Global risk of radioactive fallout after major nuclear reactor accidents J. Lelieveld et al

Atmos. Chem. Phys., 12, 4245–4258, 2012
© Author(s) 2012. CC Attribution 3.0 License.

Global risk of radioactive fallout after
major nuclear reactor accidents
J. Lelieveld1,2, D. Kunkel1, and M. G. Lawrence1,*
1Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany
2The Cyprus Institute, Nicosia, Cyprus
*now at: The Institute for Advanced Sustainability Studies, Potsdam, Germany
Correspondence to: J. Lelieveld (
Received: 7 October 2011 – Published in Atmos. Chem. Phys. Discuss.: 25 November 2011
Revised: 24 April 2012 – Accepted: 27 April 2012 – Published: 12 May 2012

Abstract. Major reactor accidents of nuclear power plants
are rare, yet the consequences are catastrophic. But what is
meant by “rare”? And what can be learned from the Chernobyl
and Fukushima incidents? Here we assess the cumulative,
global risk of exposure to radioactivity due to atmospheric
dispersion of gases and particles following severe
nuclear accidents (the most severe ones on the International
Nuclear Event Scale, INES 7), using particulate 137Cs and
gaseous 131I as proxies for the fallout. Our results indicate
that previously the occurrence of INES 7 major accidents
and the risks of radioactive contamination have been underestimated.
Using a global model of the atmosphere we compute
that on average, in the event of a major reactor accident
of any nuclear power plant worldwide, more than 90% of
emitted 137Cs would be transported beyond 50 km and about
50% beyond 1000 km distance before being deposited. This
corroborates that such accidents have large-scale and transboundary
impacts. Although the emission strengths and atmospheric
removal processes of 137Cs and 131I are quite different,
the radioactive contamination patterns over land and
the human exposure due to deposition are computed to be
similar. High human exposure risks occur around reactors in
densely populated regions, notably inWest Europe and South
Asia, where a major reactor accident can subject around 30
million people to radioactive contamination. The recent decision
by Germany to phase out its nuclear reactors will reduce
the national risk, though a large risk will still remain from
the reactors in neighbouring countries.

