current knowledge concerning the impacts of the fukushima...
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1
Current Knowledge Concerning the Impacts of the Fukushima Daiichi
Nuclear Power Plant accident on the environment
*Abubakar Sadiq Aliyu a,b
, Junwen Wu c, Timothy Alexander Mousseau
d Ahmad Termizi Ramli
a
a Department of Physics, Universiti Teknologi Malaysia, 81310 Johor Baru, Malaysia
b Department of Physics, Nasarawa State University Keffi, P.M.B 1022, Keffi, Nigeria.
c State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China
d Department of Biological Sciences and the Environment and Sustainability Program, University of South Carolina,
Columbia, SC 29208
*Corresponding author E-mail: [email protected] or [email protected]. HP:
+601116167589 (A.A Sadiq).
Abstract
It is now more than four years since the Fukushima nuclear accident; how it affects
the environment will continue to be an issue of great concern to the public and energy
policy makers. Thus, scientists around the world have been investigating the impacts
of the accident and the question of how the Fukushima nuclear accident has affected
man and his environment will remain one of great interest for centuries. Here, we
review the current understanding of the Fukushima nuclear accident’s interactions
with the environment, which includes studies on the source term assessment,
radioactive contamination of the atmosphere, soil and the marine environment. This
review addresses some fundamental scientific questions concerning the environmental
impacts of Fukushima nuclear accident with the aim of stimulating further discussion
on the topic. Finally, the paper presents a concise discussion of how the released
radionuclides from the Fukushima nuclear accident could be used as tracers for
studying environmental processes.
Keywords: Fukushima nuclear accident, environmental effect, radioactive
contamination, 137
Cs
2
1. Introduction
The confluence of a magnitude 9.0 earthquake, a subsequent gigantic tsunami,
and design flaws related to the plant site and emergency cooling systems resulted in
the Fukushima Dai-ichi nuclear power plant accident (FDNPP) on March 11, 2011. As
a result, enormous quantities of radionuclides were released into the atmosphere and
the ocean 1-3
. Major radionuclides released to the atmosphere included 131
I (t1/2=8.02
d), 134
Cs (t1/2=2.06 y), 137
Cs (t1/2=30.07 y) and 90
Sr (t1/2=28.78 y), which were then
distributed globally 4 leading to the contamination of both terrestrial and marine
environments. Additionally, large amounts of radionuclides were directly released into
the ocean as liquid wastes via discharge from the FDNPP, which widely contaminated
seawater in areas offshore of Japan. The total amounts of 137
Cs and 90
Sr directly
released to the sea have been estimated to be 1~42 PBq (1PBq=1015
Bq) 5-7
and
0.1~2.2 PBq 8, respectively. Obviously, there has been considerable uncertainty
concerning the total amounts released from the FDNPP 1, 9-11
. It is clear that currently
there is a poor understanding about some fundamental questions of the FDNPP (for
example, the release amounts and rate of radionuclides); hence, we cannot derive
reliable conclusions concerning the impacts of FDNPP on the environment.
Answering some of these fundamental questions will lead to a better understanding of
how the uncontrolled release of radioactive fission products from the FDNPP has and
will continue to affect both human and non-human biota 12
.
This article will summarize the current state of understanding about the major
radionuclides derived from the FDNPP, including the amounts released and isotopic
composition, the effects on environment (seawater, sediment, terrestrial and marine
biota), and their application as tracers for environmental processes. This paper also
aims to present the results of global field measurements of radionuclides stemming
from Fukushima, which were obtained from the literature. This field measurement
data may be used in the investigation of environmental process as well as the basis for
conducting country-wise comparative impacts assessment of the FDNPP accident.
2. Source term assessment
3
Following the Fukushima nuclear accident, there was tremendous public
concern about the release of radionuclides into the environment (i.e. atmosphere
and ocean). Several research institutes and scientists conducted many studies in an
attempt to estimate the total release amounts as a measure of the source term. For
instance, the Japanese government estimated the source term for 131
I and 137
Cs to be
160 PBq and 15 PBq ,respectively as the total atmospheric releases of radioactivity
from the FDNPP 13
. The Nuclear Safety Commission of Japan (NSCJ) 2 estimated
that the total amounts of 131
I and 137
Cs discharged into the atmosphere were 150
PBq and 12 PBq, respectively. Masson, et al. 4 estimated that
131I (153 PBq) and
137Cs (13 PBq) were released to the atmosphere. Other researchers have reported the
total emission of 137
Cs as high as 13~35.8 PBq 1, 14
. To estimate the source terms
of radioactive cesium (137
Cs) and iodine (131
I), Chino, et al. 1 adopted an
estimation method to compute the source terms using an atmospheric dispersion
model. Their results were comparable to that of the Ministry of Education,
Culture, Sports, Science and Technology, Japan 15
. The authors verified the
validity of this technique in later studies 16, 17
. In a most recent study18
, the group
improved its computational methodology to account for limitations in their earlier
studies. The new study18
considered a new scheme of parameterizations for wet,
dry, and fog-water depositions of both gaseous and particulate radionuclides.
