from Fukushima prefecture has been suspended since the disaster making the highest dose estimates very unlikely. Exposure of the Japanese public and citizens in other countries are likely to be much lower than reported by the authors.