1 Introduction
Nuclear accidents associated with the melting of the reactor
core are caused by the failure of the cooling systems, and
can have major environmental and societal consequences. In
total about 20 core melt events have occurred in military and
commercial reactors worldwide since the early 1950s (Burns
et al., 2012). An accident risk assessment of nuclear power
plants (NPPs) by the US Nuclear Regulatory Commission in
1975 estimated the probability of a core melt at 1 in 20 000
per year for a single reactor unit (NRC, 1975). A follow-up
report in 1990 adjusted this number and indicated that the
core damage frequency is not a value that can be calculated
with certainty, though an appendix presented the following
likelihood of a catastrophic accident (NRC, 1990):
a. Probability of core melt 1 in 10 000 per year;
b. Probability of containment failure 1 in 100;
c. Probability of unfavourable wind direction 1 in 10;
d. Probability of meteorological inversion 1 in 10;
e. Probability of evacuation failure 1 in 10.
The product of these possibilities is 1 in 1 billion per year
for a single reactor (this assumes that factors (a)–(e) are independent,
which is not the case, so that the actual risk of a
catastrophic accident should be higher than this). Given this,
with a total of about 440 active civilian reactors worldwide
(IAEA, 2011; Supplement), and an estimated mean remaining
lifetime of 20–25 yr (together _10 000 reactor years),
then the probability of such a major accident occurring in
this period would be roughly 1 in 100 000. In light of the
uncertainties, the simplicity of this calculation is appealing.
However, based on the evidence over the past decades one
may conclude that the combined probabilities (a) and (b)
have been underestimated. Furthermore, by using a state-ofthe-
art global atmospheric model we can directly compute
the anticipated dispersion of radionuclides, avoiding the need
to guess the factors (c) and (d). In doing so, we find that the
vast majority of the radioactivity is transported outside an
area of 50 km radius, which can undermine evacuation measures,
especially if concentrated deposition occurs at much
greater distances from the accident, as was the case for Chernobyl
in May 1986. Furthermore, even if an evacuation is
successful in terms of saving human lives, large areas around
the reactors are made uninhabitable for decades afterwards.
Therefore, we argue that such events are catastrophic irrespective
of evacuation failure or success, and exclude the factor
To gain an overview of the geographical and populationweighted
risk of contamination, we present calculations of
the amount of radioactive fallout expected to occur within
various distances around each reactor (mapping probabilistic
fields of nuclear reactor risk sites; see Baklanov and Mahura,
2004). We address the most severe NPP disasters, defined as
level 7 major accidents on the International Nuclear Event
Scale (INES). Consequently, this excludes partial nuclear
meltdowns that can possibly be contained, including for example
the Three Mile Island accident in 1979, which was
categorized as an INES level 5 accident. Since we cannot
predict the radioactivity that would actually be released by
future events, which depends on many factors such as reactor
type, capacity and fuel (and burn-up of the fuel), we focus
on the relative amount that would occur for any major accident.
To at least provide an approximation of the absolute
amount of radioactivity released, we use analyses of the welldocumented
Chernobyl reactor accident as a proxy (Smith
and Beresford, 2005; IAEA, 2006), and scale the emissions
of other reactors worldwide according to their gross capacity
to account for the different amounts of nuclear fuel involved.
In the discussion we also address preliminary estimates of the
Fukushima emissions of radioactivity and their implications
for our risk assessment.
2 The Chernobyl accident
Radioactivity from the Chernobyl nuclear reactor incident,
which occurred on 26 April 1986, impacted the entire Northern
Hemisphere. In fact, the Chernobyl accident was not triggered
by a typical core melt but rather by a strong power
excursion, leading to an explosion that destroyed the reactor
core. Anspaugh et al. (1988) estimated that the collective
dose to humans was about 930 000 Gray (the dose of ionizing
radiation, expressed by 1 Gy, is equivalent to 1 Sievert
(Sv) for gamma and beta radiation, which represents the
absorption of 1 Joule by one kg of matter). Nuclear accidents
release both gaseous and particulate radioisotopes. The
total radionuclide release of Chernobyl was influenced by
the ignition of the graphite-moderated reactor, and amounted
to >12 000 PBq (IAEA, 2006) (P is peta=1015) (Table 1).
The on-going fires released a large amount of fuel particles
(“hot” particles) carrying isotopes of cerium, zirconium,
molybdenum, neptunium and plutonium (Smith and Beresford,
2005; IAEA, 2006). These particles were relatively
large and quickly sedimented from the atmosphere, primarily
contaminating an area of about 30 km around the reactor.
Since the Chernobyl type reactor technology is now considered
obsolete, we rather focus on the radionuclides that
were emitted as gases and attached to ambient aerosol particles,
e.g., the semi-volatile isotopes of iodine, strontium,
caesium, tellurium, ruthenium and barium (131,133I, 89,90Sr,
134,137Cs, 132Te, 103,106Ru and 140Ba). These radionuclides
were mostly found on small particles with a radius of r _ 1 μm, which deposit slowly by gravitational settling and are
more effectively removed by rainfall, usually further downwind
than the large particles. For a review of measured
size distributions of radioactive aerosols we refer to Dorrian
Although only a small fraction of the radionuclides from
the Chernobyl accident was released as 137Cs, i.e., 85 PBq
(about 27 kg), 137Cs (half-life of 30 yr) is used to map the
deposition because it is straightforward to measure and is
radiologically important on a long time scale (Smith and
Beresford, 2005; IAEA, 2006). 131I (half-life of 8 days), of
which Chernobyl emitted about 1760 PBq, is also important;
especially in the first weeks after an accident, as it is released
in relatively large quantities, leading to high doses,
because it rapidly enters the food chain and is concentrated
in the thyroid (IAEA, 2006; WHO, 2006; Christodouleas et
al., 2011). To put the emissions by Chernobyl into perspective,
we list in Table 1 the known level 4 to 7 accidents and
their estimated radioactivity release to the atmosphere, partly
based on non-official information gathered from the Internet
( der Kernkraftwerke, of nuclear reactors, http:// and references therein).
Since there is little reliable information besides Chernobyl
about the release and even less about the deposition of radionuclides
from a catastrophic accident, we apply the Chernobyl
data in our model to give a first approximation, and
simulate that a major accident of each reactor similarly releases
a fraction of 85 PBq of 137Cs and 1760 PBq of 131I,
depending on the amount of reactor fuel, over a period of
one year. For each reactor we scaled the emissions to its gross
capacity (relative to that of Chernobyl) to approximately account
for the differences in fuel that could be involved in
a meltdown. Thus a reactor with half the gross capacity of
Chernobyl is assumed to emit half of the amount mentioned