Environmental data that are coupled into the new model include those from other
part of Japan and the Pacific Ocean. The new study18
methodology estimated the
total release amount of total
131I and
137Cs to be 151.0 and 14.5 PBq, respectively
considering environmental data from both land and sea (New-landsea). For the new
land data, the total release amount for total 131
I and 137
Cs are ~ 27% and ~32% ,
respectively less than the results of the WINSPEEDI-II simulation that considered
New-landsea data. The reported total release amount of 131
I is in good agreement
with the estimate of the NSCJ 2 and lies within the range estimate for total release
amount of 131
I by this review ~ (150 – 160) PBq (See Table 1).
Additionally, the important source of ocean radioactive contamination was
4
derived from direct discharge into the cooling waters at the NPP. Accurate estimation
of radioactive pollutants directly discharged into the sea is the basis for assessment of
the impact to the marine habitat. There have been many studies concerning this
question. Using a regional ocean model, Tsumune, et al. 6 estimated that the total
137Cs directly released to the ocean was 3.5 ± 0.7 PBq from March 26 to the end of
May, 2011. The total amounts of 137
Cs directly released into the ocean have been
estimated to range from 1 to 42 PBq 5-7
. We have summarized the total release
amounts of major radionuclides derived from the FDNPP and compared them with
global fallout and the source term from the Chernobyl accident (Table 1)8, 19, 20
.
Despite the uncertainty associated with the total release of fission products from
Fukushima, Table 1 depicts that the Chernobyl accident resulted in the largest
discharge of 137
Cs into the environment (85 PBq), and Fukushima resulted in the
largest accidental source term release to the ocean. Ten months later, in the winter of
2012, the mean value for water column inventories of Fukushima-derived
radiocesium (137
Cs) and bomb-derived 134
Cs were nearly equal. These values were
1020 ± 80 and 820 ± 120 Bq m-2
, respectively 21
.This suggests that the accident and
nuclear bomb testing had almost the same impacts on the Pacific Ocean during this
period.
Table1 Source terms of anthropogenic radionuclides in the atmosphere and the ocean
(PBq).
Radionuclide Global fallout Chernobyl Fukushima
atmosphere Ocean Atmosphere Ocean Atmosphere Ocean 131
I 1760 160
153
150 137
Cs 950 600 85 16 15 3.5±0.7
13 4
12 27±15 90
Sr 600 380 1 0.14 0.1
0.01 2.2 239,240
Pu 10.87 6.6 0.087 (1-2.4)×10-6
5
3. Environmental effect
3.1 Atmosphere transportation
Vast amounts of radioactivity were emitted into the atmosphere by the accident
at the FDNPP, and the radioactive plume spread over the globe in the free troposphere
by the prevailing westerly winds 22-26
. Table 2 shows the results of measurements and
detection of Fukushima-derived radioactive fission products in receptor locations
(sampling points) around the globe. The readings are for air concentrations (in Bq m-
3), soil deposition (in Bq m
-3) and concentration in both fresh water and marine
environments (in Bq l-1
). The data in Table 2 may be used in the investigation of
environmental processes as well as basis for conducting country wise comparative
impacts assessment of the FDNPP accident.
