March 2016

Situation of the contamination of the marine environment in 2015,
following the Fukushima Daiichi nuclear power plant accident

2015 Summary
The evolution in 2015 of the contamination of the coastal marine environment around the
Fukushima power nuclear plant (NPP) is characterised by a relative stability in levels: no detectablereduction in caesium-137 sediment activity and a very slow decline in seawater activity, fuelled byprobable releases into the sea by the facilities due to leaks existing on reactors that remain in thecooling phase, permanent input from leaching and soil drainage, as well as the re-suspension of
sedimentary particles and the possible desorption of associated1 radiocaesiums. Over 200 km fromthe NPP, radiocaesium concentrations have fallen to levels close to those observed prior to theaccident. Logically, the organisms living close to coastal sea beds are those showing the highest
levels of - and slowest decrease in - activity. However since 2014, outside the Fukushima
prefecture, radiocaesium levels measured in marine foodstuffs no longer exceed the marketable
limit set by the Japanese health authorities (100 Bq/kg). Within the Fukushima prefecture, this limit
is still exceeded on occasions.

1. Estimation of the marine source-term
The estimation of the direct releases ending up in the sea in April 2011 remains a subject of debate
between various authors. Numerous evaluations have been carried out concerning 137Cs inputs: 3.6 –3.7 PBq (Tsumune et al., 2013); 5.1 – 5.5 (Estournel et al., 2012); 12 – 41 PBq (Bailly Du Bois et al.,2012; 2014); 11 - 16 PBq (Charette et al., 2013; Lai et al., 2013; Rypina et al., 2013; Smith et al.,2014). The atmospheric fallout over the sea represented similar quantities (from 0.9 to 5.5 PBq near
the coast), but with an extensive distribution over the North Pacific from the first month after theaccident (Aoyama et al., 2013). For the marine environment, all of these 137Cs inputs arecomparable or superior to those that affected the seas following the atmospheric fallout of theChernobyl accident (15 - 20 PBq). The difference mainly lies with the direct inputs at sea, which led
to particularly significant concentrations close to the outfall.

1

These radiocaesiums are caesium-134 and caesium-137.

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As far as 90Sr is concerned, due to the fact that the number of measurements were far lower thanfor caesium-137, estimations of the source-term are essentially based on the 90Sr/137Cs ratio. As thisratio evolved significantly in 2011, major uncertainties exist. Povinec et al. (2012) estimate thetotal flow from direct releases at around 1 PBq. Despite major fluctuations between one
measurement and the next, the evolution of the 90Sr/137Cs ratio measured close to the facility
indicates that the residual releases since 2012 of these two radionuclides are, on average, of thesame order of magnitude.

2. Contamination of seawater
The contamination of seawater following the Fukushima accident resulted from atmosphericdeposits in an initial phase and then from massive direct releases linked to the pouring of seawaterthen fresh water onto the damaged reactors for cooling purposes. Activity of several tens of
thousands of Bq/L reached in the seawater less than 2 km from the site during the first weeks
following the accident was quickly diluted by the interaction between the two main currents thatcirculate off Japan's eastern coast: the Kuroshio, coming from the south, following the coastline,
and the lesser Oyashio, coming from the north (figure 1), which led to a rapid reduction in thecontamination of the water column. Nevertheless, during this period, part of the radiocaesium wasabsorbed by suspended matter and sediments, leading to the contamination of the latter.
Since then, within a 30 km radius of the site, water activity has remained globally stable, due to theeffect of three input categories that are nonetheless difficult to quantify: input linked to the NPP
site (leaks existing on the damaged reactors that remain in the cooling phase (approximately
320 m3/day of fresh water is injected into the reactor cores, whose outer walls are no longer
watertight), and drainage of the highly-contaminated soils at the site), leaching and drainage fromthe catchment basins whose soils are contaminated following the radioactive deposits2 (this process
is extremely effective, notably during typhoons) as well as the re-suspension of sedimentary
particles and the desorption of associated radiocaesiums.