J. Lelieveld et al.: Global risk of radioactive fallout 4247
Table 1. Radioactivity released to the atmosphere by INES 4–7 nuclear accidents (in PBq).
Location Country INES Date Total 131I 137Cs
Fukushima Japan 7 11 March 2011 >630 190–380 12–37
Chernobyl USSR 7 26 April 1986 >12 000 1760 85
Mayak USSR 6 29 September 1957 74–1850 n.d.a n.d.a
Chalk River Canada 5 12 December 1952 >0.3 n.d.a. n.d.a.
Windscale UK 5 10 October 1957 1.6 0.7 0.02
Simi Valley USA 5–6 26 July 1959 > 200a b n.d.a.
Belojarsk USSR 5 1977 n.d.a. n.d.a. n.d.a.
Three Mile Island USA 5 28 March 1979 1.6c <0.0007 n.d.a.
Chernobyl USSR 5 1 September 1982 n.d.a. n.d.a. n.d.a.
Idaho Falls USA 4 29 November 1955 d d d
Idaho Falls USA 4 3 January 1961 n.d.a. n.d.a. n.d.a.
Monroe USA 4 5 October 1966 d d d
Lucens Switzerland 4–5 21 January 1969 d d d
Windscale UK 4 1973 n.d.a. n.d.a. n.d.a.
Leningrad USSR 4–5 6 February 1974 e n.d.a. n.d.a.
Leningrad USSR 4–5 October 1974 55 n.d.a. n.d.a.
Jaslovsk´e Bohunice CSSR 4 22 February 1977 n.d.a. n.d.a. n.d.a.
Saint-Laurent France 4 13 March 1980 n.d.a. n.d.a. n.d.a.
Buenos Aires Argentina 4 23 September 1983 n.d.a. b n.d.a.
Tokaimura Japan 4 30 September 1999 n.d.a. n.d.a. n.d.a.
INES 0–3 events are indicated as deviations, anomalies and incidents;
INES 4 is an accident with local consequences; INES 5 is an accident with wider consequences;
INES 6 is a serious accident; INES 7 is a major accident (major release of radioactive material with widespread health and
environmental effects requiring implementation of planned and extended countermeasures);
n.d.a. no data available; a Substantial emissions of 85Kr, 133Xe assumed though n.d.a.; b Substantial 131I emissions assumed, though
n.d.a.; c Mainly 85Kr emitted; d No strong source of radioactivity to the atmosphere; e Release of radioactive sludge from filter
powder to the environment.
By integrating over a year we capture the annual range of
meteorological conditions, thus providing a stochastic representation
of the atmospheric transport and deposition pathways,
accounting for the different seasons. During an actual
accident the total deposition likely occurs over a much
shorter time period, as was the case with the Chernobyl
reactor. To assess the effect of individual accidents one
would need to simulate the actual emissions and meteorological
conditions in a deterministic approach (http://flexrisk. Such calculations can be performed when the
emissions and meteorological conditions are known, which
has been done for Chernobyl and needs to be done for
Fukushima. For our risk calculations the total deposition over
all meteorological conditions is relevant, rather than the actual
time period of any individual accident. For this reason
we assess the contamination risk per year in Sect. 4. To illustrate
the intra-annual variability, we also present monthly
contamination risk maps in Sect. 4 by assuming that the same
emission occurs within one month rather than one year. Furthermore,
in Sect. 6 we test our approach by comparing the
continuous annual simulations with those in which we release
the radioactivity in weekly periods for two selected locations
3 Global model calculations
Simulations of particulate 137Cs and gaseous 131I have been
performed with the ECHAM5/MESSy Atmospheric Chemistry
(EMAC) general circulation model (Roeckner et al.,
2006; J¨ockel et al., 2006; Kerkweg et al., 2006, 2007; Tost
et al., 2007; Burrows et al., 2009; Pringle et al., 2010). The
model has been extensively evaluated against observations,
including satellite measurements (Kerkweg et al., 2008; Liu
et al., 2011; Pozzer et al., 2010, 2012). We applied it at
T106 horizontal resolution (corresponding to a quadratic
Gaussian grid of about 1.1°—1.1_ in longitude and latitude,
roughly 100°—100 km) and 31 vertical layers up to 10 hPa.
The emissions of all NPPs are represented by separate 137Cs
and 131I tracers. The applied sub-models, which simulate all
relevant meteorological processes for the dispersion of the
aerosols, are: CLOUD, CONVECT, RAD4ALL, PTRAC,
CVTRANS, ONLEM, DRYDEP, SEDI, SCAV (for a description,
see For a similar
application, using the EMAC model at T106 resolution
to represent a series of point sources (i.e., major population
centres worldwide), see Kunkel et al. (2012). We simulated
the year 2005, which we consider representative, i.e., without
strong annular modes (e.g., El Ni˜no, North Atlantic Oscillation),
by using meteorological analyses from the European
Centre for Medium-range Forecasts (ECMWF) to nudge the
general circulation model (Lelieveld et al., 2007). It will be
interesting to also study the effects of annual modes on the
transport of radioactivity in future work.
To test the representativeness of the year 2005 we performed
multi-year tracer tests for a sub-set of the tracers,
and find that the transport patterns and deposition fields are
not significantly sensitive to this assumption. We simulated
1.5 yr in total, starting in 2004. The model setup has been
designed such that first the airborne tracer mass is in steady
state (which takes about 3–4 months). Therefore, the first six
months are used for spin-up and the last twelve for the analysis.
At the beginning of our analysis there is already tracer
mass present in the atmosphere, which is consequently deposited
within the analysis period and is about the same as
the tracer mass that is still airborne at the end of the analysis
The mean aerosol lifetime of 137Cs in the atmosphere,
about one week, is much shorter than the decay time of the
radionuclei; therefore the removal of particulate radioactivity
is controlled by wet and dry deposition of the particles,
rather than atmospheric decay. 131I has a much shorter halflife,
so that a significant amount can decay radioactively before
depositional loss; we assume that it travels in the gas
phase as 131I2 and have represented its dry deposition using
the method of Ganzeveld et al. (1998), based on the concept
ofWesely (1989).We apply a Henry’s law coefficient of
3.1 mol l−1 atm−1, indicating a low solubility. For a review of
deposition modelling methods we refer to Sportisse (2007).
A small fraction of the 131I may be incorporated in other
gases than I2 and in aerosols, and we thus implicitly assume
that their deposition processes are the same.
In the model we emit the 137Cs on aerosol particles with a
mean radius of 0.5 μm by introducing them into the surface
layer of about 60m depth. We assume that 131I and 137Cs
are released gradually at the surface and not explosively or
by large fires, which would increase the effective emission
height (Chernobyl burned during 10 days). This would be
particularly important when the effective emission height exceeds
that of the well-mixed boundary layer (typically 1–
3 km). Regarding the sensitivity that could be expected to
explosive versus gradual emissions, two dimensions need to
be considered. Temporally, we have demonstrated that the
average deposition from the continuous emissions is comparable
to the expected value of short-term emissions occurring
during any week of the year. Spatially, we can refer to
Pozzer et al. (2009), who applied our model to compare air
pollution emissions released from the surface and according
to a height profile, the latter being relevant for emissions by
biomass burning and from high stacks. The results indicate
that the impact on atmospheric chemistry is generally small,
typically less than 5% in the free troposphere and less than
30% in polluted regions. Since we neglect elevated sources,