6
Table 2 Fukushima-derived anthropogenic radioisotopes detected in locations around
the world
Location,
Country
Media (Soil Air or Water): Activity concentrations of
radionuclide, ( r e f )
Japan
Air (Bq m-3
): 132
Te: 9.9, 131
I: 16.2, 134
Cs: 10.13, 137
Cs: 10.27, 129m
Te: 4.0, 136
Cs: 123 27
Soil(Bq m
-2):
137Cs: 9 0 0
28, 29
South Korea
Air (Bq m-3
): 131
I: 3.12 x 10-3
, 134
Cs: 1.19 x10-3
, 137
Cs: .25 x 10-3
30
Soil(Bq m
-2):
137Cs:649,
134Cs: 802
31
United States Air (Bq m-3
): 134
Cs: 1.11x 10-3
, 137
Cs: 1.18 x 10-3
, 131
I: 3.85 x 10-3
32
Krasnoyarsk (Russia) Water (Bql-1
): 131
I: 0.62, 137
Cs: 0.075, 134
Cs: 0.095 23
Thessaloniki (Greece)
Air (Bq m-3
): 131
I: 497 x 10-6
, 137
Cs: 145 x 10-6
, 134
Cs: 126 x 10-6
24
Water (Bql
-1):
131I: 0.7
23, 24
Ibaraki (Japan) Soil (Bqm-2
): 137
Cs: 1.4 x 104 33
China
Air (Bq m-3
): 131
I: 1720 x 10-3
, 134
Cs: 247 x 10-3
, 137
Cs: 289 x 10-3
, 136
Cs:
23 x 10-3
34
Soil(Bq kg-1
): 131
I: 2.68 34
Milan (Italy) Air (Bq m
-3):
131I: ~4.2 x 10
-4,
137Cs: ~9.4 x 10
-5,
134Cs: 8 x 10
-5
35
Czech Republic Air (Bq m-3
): 131
I: 5.6 x 10-3
, 137
Cs: 0.72 x 10-3
, 134
Cs: 0.64 x10-3
36
Lithuania Air (Bq m-3
): 131
I: 3700 x 10-6
, 137
Cs : 1040 x 10-6
37, 38
France Air (Bq m-3
): 131
I: 2.4 x 10-3
, 134
Cs: 0.2 x 10-3
, 137
Cs: 0.7 x 10-3
39, 40
Bratislava (Slovakia) Air (Bq m-3
): 131
I: 0.5 x 10-3
, 137
Cs: 0.07 x 10-3
41
7
Spain Air (Bq m
-3):
131I: 3080 x 10
-6,
137Cs: 690 x 10
-6,
134Cs: 620 x 10
-6,
132Te:
330 x 10-3
42
Serbia Air (Bq m-3
): 131
I: 2.8 x 10-3
, 137
Cs: 0.21 x 10-3
43
Vietnam Air (Bq m-3
): 131
I: 193 x 10-6
, 134
Cs: 33 x 10-6
, 137
Cs: 37 x 10-6
44
UK Grass (Bq kg-1
): dry matter for 131
I: 55, dry matter for 137
Cs: 8 45
Romania Water (Bq l-1
): 131
I: 1.61 46
Svalbard (Norway) Air (Bq m-3
): 131
I: 810 x 10-6
, 134
Cs: 659 x 10-6
, 137
Cs: 675 x 10-6
47
Lisbon (Portugal)
Air (Bq m-3
): 131
I: 1.47 x 10-3
, 137
Cs: 134 x 10-3
48
Soil (Bq m23
): 131
I: 1.03, 134
Cs: 0.65, 137
Cs: 0.74 48
Vancouver Island
(Canada)
Air (Bq m-3
): 133
Xe: 70 49
Finland Air (Bq m
-3):
131I: 11 x 10
-3,
134Cs: 0.39 x 10
-3,
137Cs: 0.47 x 10
-3,
136Cs: 30
x 10-3
, 129
Te: 148 x 10-3
, 129m
Te: 254 x 10-3
, 132
Te: 54 x10-3 50
Iberian Peninsula
Air (Bq m-3
): 131
I: 3.88 x10-3
, 132
I: 0.175 x 10-3
, 134
Cs: 0.193 x10-3
, 137
Cs:
1.0 x 10-3
51
Istanbul (Turkey) Air (Bq m-3
): 131
I: 1.11 x 10-3
, 134
Cs: 0.28 x 10-3
, 137
Cs: 0.26 x 10-3
52
Taiwan
Air (Bq m-3
): 131
I: 0.1 x 10-3
, 134
Cs: 1.13 x 10-3
, 137
Cs: 1.45 x 10-3
26
N:B Only maximum detected values were considered for all sampling
points.