2

For more information, the reader can consult the following information sheet: "How are radioactive deposits redistributed
within catchment basins in post-accident situations? The lessons learned from the Chernobyl and Fukushima accidents".

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Figure 1: Depth and surface currents facing the Japanese east coasts

Since the summer of 2012, caesium-137 seawater concentrations are therefore, overall, relatively
stable within the 30 km radius around the site, with a slight downward trend (figures 2 and 3). Less
than 2 km from the NPP (figure 2), concentrations lie within a range of 0.01 to 5 Bq.L-1. They arearound 10,000 times lower than those detected in April 2011. Between the zone close to the facility
and a distance of 30 km along the coast, caesium-137 concentrations lie within a range of 0.007 to
0.2 Bq.L-1. With the renewal of marine water being comparable from one year to the next, thestability over the time of these levels can only be explained by a continuation of 137Cs inputs in theseawater in accordance with the sources indicated previously. It is not possible to precisely
determine the respective share of these contributions. Nevertheless, the difference inconcentrations by approximately factor 10 to 100 between the zone close to the site (less than 2km) and zones further away (2 to 30 km and 30 to 200 km respectively) (figure 4) indicates thatthese inputs remain significant.

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Figure 2: Evolution over time of caesium-137 seawater activity along the coast for distances from thenuclear power plant of: less than 2 km (dark blue, excluding the port zone) and between 2 and 30 km(light blue).

Figure 3: Evolution over time of caesium-137 seawater activity for distances from the damaged nuclear
power plant of 30 to 100 km (dark blue), 100 to 200 km (medium blue) and over 200 km (light blue).

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1000

Cs-137 Bq.L-1

100
10
1
0,1

0,01
0,001

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Cs-137 0.7 - 2 km

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Date

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Cs-137 2 - 30 km

Figure 4: Evolution of average quarterly caesium-137 seawater activity for distances from the damaged nuclear power plantof 0 to 2 km, 30 to 100 km (dark blue) and 100 to 200 km (medium blue) (light blue).

Caesium-137 concentrations measured in 2015 at over 100 km from the nuclear power plant vary
between 0.002 and 0.007 Bq.L-1. The minimum values correspond to the concentrations measured
prior to the accident (0.001-0.002 Bq.L-1) that resulted from the atmospheric fallout from aerial
nuclear weapons tests. Since 2013, at over 30 km, the maximum values have fallen below thosemeasured in this area of the North Pacific during the 1960s (0.02 Bq.L-1 at the peak of this fallout(Povinec et al., 2013)).
The plume marked by the Fukushima releases was notably detected along the American coastline
4,000 km to the east at a rate of 0.001 to 0.005 Bq.L-1 according to Smith et al. (2014) and in thesubtropical zone of the North Pacific, up to 2,000 km to the south of Japan (Kaeriyama et al., 2014;
Kumamoto et al., 2015). These plumes were identified as having a depth of between 0 and 500
metres. Works published do not show any marking in the Arctic Ocean, via the Bering Strait.
Radionuclides other than radiocaesiums were measured, associated with the dissolved phase in thewater column, but in a far more sporadic manner. These consist mainly of strontium-90 and tritium
which, in comparison to radiocaesiums measurements, can reveal different origins. Caesium-137 and134 were released in similar quantities at the time of the accident, and the results obtained forthese two radionuclides are therefore comparable if their respective radioactive decay periods aretaken into account (caesium-137: 30 years; caesium-134: 2 years). The concomitant evolution of the
Sr-90/Cs-137 ratio close to and further away from the facility indicates a similar origin, coming fromthe facility area. For the year 2014, tritium marking is comparable to that measured in French rivers(1 - 10 Bq.L-1). These results confirm earlier data indicating that tritium releases following the
Fukushima accident were very moderate compared to those of caesium-137.