our estimates of long-distance transport should be considered
as conservative (see also Kunkel et al., 2012).
Since the chemical composition of the particles is not
known, we adopted two categories, one “soluble” and one
“insoluble”, which represent the extremes of possible particle
characteristics. The main difference between the two is that
only soluble particles are removed by nucleation scavenging,
i.e., they can act as cloud condensation nuclei (CCN), though
do not exert feedbacks on cloud formation processes. Both
soluble and insoluble particles are removed by impaction
scavenging (Tost et al., 2007), aerosol sedimentation and dry
deposition (Kerkweg et al., 2006). Thus, the insoluble particles
have a longer lifetime due to reduced wet deposition
and the consequently extended transport in the atmosphere.
The difference between the results from the two simulations
is typically less than 30 %.
To test these assumptions and the model setup, we simulated
the Chernobyl accident in 1986 to study the distribution
of 137Cs deposition and compare it with a compilationinterpolation
of measurement data presented by Smith and
Beresford (2005), based on the original work of De Cort et
al. (1998). The source profile of 137Cs, including the emission
height, has been adopted from Brandt et al. (2002),
based on several publications. Since the deposition data
could not be made available from the Radioactivity Environmental
Monitoring (REM) Action of the European Commission,
we evaluated the model output by comparing with a
deposition map based on all available data (Smith and Beresford,
2005; Peplow, 2006), applying the same colour scaling
in Fig. 1. We find that that our results agree well with the
map of Peplow (2006) (see also IAEA, 2006), especially of
our soluble tracer, indicating 137Cs deposition in excess of
40 kBqm−2 in the Ukraine, Scandinavia, Northern Greece,
southern Germany and Austria (Fig. 1).
This qualitative agreement is quite satisfactory, especially
because quantitative agreement cannot be expected due to the
neglect of the large particles emitted by Chernobyl fires and
the difficulty of accurately simulating the emission time and
height profiles. Considering that we achieve the best agreement
with the soluble tracer, and since the majority of particles
acts as CCN (Pruppacher and Klett, 1997), we henceforth
assume that 75% of the aerosol is removed by nucleation
scavenging (75% soluble and 25% insoluble particles).
Our total 137Cs deposition distribution also compares
favourably with the high-resolution regional model results of
Brandt et al. (2002), notably the map for which the most sophisticated
deposition method was applied (their Fig. 7).
Subsequently we emitted gaseous 131I and particulate
137Cs from each of the 440 active reactors worldwide for
the year 2005 (the reactors, locations and operational lifetimes
are listed in the Supplement). The calculations indicate
that on average only about 8% of the 137Cs deposits within
50 km of the source, _30% within 500 km, _50% within
1000 km, and that about 25%is transported beyond 2000 km.
Clearly, radionuclides from a reactor accident anywhere in
the world would not only be deposited locally. The variability
of these fractions for different regions, e.g., in Europe, the
Fig. 1. Model calculated fallout of 137Cs after the Chernobyl accident,
assuming a soluble particle tracer (a) and an insoluble one (b).
The location of Chernobyl is indicated by the black square. The
colour scale is the same as in Peplow (2006); see http://www.nature.
USA and Asia is relatively small, even though the retention
of emissions near the surface in temperate and high latitudes
is typically stronger than in the tropics where the intensity of
convection is greater (Lawrence et al., 2007). Risks for human
exposure in regions with NPPs are much more variable,
largely determined by population densities, and for individual
major accidents they are highest in South Asia and West
Europe, and lowest in North Europe.
4 Deposition risk assessment
To evaluate the global risks, we can use empirical evidence
to estimate the factors (a) and (b) from above. In the past
decades, four INES level 7 catastrophic nuclear meltdowns
have occurred, one in Chernobyl and three in Fukushima.
Note again that we are not considering INES 6 and lower
level accidents with partial core melts such as Three Mile Island
(USA), Mayak (a plutonium production and reprocessing
plant in Siberia) and Sellafield (UK). The total number of
operational reactor years since the first civilian nuclear power
station in Obninsk (1954) until 2011 has been about 14 500
(IAEA, 2011; Supplement). This suggests that the probabil
of a major reactor accident, i.e., the combined probability
of the factors (a) and (b), is much higher than estimated in
Simply taking the four reactor meltdowns over the 14 500
reactor years would indicate a probability of 1 in 3625 per reactor
per year, 275 times larger than the 1990 estimate (NRC,
1990). However, since 2011 is at a junction in time with impacts
of a catastrophic meltdown still unfolding, this direct
estimate is high-biased, and we round it off to 1 in 5000 per
reactor per year for use in our model simulations. This is
actually only a factor of two higher than the estimated core
melt probability noted above, factor (a), although originally
this factor also represented partial core melts, which have
occurred more frequently. Based on the past evidence, this
principally assumes that if a major accident occurs, the probability
of containment before substantial radioactivity release
is very small.We thus argue that including the factors (b)–(e)
can distort the risk perception. Our rounded estimate implies
that with 440 civilian reactors worldwide a major accident
can be expected to occur about once every few decades, depending
on whether we count Fukushima as a triple or a single
Although an objective measure for dangerous radioactive
contamination is debatable, a level of >37 kBq137Csm−2 or
_40 kBqm−2 for beta- and gamma-emitters has been suggested
after the Chernobyl accident as a threshold contamination
level (IAEA, 2005, 2006). The reasons given by
IAEA (2006) are that:
– This level was about ten times higher than the 137Cs
deposition in Europe from global fallout;
– At this level the human dose during the first year after
the major accident was about 1 mSv and was considered
to be radiologically important.
Here we define _40 kBqm−2 as “contaminated”, following
the definition by IAEA (2005). Subsequently,
the risk of contamination has been calculated based
on the expression: (modelled total 137Cs deposition
(kBqm−2 yr−1)/40 kBqm−2)°—(probability of a major accident).
Hence the presented risk maps correspond directly to
the model calculated 137Cs deposition distribution.
Figure 2a shows the modelled annual risks of contamination,
from lower than 0.01%yr−1 in Alaska and eastern
Canada, much of Africa and Australia, to higher than
2%yr−1 in some areas around multiple reactors in the northeastern
USA,West Europe and Japan. These numbers signify
the expected values, defined as the weighted average (first
moment) of an independent variable of all possible values
it can take. The weights correspond to the probabilities of
these values. This means, for example, that on average in
the Northeast USA, West Europe and Japan contamination
by major accidents is expected at least every 50 yr, which is
in accord with the frequency of past events in Europe and
Japan. In Fig. 3 we present regionally enhanced sub-sections