Austria
Air (Bq m-3
): 131
I: 1.2 x 10-3
53
Rain water (Bq l-1
) : 131
I: 5.2 53
Hungary Air (Bq m-3
): 7Be: 10
-2,
131I: 3 x10
-3,
137Cs: 10
-4, 134
Cs: 10-4
54
Monaco Air (Bq m-3
): 131
I: 354 x 10-6
, 134
Cs: 30 x 10-6
, 137
Cs: 37 x 10-6
55
8
Germany
Air (Bq m-3
): 131
I: 2.1 x10-3
25
Soil (Bq kg-1
): 131
I: 0.093, 137
Cs: 3.07, 134
Cs: 0.69 25
Darwin (Australia)
Air (Bq m-3
): 133
Xe: 12 x 10-3
No Fukushima-derived radio iodine and
cesium was detected in Australia 56
Krakow (Poland) Air (Bq m-3
): 131
I: 6.08 x 10-3
, 134
Cs: 0.50 x 10-3
, 137
Cs: 0.474 x 10-3
3 57
It has been estimated that it took radionuclides from FDNNP ~18 days to reach
locations around the globe 58
. This is much longer than the time (~10 days) it took for
the radiation clouds emitted from the Chernobyl accident to do the same 59
, and it is
longer than the time (~13 days) for Asian dust to go around the world 60
. It is known
that the largest portion of the radioactive plume from Fukushima was transported
through the atmosphere and deposited over the Pacific Ocean. This might have
resulted in the longer time for travel around the world as the space and temporal
variation of the boundary layer over the oceans is relatively slow. This is because the
surface temperature of the sea varies little during the diurnal cycle owing to large
mixing within the top layer of the ocean 61-63
.
Airborne radioactivity from FDNPP accident was detected at many laboratories
around the world and possible health effects from the radioactivity transported by the
radioactive plumes from Japan was of concern. In Japan, as of October 21 2011 (7
months after the accident), the air concentrations of 134
Cs and 137
Cs in Fukushima city
were still elevated to 613 µBq m-3
and 829 µBq m-3
, respectively 64
. Other prefectural
governments also conducted environmental radioactivity monitoring, which included
45 stations covering the Japanese Islands 65
. Outside Japan, as shown in Table 2,
radioactive materials were detected in Korea, Vietnam, Taiwan, U.S., Canada, and
Europe 4, 22, 26, 30, 66
. In Europe, the first signs of the radioactive cloud were detected on
March 19th, 2011 (slightly more than one week after the accident) while the first peak
9
of activity level was observed between March 28th and 30th. Measurements have
shown that airborne activity levels are now of minimal concern for public health in
Europe 4. The activity concentrations of the radionuclides in the plume exponentially
decreased with downwind distance 58
due to deposition (dry and wet) and scavenging
67.
3.2 Soil deposition
Deposition of Fukushima-derived radionuclides on land mostly stemmed from
the release to the atmosphere of relatively volatile fission products, most importantly
137Cs,
134Cs and
131I and radioactive noble gases
68, which present a large risk for
internal radiation exposure via inhalation and ingestion of contaminated agricultural
crops, stock farming, and thus human life . The soils around the FDNPP and
neighboring prefectures have been extensively contaminated with 137
Cs and 134
Cs
depositions of more than 105 and 10
4 Bq m
-2, respectively
28, 68. Similarly,
Kinoshita, et al. 29
has also reported that the top soil deposition of 137
Cs in the
central-east Japan due to the Fukushima accident ranged from 0.4 to 900 kBq m-2
,
which are 0.06-130 times the cumulative deposition from the atmospheric nuclear test
(100-1000 Bq m-2
). It is estimated that a total of more than 5.6 PBq 137
Cs was
deposited over the Japanese Islands 28
. High activity concentrations of radionuclides
have been reported. For example, soil activities for 134
Cs and 137
Cs in the Iitate Village
area (located 25-45 km northwest of the Fukushima site) were 62400 and 72900 Bq
kg-1
, respectively 69
. As of March 2011, the top soil concentration of 137
Cs was
measured to be 80 kBq m-2
. The environmental external radiation dose rate before the
accident was approximately 0.05 µSv h-1
29
. Additionally, the radiation dose rate of
other prefectural areas due to the Fukushima accident have also been estimated, for
example, 1.14 µSv h-1
in Naka-Dori, 4.57 µSv h-1
in Iitate, 0.02 µSv h-1
in the region
between northern Ibaraki and eastern Saitama, and 0.23 µSv h-1
in southern Ibaraki
and northern Chiba prefectures 29
. This clearly demonstrates that large areas in eastern
and northern Japan were strongly contaminated by 137
Cs derived from Fukushima,
while in contrast, the western part of Japan was shielded by its mountainous terrain.