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3. Contamination of sediments
With the exception of the port zone to the right of the Fukushima Daiichi site, surface sedimentactivity shows little evolution between the immediate surroundings of the NPP and up to 80 km
away, ranging between around 1 and 1,000 to 5,000 Bq/kg of dry sediment. They also remain
constant since 2012 (figure 5) without any significant trend, either upward (which could result fromadditional inputs) or downward (likely to result from a dispersion of sediments through erosion thenlateral transport, or from the burial of the most contaminated particles through bioturbation). The
decline linked to the radioactive decay of caesium-137 is not perceptible over the five years given
the high variability in concentrations.
Further away, between 80 and 280 km, activity is lower and ranges between approximately 1 and100 to 500 Bq/kg dry weight.

Figure 5: Evolution over time of the caesium-137 concentration in sediments up to 280 km from the coast (samples taken at
sea and in the port zone of Fukushima Daiichi) until 27/10/2015; in the absence of information (particularly during the firstmonths following the accident), it is possible that some activity was reported on non-dried matter; data collected by the
IRSN Sea Committee (Cellule Mer)

In general, caesium-137 concentrations in sediments are higher to the north of the damaged NPP
than to the south. This can be explained by the fact that the contamination of the sediments mainly
took place during the first six months following the accident and that, during this period, all of thewaters contaminated by caesium-137 would have shifted northwards at the beginning of May 2011,
before some returned along the south coast at the end of May 2011.
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Although at a regional level (up to 280 km from the coast) the variation range of caesium-137
concentrations is homogeneous, significant local variations in activity were measured by Thornton
et al. (2013) within a 20 km radius of the NPP using a spectrometer towed over the sediments by avessel (figure 9). These measurements reveal preferential accumulation zones of contaminatedparticles related to bathymetry. Zones of activity exceeding average surrounding values by factor 5
to 10 are called "hot spots" (Black and Buesseler, 2014). The concentrations measured in thesediments there reach 5,000 Bq/kg wet weight. The ʺhot spotsʺ are most often located in areas witha depth of less than 150 m and their characteristic dimensions vary from a metre to around ahundred metres.

(a)

(b)

Figure 6: Examples of "hot spots", points of high concentrations of caesium-137 in sediments, corresponding toaccumulations in bathymetric "low points", observed within a radius of 20 km around the Fukushima power plant (adaptedfrom Thornton et al., 2013).

Radionuclides other than radiocaesiums are measured on a very occasional basis. Since theaccident, strontium-90 concentrations measured vary between 0.045 and 63 Bq/kg dry weight. The
average value is 2.3 Bq/kg dry weight.
In the north-western Pacific, the two main sources of plutonium identified to date have been thefallout from aerial nuclear weapons tests. The analysis of a 10 cm long sediment core, sampled in
January 2013 at a depth of 300 m and 100 km to the east of the nuclear power plant, suggests theabsence of a detectable influence of the Fukushima accident for this radioelement (Bu et al., 2014).
Plutonium-239+240 activity measured in surface sediments, of between 0.48 ± 0.01 and 3.53 ± 0.10
Bq/kg dry weight, are close to those measured to the north of Fukushima between April 2008 andJune 2011: between 0.37 and 4.1 Bq/kg dry weight. However, due to the high affinity of plutonium
for particles and the absence of specific measurements, a marking of sediments located at distances
closer to the NPP cannot be excluded.