Fig. 2. Global risk of radioactive contamination by 137Cs. (a) Based
on the modelled deposition of _40 kBq 137Csm−2 yr−1. The risk
is the expected value normalized by 40 kBqm−2. (b) Modelled risk
of human exposure to 137Cs deposition.
of the same depiction as Fig. 2. There are extended regions
with a risk of >1%yr−1; and large parts of North America,
East Asia and especially Europe have risks of more than
1‰yr−1, as indicated by the orange colour scale. In Fig. 2b
and the lower panels of Fig. 3 we weigh the expected deposition
with population density (SEDAC, 2011), which underscores
the disproportional risk of contamination for people in
regions with many reactors in Europe, parts of the USA and
Figures 4 and 5 show the results for 131I, again defining
_40 kBqm−2 as “contaminated”. Interestingly, the deposition
patterns over land and the risks of contamination and
human exposure are quite comparable to 137Cs. Even though
the amount of radioactivity released by Chernobyl as 131I
was _20 times that of 137Cs, the low solubility of iodine
and its _1400 times shorter half-life limit the deposition,
especially on water surfaces. Recall that 131I and 137Cs are
used as proxies for the total release and deposition of radionuclides,
also including 90Sr and 134Cs, for example. By
adding the risks of the two tracers in Figs. 2 and 4, the total
risk of contamination by _40 kBqm−2 is roughly twice
that indicated by the individual contamination risks. Furthermore,
the risks scale with the addition of other tracers such
as 134Cs and 90Sr. However, also recall that we have taken
the emissions by Chernobyl, scaled by gross reactor capacity,
as a proxy for all 440 civilian reactors. Preliminary estimates
of the release of radioactivity during the first three
weeks after the Fukushima accident are substantially lower
than for Chernobyl, i.e., _140 PBq of 131I and _10 PBq of
137Cs (Chino et al., 2011). Winiarek et al. (2012) estimated
that the 137Cs and 131I releases by Fukushima were about
a factor of 5–10 less than of Chernobyl. Stohl et al. (2012)
calculated that during a period of 40 days after the accident
_37 PBq of 137Cs were released, about 43% of the emission
by Chernobyl. If the latter would be more representative
of accidents associated with core melts than Chernobyl, the
numbers in Figs. 2–5 would decrease by about a factor of
seven. Since these are only preliminary values, not including
the full time period of release and only apply to one single
radionuclide, it seems likely that these estimates bound the
actual probabilities.
It is important to underscore the different effects of 131I
and 137Cs. In the initial period after accidents 131I is of greatest
concern as it deposits on agricultural crops, contaminating
fruits, herbs and vegetables, and on grasslands where
dairy cows graze (IAEA, 2006; Christodouleas et al., 2011).
The contaminated grass ingested by the cows is transferred
to the milk in about a day (Beresford et al., 2000). Even
though the half-life of 131I is only 8 days, it may elevate longterm
cancer risks (Christodouleas et al., 2011). According to
Figs. 2–5 the integrated deposition loads of 131I and 137Cs are
similar, while the latter exerts long-term effects through its
cycling and re-suspension in soils, groundwater and the vegetation.
The half-life of 137Cs (and 134Cs and 90Sr) is years
to decades, and the main problem is the enduring exposure
of the fauna through forage and humans through meat, milk
and to a lesser extent vegetables (IAEA, 2006; WHO, 2006).
We emphasize that our results represent a probabilistic assessment
that is representative for an entire year and integrate
the accumulated deposition over the annual range of weather
conditions, thus accounting for the temporal and spatial variability
of transport and removal processes (Baklanov and
Mahura, 2004). However, Chernobyl and Fukushima have
demonstrated that most of the radioactivity emissions occur
within the first month after the accident. To demonstrate the
difference between the annually and monthly integrated risks
of contamination by 137Cs we also performed simulations
that condense the same release of radioactivity within single
months (Fig. 6). Hence the total deposition during each
month in Fig. 6 is approximately the same as in Fig. 2, but
the distribution differs depending on the meteorological conditions
specific to the month of release.
The monthly deposition patterns in Fig. 6 are similar to
the annual ones though differ in specific aspects. In most locations
downwind of NPPs in the Northern Hemisphere – notably
at middle latitudes – the risks are highest in late spring
and summer; in continental Europe in May and July, for example.
The risk of contamination in the USA, Europe and
Japan is generally lowest in the winter. In Japan it is highest
from June to September and in East Canada during late
summer and autumn (November). The westerly outflow from
Fig. 3. Regional risks of radioactive contamination by 137Cs. Same as Fig. 2 for selected regions.