10
However, as with Chernobyl, deposition was highly heterogeneous even within the
most contaminated areas e.g. 70, 71
with ground level readings ranging from 0.5 to >30 µSv h-1
within Namie township. The information on soil contamination which was detected in
countries in Asia, Europe and in North America is presented in Table 2. The advantage
of the information in Table 2 is that it is based on empirical environmental data and
not computer models. The advantages of empirical data stem from the fact that
dependence solely on environmental models may result in over or under estimation of
the impacts of the accident as it has been observed in the study of Hoeve and
Jacobson 11
. The authors 11
, estimated that there would be an additional 130 (15-1100)
and 180 (24-1800) cancer related mortalities and morbidities related to the Fukushima
radiological disaster, respectively with over 90% of the projected impacts occurring in
Japan. However, Beyea, et al. 72
argued that the failure by Hoeve and Jacobson 11
to
consider the long-term dose estimate from radiocesium in their studies may lead to
under estimation of the global effects of the accident. By including a more realistic
population perspective, Beyea, et al. 72
suggested that the mid-range estimate for the
number of future cancer mortalities due to Fukushima accident is probably closer to
1000 than 125. Recently, Evangeliou, et al. 73
developed a pioneering environmental
modeling study that considered numerous fission products. This study estimated that
cancer fatalities are likely to be fewer than 2000.
3.3 Marine environment
Other than fallout from atomic bomb testing, contamination of the marine
environment following the FDNPP accident represents the most important
anthropogenic radioactive release into the sea ever known. The radioactive marine
pollution came from atmospheric fallout onto the ocean, direct release of
contaminated water from the plant, and the transport of radioactive pollution from
leaching through contaminated soil. For instance, the total amount of 137
Cs directly
released to the ocean has been estimated as 1~42 PBq 5-7
. In the immediate vicinity of
the plant, the Cs levels of surface ocean at the discharge point are up to 103 times
higher activities than it has been (~1-2 Bq m-3
) 74
, with a peak of 68 kBq L-1
on
11
April 6, 2011 75
. Certainly, dilution of the primary contaminant signal occurs quite
rapidly due to ocean mixing, particularly in the energetic coastal waters off Japan
where the Oyashio waters move south and interact with the rapidly flowing and
offshore meandering of the Kuroshio Current. These currents, tidal forces and eddies
mix the waters quite rapidly offshore. As a consequence, one month after the accident
the 137
Cs level decreased by a factor of 103.
Honda, et al. 76
reported that the 137
Cs activity in surface seawater ranged from
0.004 to 0.284 Bq kg-1
in the western North Pacific on 14 April to 5 May. Tsumune, et
al. 6 argued that if no additional releases occur, the effect of the
137Cs might be
smaller than that of global fallout in the North Pacific. The timescale of transport to
other basins is several decades. Thus, the effect of the 137
Cs release on other oceanic
basins might be negligible. With respect to dose effects on humans, Buesseler, et al. 75
calculated the dose due to direct exposure during human indirect contact with the
water to be 0.1 µSv d-1
when the 137
Cs level approaches 102 kBq m
-3, which is lower
than the average dose from all sources to the Japanese population of about 0.17µSv d-
1. Thus there is unlikely to be any significant direct external dose effect on humans via
this pathway.
Cesium-137 concentration in suspended solids (SS) from surface and subsurface
waters off western North Pacific has been measured by Honda, et al. 76
; it ranged
from 5.92 to 32.42 Bq kg-1
on a dry weight basis (kg-dw-1
). The same is true for 134
Cs.
Thus, it is likely that SS were contaminated with radionuclides from the FDNPP 76
.
The SS-seawater distribution coefficient (Kds: Bq kg-dw-1
for SS/Bq kg-1
for
seawater) was further estimated to be 200~4400, with an average of 2100. This
average Kds is comparable to the recommended Kds for open ocean (2000) reported 19
.
In terms of potential biological impacts, radiation doses in marine organisms are
generally dominated by the naturally occurring radionuclides 210
Po ( an alpha emitter)
and 40
K, even when organisms are exposed to anthropogenic radioactivity discharged
to coastal waters 77
. Fukushima-derived radiocesium was detected in zooplankton and
mescopelagic fish 78
, with the 137
Cs concentration in zooplankton off western North
12
Pacific at 13.46~71.46 Bq kg-dw-1
, which is significantly higher than that of the
southern Baltic Sea and the northern Adriatic Sea one month after the Chernobyl
accident (0.4~5 Bq kg-ww-1
) 79, 80
. In contrast, the 137
Cs concentration in zooplankton
around Japan was less than 0.1 Bq kg-ww-1
during the last decade 81, 82
. Thus, the
observed concentrations were two orders of magnitude higher than before the
accident. Meanwhile, the concentration factor (CF, i.e. the ratio of the Cs
concentration of zooplankton to that of ambient seawater (Bq kg-ww-1/Bq kg-1
) was
calculated to be 200~840, which is an order of magnitude higher than the 10~100 of
previous observations 19, 81, 82
.