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4. Effects on marine species
Between October 2012 and 30 November 2015, 1,200 samples were taken in the port of the
Fukushima nuclear power plant. Thanks to the structures put in place in order to restrict the entry
and exit of the port's fish, demersal species (those feeding on the sea bed) were those that mainly
remained present in the enclosure. Since the beginning of 2015, radiocaesium concentrations ofover 10,000 Bq/kg fresh weight have been measured in three demersal fish species: the jacopever,Japanese black rockfish (or Sebastes cheni) and the brass blotched rockfish. Figure 7 shows thecomparative evolutions of the activities of the latter two species sampled either on the coastline orin the port of Fukushima.
Since May 2012, in this coastal zone off the Fukushima prefecture, TEPCO has implemented
monitoring of contamination levels in marine organisms at several stations in the coastal zone close
to the nuclear power plant. Generally-speaking, stations at sea, over 10 km from the coast, show adecrease in radiocaesium concentrations with, at 28 November 2015, values close to 10 Bq/kg fresh
weight (figure 7).

Figure 7: Representation of radiocaesium concentrations in two demersal fish species: Japanese black rockfish (dark blue)and brass blotched rockfish (dark green) inside the port of the nuclear power plant and within a radius of 20 km, in acoastal zone of 20 km (in light blue and light green for each species respectively) (Source TEPCO).

Two coastal stations were sampled to the north of Fukushima: 1 km from the mouth of the Ota
river, and 3 km off the coast of Odaka. Despite a steady decline, radiocaesium concentrations on
occasions reached values of over 100 Bq/kg fresh weight in 2015 (marketable limit). This content
must certainly be linked to inputs from the rivers.
In 2015, radiocaesium concentrations exceeding the 100 Bq/kg fresh weight limit were still observed
from time to time. As an example, two of the demersal fish species most sampled by TEPCO, bothover time and space, are the Japanese black rockfish (Sebastes cheni) and the spotbelly rockfish (or8/12March 2016
brass blotched rockfish, Sebastes pachycephalus), with 204 and 601 samples respectively.
Radiocaesium concentrations in these species (figure 7) enable two trends to be highlighted. Since
the start of the sampling carried out by TEPCO (May 2012) in the coastal zone and within a 20 km
radius around the NPP, radiocaesium concentration is slowly decreasing in relation to the sediments
sampled in this same zone. Contrary to 2012, the variability between measurements at a same datehas decreased (figure 7).
The Fukushima accident was originally characterised by a 137Cs/134Cs ratio of 1. This ratio increases
over time due to the quicker decay of 134Cs than 137Cs; it is currently around 4. This ratio ismeasured in all samples taken by TEPCO inside or outside the port; it is in line with expectations.
Demersal fish are more marked than pelagic fish.

Figure 8: Representation of total caesium concentrations in demersal fish (grey), pelagic fish (blue) and marine
invertebrates (red) sampled from the ocean to the north-west of Japan.

The data in figure 8 shows that fish whose lifestyles are linked to sediment (grey) show higher levels
than pelagic fish (blue). Since September 2015, radiocaesium concentrations are below the 100
Bq/kg fresh weight marketable limit.
Figure 9 shows that in 2015, off the coast of prefectures other than Fukushima, radiocaesium
concentrations in fish arriving to these other prefectures are below the 100 Bq/kg fresh weightmarketable limit.

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Figure 9: Temporal evolution of the proportion of samples exceeding the radiocaesium marketable limit in fish arriving toprefectures other than Fukushima (http://www.jfa.maff.go.jp/e/inspection/index.html).

Figure 10 shows the evolution of radiocaesium concentrations in demersal fish (those feeding at thesea bed) in six prefectures in East Japan. As expected, the highest levels concern fishing off thecoast of Fukushima. More than five years after the accident, demersal species are still close to thepermitted limits (100 Bq/kg fresh weight) in the Fukushima prefecture.
In zones far away from the nuclear power plant, the evolution of the activity levels of marineorganisms is doubtless linked to the lasting contamination of sediments and inputs via rivers
following the leaching of land contaminated by atmospheric fallout.

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Figure 10: Temporal evolution of radiocaesium concentrations in different species of demersal fish between March 2011 andNovember 2015, sampled off the coast of the prefectures of Chiba, Ibaraki, Fukushima, Miyagi, Iwate and Aomori.

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