Fig. 4. Global risk of radioactive contamination by 131I. (a) Modelled
deposition of _40 kBq 131Im−2 yr−1. The risk is the expected
value normalized by 40 kBqm−2. (b) Modelled risk of human exposure
to 131I deposition.
North America and East Asia is strongest from December to
March, diverting the risks to the Atlantic and Pacific Oceans,
respectively, while transport from Europe to the Arctic region
and across Russia is strongest in winter and early spring.

5 Footprint of catastrophic accidents
Next we calculated the footprint of major nuclear accidents
and find that the average surface area onto which
_40 kBq137Csm−2 would be deposited in regions around
a reactor after a catastrophic core melt is on average about
138 000 km2 with a typical range of ±20 %. The area that
is estimated to be at risk of contamination by a single accident
is largest in West Europe, about 165 000 km2, while
in East Asia it is 153 000 km2, followed by East USA with
148 000 km2, 140 000 in Midwest USA, 129 000 km2 in
North Europe and 102 000 km2 in South Asia.
Much of the variability is related to the strength of vertical
transport in convective storms and removal by precipitation,
for example during the monsoon. This is consistent
with the findings of Lawrence et al. (2007), who showed that
the venting of pollutants from the boundary layer is generally
strongest in the tropics (e.g., in South Asia), whereas
in the extra-tropics, where most NPPs are located, the radionuclides
will tend to remain more in the boundary layer
over longer near-surface transport distances. Our model results
indicate that the average number of people that would
be affected by the radioactive contamination of 137Cs due to
a single reactor accident varies strongly by region: about 3M
in North Europe, 8M in the Midwest USA, 8M in East Europe,
14M in East USA, 21M in East Asia, 28M in West
Europe, and 34M in South Asia (M is mega=106).
For Chernobyl we computed a 137Cs-contaminated area of
about 113 000 km2, endangering about 8 million people (note
that this refers to 2005 mean meteorology and population
statistics, not the 1986 simulation). This is not far from the
published estimate for the Chernobyl accident, suggesting