Buesseler, et al. 78
suggested that radiation risks of 137
Cs derived from the
FDNPP to marine organisms and human consumers of seafood are well below those
from natural radionuclides. Meanwhile, the total cesium levels in demersal (bottom-
dwelling) fish off Fukushima were investigated 83
, where the cesium levels in 40% of
fish were above the new regulatory limit (100 Bq kg-1
wet) set by Japan Ministry of
Agriculture 84
. However, this is lower than the new regulatory limit of cesium in fish
products in four prefectures (Iwate, Miyagi, Ibaraki and Chiba).
The effect of the FDNNP accident on marine products in Fukushima was also
assessed by Wada, et al. 85
. The authors concluded that the total activity
concentrations of radiocesium in marine products have decreased significantly.
However, the authors noted that higher activity concentrations have been observed in
shallower waters south of the FDNPP. Long-term monitoring of marine products
around the FDNPP is important before restarting the coastal fisheries 85
. For other
prefectures that are adjacent to Fukushima, Chen 86
reported the radioactivity level in
the fish products from these prefectures based on monitoring data of July 2013. The
activity concentrations of 134
Cs, and 137
Cs varied from 0.1 to 338 Bq kg-1
(average: 11
Bq kg-1
) and from 0.1 to 699 Bq kg-1
(average: 18 Bq kg-1
), respectively 86
. Three
years later, the dose rate to the most impacted fish species near the FDNPP have
remained above baseline levels and a theoretical human consumer of 50 kg of fish,
gathered a short distance (3 km) from the FDNPP in 2013, would have received a
13
total committed effective dose of approximately 0.95 mSv y-1
with FDNPP
contributing ~14% of total committed effective dose 87
.
Numerous studies have assessed the impacts of the Fukushima accidents on the
marine and environment in both Japan and other countries 30, 88-93
. For instance, Aliyu
and co-authors concluded that within Japanese Coastal areas, the accident had heavier
damage to marine bionts compared to terrestrial flora and fauna. Even though many of
the studies outside Japan have demonstrated that the levels radiocesium in seafood
were not likely of discernible health effects 88
, further investigations of the marine
environment have continued in order to address public concern about the impact of
the accident.
Buck 90
reported that the radiation in the ocean will be diluted along the Japanese
coast, however exposed marine organisms and contaminated tsunami debris will
likely reach the U.S. Time series measurement of 134
Cs and 137
Cs in seawater showed
that the first arrival of the Fukushima derived radio-cesium in North America in a
location was in June 2012, which is 1.3 y after the accident 94
. This was in a location
that is 1ocation that is 1500 km west of British Columbia, Canada.
An assessment of the impact of the accident on marine environments in China
Sea showed that the activity concentration of 137
Cs ranged from 0.75 ± 0.07 to 1.43 ±
0.08 Bq m-3
, which is below the regulatory standard in China 92
.
Strontium-90 is of significant health and environmental concerns given that it is
a calcium analog and thus may bioaccumulate in the bones and teeth of many
organisms. The activity concentrations of 90
Sr have been measured in selected hotspot
in Japan 95
. The author reported that the 90Sr activity concentration in soil and
vegetation samples from the hot spots is relatively lower than the Japanese regulatory
limit. The activity concentrations of 90
Sr were less than 10% of 137
Cs.
4. Effects of FDNPP accident on some terrestrial species
Hiyama, et al. 96
used the Pale Blue Grass butterfly, Zizeeria maha, as indicator
species to evaluate the environmental impact of ionizing radiation. Observed
morphological abnormalities were detected on some individuals of adult butterflies
14
collected in May 2011 increased in number and with time. Genetic studies indicated
that some abnormalities were inherited in the F2 generation. These abnormalities may
be explained by random mutation in important genes or by epigenetic mechanisms.
By comparing their data with that obtained from control populations, the authors
concluded that Fukushima-derived anthropogenic radionuclides likely caused the
morphological and genetic damage to the butterfly species. The study by Hiyama, et
al. 96
was significantly strengthened by laboratory experiments that used both internal
and external radiation sources, and these findings validated observations of the
elevated mutation rates and phenotypic effects observed in the field 97
. However, a
recent article by Copplestone and Beresford 98
has highlighted some major drawbacks
in the work of Hiyama and colleagues. Copplestone and Beresford 98
argued that
based on data reported by Hiyama, et al. 96
, the lethal dose required to kill 50% of
exposed individuals (LD50) is equivalent to the radiation levels that organisms receive
from exposure to background natural radiation sources. This disconnect between
empirical findings and predictions based on theoretical models highlights the many
uncertainties associated with accurate estimation of dose in wild populations 99
and
the likelihood that wild animals are significantly more sensitive to the effects of
radiation than predicted by conventional approaches 100
. In a later study 101
, the
authors clarified some of the questions raised concerning their earlier study and
presented some new data on the study.
