Concentrations In the air
In 2015, measurement results relating to the monitoring of radionuclides present in the air showed that traces are still perceptible in the ambient air five years after the accident.
In Tsushima, within the evacuated zone, measurements show that concentrations are significantly higher than in Tsukuba, an agricultural town located 170 km from the nuclear power plant (NPP).
The main driving forces behind this atmospheric remanence are the
suspension by wind erosion
of particles from the soil and from the ash coming from biomass fires that were more or less contaminated at the time of the accident. These particles and ash can be
transported by the wind towards territories where deposits were lower and thus lead to a temporary increase in concentrations in the air.
Download "Radionuclide concentrations in the air in Japan, five years after the Fukushima accident" (PDF, 347 Ko)
Radioactive deposits in the environment
The evolution of caesium-134 and caesium-137 deposited on the territories, and the ensuing dose rates in the air, are decisive elements in guiding strategies with regard to the return of inhabitants to the evacuated zones and decontamination measures.
The evolution of the radiological situation has been characterised by multiple air dose rate measurement campaigns. Regardless of the device used, the dose rate in the air fell by an average of factor 2 to 3 between mid-2011 and end-2013 in an 80 km radius around the Fukushima NPP.
This decrease was
relatively sustained during the first two years following the accident, and has taken place at a quicker pace than expected through the radioactive decay of caesium-134 (period of two years) and caesium-137 (30 years). This decline
has slowed since 2013, more or less complying with the expected theoretical evolution.
Numerous natural or anthropogenic mechanisms have been put forward in order to explain this decrease, such as the effect of leaching by rainfall, migration into the soil, the effect of ploughing and decontamination measures. The contribution made by these processes is most certainly significant in urban and agricultural environments.
Nonetheless, none of these mechanisms help to explain the decrease in dose rates at forest surface level. An IRSN study has shown that the progressive decontamination of conifer canopies could noticeably alter signals measured at altitude, notably due to the screening effect of vegetation on radiation emitted at ground level.
Download "Activity of caesium deposited on the territories and associated ambient dose rates following the Fukushima accident" (PDF, 1,79 Mo)
Contamination of Japanese foodstuffs
One of the radiological pathways to reach humans is the ingestion of contaminated foodstuffs. Since the accident, the Japanese Ministry of Health, Labour and Welfare has regularly published the radionuclide measurement results of foodstuffs produced in Japan on its website.
Between March 2015 and February 2016, over 270,000 samples were analysed across the whole of Japan. These analyses primarily concern foodstuffs from animal sources (86% of samples), agriculture (10%), dairy (1%), game (0.2%) and other foodstuffs (3%).
Over this period,
there was a decline in the activity measured in these foodstuffs. This can be explained by the radioactive decay of the radionuclides, various natural processes (caesium migration into the soils) as well as by different anthropogenic measures (soil decontamination, the use of potassic fertilisers reducing the transfer of radioactive caesium, a stop to - or change in - farming).
As a result,
less than 0.1% of the samples tested exceed the Maximum Permissible Levels (MPLs) set at 100 Bq/kg fresh weight for the marketing of foodstuffs. In 2015, with very few exceptions, the foodstuffs exceeding the MPLs only concerned:
wild foodstuffs (game, seasonal mushrooms) or those cultivated in a natural environment (shiitake mushrooms);plant-based foodstuffs that were in leaf in March 2011 at the time of the radioactive fallout (bamboo shoots, tea leaves, aralia and koshiabura shoots, ostrich ferns...).
The Chernobyl accident had already shown that forest products such as mushrooms and game can reach high contamination levels that only decrease slowly over the years. Fukushima has confirmed this characteristic, extending it to plant-based shoots consumed in Japan and to cultivated mushrooms (shiitakes).
Finally, despite the notable fall in activity measured in the agricultural production of the Fukushima prefecture, and the low percentage of measurement results exceeding the MPLs,
the agricultural sector is registering a significant decline in demand, leading to overproduction and a fall in prices. Lower incomes for farmers has led to 20,000 of them ceasing their activity. Only a few products benefiting from a strong brand image have got back to sales prices equivalent to or higher than those of 2010.
Download "Contamination of Japanese foodstuffs following the Fukushima accident" (PDF, 1,75 Mo)
Contamination of the marine environment
The evolution in 2015 of the contamination of the coastal marine environment around the Fukushima NPP is characterised by a relative stability in radionuclide concentrations. In detail, we can observe the absence of a detectable decline in caesium-137 activity in sediments, and a very slow reduction in the contamination of seawater.
These phenomena are fuelled by probable releases to the sea due to leaks existing on reactors that remain in the cooling phase, permanent input from leaching and soil drainage, as well as the re-suspension of sedimentary particles and the possible desorption of associated radioactive caesium.
Within a 30 km radius of the NPP, contamination of seawater is maintained due to the effect of three input categories that are, nonetheless, difficult to quantify:
potential releases from the facilities (leaks on damaged reactors, runoff of the water injected in order to cool the reactors whose outer walls are no longer watertight, and drainage of highly-contaminated soils from the site); leaching and drainage from the catchment basins whose soils are contaminated following the radioactive deposits. This process is particularly visible during typhoons; re-suspension of sedimentary particles.
More than 200 km from the NPP, radioactive caesium concentrations have fallen to levels close to those observed prior to the accident.
Evolution of Cs-137 activity in seawater in relation to the distance from the plant from 0 to 2 km, 2 to 30 km, 30 to 100 km (dark blue) and 100 to 200 km (medium blue and light blue).
Five years after the accident, radioactivity levels in surface sediments are evolving only slightly between the immediate surroundings of the power plant and up to 80 km away. They have also been constant since 2012. Beyond that distance, between 80 and 280 km, activity is lower.
Logically, the organisms living close to coastal seabeds are those showing the highest levels of - and slowest decrease in - activity. Outside the Fukushima prefecture, levels of radioactive caesium measured in marine foodstuffs no longer exceed the marketable limit set by the Japanese health authorities (100 Bq/kg) since 2014. Within the Fukushima prefecture, this limit is still exceeded on occasions.
Temporal evolution of radiocaesium concentrations in different species of demersal fish (feeding at the seabed).
Download "Situation of the contamination of the marine environment following the Fukushima Daiichi nuclear power plant accident" (PDF, 2,03 Mo)