that an area of about 200 000 km2 was contaminated, affecting
more than 5 million people (Smith and Beresford, 2005),
which provides additional support for the validity of our approach
and its representativeness for other time periods. The
model calculations suggest that some regions have a particularly
high risk of contamination, due to the numerous reactors
in operation, for example the region between Washington
D.C. and New York, and around Atlanta, Toronto, Tokyo
and Osaka. Many regions are densely populated and associated
with a high human exposure risk, for example also the
Shanghai region and Hong Kong. The highest risk of radioactive
contamination occurs in West Europe, especially around
the borders between Germany, Belgium and France, with the
greatest human exposure risk in southwestern Germany in
the area between Stuttgart and Cologne.
6 Sensitivity to short-term and annual emissions
In our global model simulations we have assumed a continuous
source of radioactivity throughout the year 2005 from
each of the 440 reactors, from which we calculated the fallout
and subsequently the risk of contamination, thus accounting
for the annual range of meteorological conditions. In Sect. 4
and Fig. 6 we also presented results of monthly simulations
for comparison. One might argue that employing a continuous
source does not lead to the same result as a more realistic
simulation of a short-term emission of radioactivity, since
Chernobyl and Fukushima released most radioactivity within
weeks (see the interactive discussion of the ACPD version of
our manuscript:
To further test the sensitivity to this assumption we have
performed additional model calculations in which we released
the same amount of radioactivity within one week,
during each of the 52 weeks of the year, from two locations:
Chernobyl and Fukushima. For both source locations we obtain
deposition fields for each weekly release which are integrated
between the beginning of 2005 and end of March
2006. Then we calculate a mean deposition field of these 52
single deposition fields to compare with the deposition from
the continuous annual emissions.
Figure 7 depicts the total atmospheric load of 137Cs resulting
from each of the weekly emission events, showing
that most of the aerosol-borne reactivity is removed within
about a month. The longest aerosol lifetimes and largest atmospheric
loads occur in periods when rain events downwind
are relatively infrequent, e.g., for Chernobyl in early spring
(April) and autumn (September). For Fukushima the variability
is generally less, with a minimum atmospheric load (i.e.,
most efficient removal) in summer (July–August). The maximum
atmospheric loads per week from Chernobyl are typically
larger (_20–70 PBq 137Cs) than from the same emission
in Fukushima (_10–40 PBq 137Cs), indicating that atmospheric
removal of the aerosol-borne radioactivity is generally
more efficient for Fukushima.
Figures 8 and 9 compare the 137Cs fallout patterns of the
weekly and annual emission simulations. The differences
(i.e., annual – weekly deposition) are generally small, especially
in the region around Chernobyl where they are typically
within ±10 %, which corroborates the validity of our
approach. In the Fukushima area they are somewhat larger,
and the annual emission calculations lead to slightly higher


Fig. 6. Global risk of radioactive contamination by 137Cs based on the modelled deposition of _40 kBq 137Csm−2/month, applying the
same emission as in Fig. 2 though monthly instead of annually.

137Cs fallout in the Pacific Rim vicinity of the reactor (i.e.,
the domain shown in Fig. 9). It thus appears that our stochastic
approach is appropriate, though possibly leading to conservative
estimates of the contamination impact after severe
reactor accidents.

7 Phase-out of NPPs in Germany
In the wake of the events in Fukushima the German government
has decided to phase out all nuclear power plants over
the next decade (in accord with the theory that large-impact
and rare events are leading causes of societal change; see
Taleb, 2010). Figure 10a presents calculations in which the
17 German reactors have been switched off. This reduces the


Fig. 7. Atmospheric load of 137Cs by applying the emissions of
Chernobyl (85 pBq) each week. The simulations were performed
for the locations of Chernobyl and Fukushima, applying the same
source term.
expected 137Cs deposition in Germany by about a factor of
two, though it nevertheless continues to be among the highest
worldwide. Only in France (58 reactors) and Belgium (7 reactors)
does the overall risk remain higher. Figure 10b shows
that by switching off the reactors in neighbouring countries,
i.e., France, Belgium, the Netherlands, Switzerland and the
Czech Republic, but leaving all 17 German reactors active,
the deposition risk in Germany would be even more effectively
reduced than by the national efforts alone. This emphasizes
that, for the sake of reducing the risk of exposure
to radioactivity, there is a need to coordinate phase-out decisions
on an international level, especially when NPPs are
located in the vicinity of international borders.
8 Conclusions and recommendations
Using our stochastic approach, it will be possible to evaluate
other risks besides direct human exposure, including
biological and indirect health risks. For instance, linking to
agricultural information will allow evaluation of the risk that
radioactivity enters the food chain. Clearly there is a need
to perform an extensive re-assessment of the factors (a) and
(b), discussed above, based on the evidence from Chernobyl,
Fukushima and other less catastrophic accidents. In particular,
a better understanding of reactor risk profiles and expected
release of radioactivity in case of a meltdown is required.
This depends on several aspects, including:
– Type of reactor and capacity;
– Reactor maintenance, safety culture and other human
– Safety improvements with the progress of technology;
– Degradation of the concrete reactor shell with age;
– Enhanced risk for NPPs with multiple reactors and