Low blood cell counts were reported in wild monkeys from the forests of
Fukushima City compared with the counts in monkeys from Aomori prefecture which
is 400 km away 102
. The total radiocesium concentration in the muscle of Fukushima
15
wild monkeys was measured to range from 78 to 1778 Bq kg-1
, while the cesium
concentration in the control population was reported to be below detection limits. A
negative correlation was observed between activity concentrations of Cs and low
blood counts in juvenile Fukushima monkeys. Ochiai, et al. 102
concluded that the
observed hematological aberration was due to exposure to Fukushima-derived
anthropogenic radionuclides. This study is based on ecological design that is rather
limited. Firstly, the population size of the monkeys (61 from Fukushima and 31 from
Shimokita) considered in the study is rather small and does not include individuals
from the more contaminated areas of the region. A similar result which showed
hematological variations in the Fukushima monkeys was presented by the same group
of authors in an earlier study 103
. The level of radiocesium in the Fukushima wild
monkeys was similar to that found in sheep in some parts of the UK following the
Chernobyl accident, i.e. extremely low in terms of damage to the animals themselves
104. The greatest strength of this study comes from comparisons to children living in
contaminated areas of northern Ukraine near Chernobyl where similar hematological
effects were also observed 105
.
A recent study of bull sperm and testis from the Fukushima region found no
evidence for significant histological changes in the testes or sperm morphology 106
although this study was very preliminary with only two bulls from a relatively
uncontaminated part of Fukushima represented for the analysis of sperm.
Conservation biologists are concerned with the effects of environmental
perturbances on population abundances and biodiversity as these are indicators of
overall ecosystem “health”. Initial studies of the Fukushima region in July 2011
surveyed abundance and diversity at 300 locations that varied from 0.5 usv/h to more
than 30 usv/h. It was found that several bird species as well butterflies and cicadas
showed significant reductions in numbers in the more radioactive areas even after
correcting for other environmental factors known to influence population sizes 107, 108
.
Following an additional three years of biotic inventories at 400 locations, it was found
that bird abundance and diversity had continued to fall at contaminated locations 70,
16
71 . A more detailed analysis of barn swallow (Hirundo rustica) populations found
strong evidence of population declines although preliminary molecular analysis found
no evidence for significant increases in genetic damage 109
. Overall, these
investigations of population effects bear a striking resemblance to those found for
Chernobyl-affected regions of Ukraine and Belarus, providing some support for the
hypothesis that radioactive contaminants are the likely cause for the observed declines
e.g. 110, 111-116
.
The radioloecological consequence of the Fukushima accident has been assessed
using ERICA assessment tool for different biota in two known studies 69, 89
. The
exposure profile to 131
I, 134
Cs, and 137
Cs, one month after Fukushima was used by
Garnier-Laplace, et al. 69
for the dose rates (mGy d-1
) calculations. The dose rate to
selected reference species were 1.5 (mGy d-1
) for birds, 2.3 (mGy d-1
) for soil
invertebrates, and 3.9 (mGy d-1
) for forest. Such dose rates could be biologically
significant especially for longer-lived or radiosensitive organisms.
5. Application of isotopes as tracers for environmental processes
In the oceans, the behavior of cesium is thought to be conservative, i.e. it is
soluble (less than 1% adsorbed to marine particulates) and is carried primarily with
ocean waters and as such has been used as a tracer of water mass mixing and transport
117-119. Following the Fukushima nuclear accident, cesium activity and the
134Cs/
137Cs
activity ratio have been measured to assess its environment effect and to study
oceanographic processes. Studies have shown that the 134
Cs/137
Cs activity ratio in the
Fukushima derived radionuclides was nearly 1.0 65, 75
, which is based on the samples
of atmosphere, soil, seawater and marine biota off Fukushima. It is higher than that in
the Chernobyl fallout (134
Cs/137
Cs activity ratios:0.5) 120, 121
. This makes the tracking
of Fukushima derived radionuclides in the ocean quite straightforward, since given its
relatively short half-life (2 yr), the only source of 134
Cs in the North Pacific at this
time would be the Dai-ichi NPPs. Hence in addition to the elevated cesium activities,
the presence of 134
Cs is a unique isotopic signature for tracking these waters and
17
providing a means to study the rates of vertical and horizontal mixing processes in the
Pacific Ocean.