shared technical facilities;
– The likelihood of natural disasters such as earthquakes
and tsunamis;
– Susceptibility to aircraft impacts, sabotage and terrorist
Key information in the assessment of exposure risks is
the emission strength of radioactivity by major nuclear accidents.
Table 1 illustrates that this information is generally
poor or lacking, also for Fukushima (Taira and Hatoyama,
2011), except for Chernobyl. Therefore, we have used the
available reports of the Chernobyl disaster – by scaling the
emissions of the other reactors according to their gross capacity.
It will be important to also evaluate the Fukushima
accident with the same level of scrutiny as Chernobyl to complement
the available data. In a future publication we will report
simulation results from the Fukushima accident, which
will be tested against radionuclide measurement data from
the Comprehensive Nuclear-Test-Ban Treaty Organization
(CTBTO). Preliminary estimates suggest that the Fukushima
emissions of radioactivity are up to an order of magnitude
less than of Chernobyl. If representative for NPPs, this would
reduce the risks of contamination by 137Cs presented here
correspondingly and the present risk assessment may be considered
as a worst-case scenario. However, since most of the
radioactivity from Fukushima has been deposited to the Pacific
Ocean, remote from measurement stations, the inverse
computation of emissions based on observational data will be
difficult, although the back-calculation of trans-Pacific transport
based on measurements along the US west coast holds
promise (Priyadarshi et al., 2011).
The transport and deposition calculations presented here
may be regarded as “scalable” because the modelling of
137Cs is a near-linear tracer experiment, as long as the
tracer lifetime is significantly longer than the lifetime of the
aerosols on which they travel and the statistics of the meteorological
conditions are representative. For example, if 50%
or 10% of the 137Cs release by Chernobyl would be more
representative for modern NPPs, the deposition and contamination
risks could be scaled down by a factor of two or ten,
respectively. On the other hand, if one wishes to account for
additional radionuclides such as 134Cs (the Chernobyl release
of radioactivity by 134Cs was about 50% of 137Cs) the 137Cs
results could be scaled up by a factor of 1.5 based on Chernobyl.
Furthermore, it will be important to re-think the level
of “dangerous” contamination. We applied 40 kBqm−2 of
137Cs, but this does not do justice to the dangers by the
short-lived 131I and the additional contamination by other
long-lived radionuclides. Since Chernobyl released many
other beta- and gamma-emitters to which the contamination
threshold of 40 kBqm−2 applies, including hot particles, we
feel that using the Chernobyl emissions of 137Cs is a reasonable
compromise between the prospect that modern NPPs
release less radioactivity and the use of reasonably accurate


Fig. 8. Yearly-integrated fallout of 137Cs (year 2005), comparing annually continuous to weekly emissions (both the same total source
strength of Chernobyl), released from the location of Chernobyl.

Fig. 9. Yearly-integrated fallout of 137Cs (year 2005), comparing annually continuous to weekly emissions (both the same total source
strength of Chernobyl), released from the location of Fukushima.
emission estimates of 137Cs, even though they may represent
an upper limit.
Going beyond the factors (a) and (b), the probabilities of
a core melt and containment failure, we have provided a better
understanding of the impact of atmospheric dispersion,
providing evidence that the widespread risks to humanity of
nuclear accidents are much larger than suggested in official
reports several decades ago. Although improved risk assessments
may have been performed for some reactors more recently,
unfortunately their results and the basic assumptions
about the release of radioactivity after major accidents are not


Fig. 10. Phasing out nuclear power plants. (a) Same as Fig. 3b, indicating the regional risks of contamination by 137Cs with German reactors
switched off. (b) Same as Fig. 3b with reactors in the neighbouring countries of Germany switched off (i.e., France, Belgium, the Netherlands,
Switzerland and the Czech Republic).
publicly available. Publication in the peer-reviewed literature
would help scientific scrutiny as well as the public debate.
It is important to select reactor sites by accounting for
the proximity to large population centres, as there appears
to be a tendency to build NPPs in the vicinity of electricity
consumers in urban regions. NPPs in densely populated
areas in Europe, the USA, eastern Asia and southern Asia
bear a high risk of exposing large numbers of people to radioactive
contamination after reactor accidents. In western
Europe (notably France), the concentration of NPPs is relatively
high, and although the phase out in Germany in the
next decade will halve the national risk of radioactive contamination
by major accidents, international efforts will be
necessary to achieve the German safety objectives. Worldwide
more than 60 reactors are currently under construction
(Supplement) and many more are planned, hence the global
risks may change accordingly in the coming years.
Supplementary material related to this article is
available online at:
Acknowledgements. We thank the anonymous contributors to
Wikipedia for the list of nuclear power plants and information
about radioactivity releases, Sarah Alznauer and Bettina Kr¨uger
for for tabulation and editing help, and the Computing Centre (RZ)
Garching (in particular Hermann Lederer) for computer support.
We also thank Henning Rodhe, Falk Schmidt, Costas Papanicolas,
Greet Maenhout and Paul Crutzen for comments and discussions.
The research leading to these results has received funding from the
European Research Council under the European Union’s Seventh
Framework Programme (FP7/2007-2013)/ERC grant agreement
no 226144.
The service charges for this open access publication
have been covered by the Max Planck Society.
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