Compared with cesium, Pu has high particle affinity, which thus provides a tool
for studying a variety of processes in the marine environment such as particle fluxes
and scavenging, the validation of various biogeochemical ocean models. However, the
most useful aspect of Pu as an environmental tracer is its well-defined sources terms
122. This is because the composition of Pu isotopes from different sources in marine
environment varies significantly due to the characteristic isotopic composition
corresponding to the means of production, for example, reactor-grade Pu typically
contains more than 35% 239
Pu with a 240
Pu/239
Pu atom ratio of 0.2-1.0 after fuel burn-
up 123
. The 240
Pu/239
Pu atom ratio in weapon-grade Pu is much lower (0.02-0.06)
because burn-up is kept low to minimize the production of higher Pu isotopes 124
. The
current study shows the Fukushima derived 240
Pu/239
Pu ratio is 0.216~0.255, which
is higher than that of global fallout (0.18), but lower than that of Chernobyl. A
summary of the composition of Cs and Pu isotopes for Fukushima, Chernobyl and the
global fallout is presented in Table 3.
Table 3 Anthropogenic nuclide isotope ratio characteristic
Source Fukushima Chernobyl Global fallout
134Cs/
137Cs(activity ratio) ~1 0.5
137Cs
/239,240
Pu (activity ratio) 1.95×105~2.53×10
7 38.5
238Pu/
239,240Pu(activity ratio) 1.2 0.5 0.030
241Pu/
239,240Pu(activity ratio) 107.8 13～16
240Pu/
239Pu(atom ratio) 0.216~0.255 0.408 0.180
241Pu/
239Pu(atom ratio) 0.132 0.00194
In soil, Fukushima-derived 137
Cs has been deposited in Japan and other parts
of the globe. Atmospheric and soil measurements of Fukushima-derived 137
Cs
activity concentrations, revealed that the detection of 137
Cs in the ambient aerosol
ended 2–3 months after the initial emissions 4, 47, 125, 126
. This indicates that topsoil
activity concentrations of 137
Cs that are obtained after this period are reliable for
18
monitoring soil processes as most of the Cs in the air has been deposited on the soil
and surface of water bodies. Cesium is absorbed on soil particles, thus restraining its
movement by either biological or chemical means. The movement of 137
Cs in the
environment is by physical processes hence it can be used as a tracer for studying the
processes of sedimentation and erosion 127
. By determining the spatial distribution
patterns of 137
Cs in vertical and horizontal planes across the landscape, the rates of
soil loss or deposition can be measured for different parts of a watershed. As for
sedimentation rate, it can be measured by matching the vertical distribution of 137
Cs in
sediments with the temporal deposition of fallout 137
Cs from the atmosphere to locate
sediment horizons 128, 129
. Highlights of some of the state-of- the-art methods for
using anthropogenic radionuclides as tracers in soil process are presented in Zapata
130. The soil retention mechanism of Cs derived from FDNPP was investigated by
Mishra, et al. 131
. The authors reported that Cs is sorbed on the surface of clay
particles; and analysis of the vertical profile of deposited Cs showed that the
radionuclide largely retained in the top 5cm of the soil.
Conclusions
In this paper we have reviewed the current state of knowledge concerning the
environmental impacts of the Fukushima nuclear accident. First, we assessed the
source term for the Fukushima nuclear accident and concluded that the FDNPP
released a total amount of 10~15 PBq of 137
Cs to the atmosphere and 1~42 PBq to the
ocean. Next, we analyzed the radioactive contamination of atmosphere, soil and
marine environments (seawater, suspended solid and biota), whereby global
contamination was mediated through atmospheric dispersion. High deposition rates of
fission products have been observed in Japan. The radionuclides directly released into
the ocean caused activity levels and total inventory to significantly increase. The
concentrations of radionuclides in SS and marine biota have also been discussed. We
also summarized recent studies concerning the hematological, morphological and
genetic impacts of the accident on Fukushima wild monkeys, cows, birds, and
19
butterflies. Finally, we discussed the application of anthropogenic radionuclides (for
example, 134
Cs/137
Cs activity ratio and 240
Pu/239
Pu atom ratio) as a tracer to study
marine and soil processes and/or identify sources.
Could the atmospheric and oceanic dispersion of anthropogenic radionuclides
provide a basis for climate change monitoring? This question should be considered by
climate change scientists as data from the CTBTO global monitoring facilities may
provide clues concerning mechanisms of global climate change.
Acknowledgements
A.S. Aliyu wishes to acknowledge the support of the Research Management
Center of Universiti Teknologi Malaysia for its support through the Post Doc
fellowship scheme under the project number (Q.J130000.21A2.01E98).
20
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