The activity concentrations of 238,239,240Pu and 241Am (for
determining its mother nuclide, 241Pu) as well as activity ratios of
238Pu/239+240Pu, 241Pu/239+240Pu and
239+240Pu/137Cs and the mass ratio of 240Pu/239Pu were
determined from air filter samples collected in Rovaniemi (Finnish Lapland)
in 1965 to 2011. The origin of plutonium in surface air was assessed based on
these data from long time series. The most important Pu sources in the
surface air of Rovaniemi were atmospheric nuclear-weapon testing in the
1950s and 1960s, later nuclear tests in 1973–1980 and the SNAP-9A
satellite accident in 1964, whereas the influence from the 1986 Chernobyl
accident was only minor. Contrary to the alpha-emitting Pu isotopes,
241Pu from the Fukushima accident in 2011 was detected in Rovaniemi.
Dispersion modeling results with the SILAM (System for Integrated modeLling of Atmospheric composition) model indicate that Pu
contamination in northern Finland due to hypothetical reactor accidents
would be negligible in the case of a floating reactor in the Shtokman
natural gas field and relatively low in the case of an intended nuclear
power plant in western Finland.
Introduction
The distribution of anthropogenic radionuclides in global fallout from
nuclear-weapon testing is uneven, and even more inhomogeneous is their
distribution in regional and local fallout from different sources. It is
known that Subarctic and Arctic regions have received radionuclide
deposition with radioactivity levels and composition different to the more
temperate areas of the Earth. Subarctic and Arctic ecosystems have a special
combination of harsh climate; often sparse vegetation; lack of nutrients;
and, in the case of humans, dependence on traditional livelihoods and
lifestyles like hunting, fishing, reindeer herding, and gathering mushrooms
and berries. Consequently, these Nordic ecosystems are highly vulnerable to
toxic agents, including radionuclides. Still, there are only a few
contiguous long-term radioactivity data series from Subarctic and Arctic
areas where the changes in concentration levels and isotope ratios can be
followed and nuclear events can be identified as contamination sources in a
particular environment.
In total, the radionuclides of 137Cs and 90Sr and total beta activity of
238,239,240Pu and 241Am were determined from the air filter
samples that were collected in Rovaniemi (Finnish Lapland) in 1965–2011.
241Am (t1/2 – half-life – of 432.2 years) was analyzed for calculating the activity
concentration of its mother nuclide, the relatively short-lived beta emitter
241Pu (t1/2 of 14.35 years). The major part of 241Am in the
samples originates from the decay of 241Pu after the sampling, and only
a minor part of 241Am originates directly from nuclear events. The
results for 137Cs, 90Sr and total beta activity have been
reported elsewhere (Salminen-Paatero et al., 2019). The activity ratio
of 238Pu/239+240Pu and the mass ratio of 240Pu/239Pu in
Rovaniemi have been presented pictorially with other global ratio values in
an article by Thakur et al. (2017), which did not, however, discuss the
ratio values of Rovaniemi in detail.
In this study, radionuclide concentration and isotope ratio data from
1965 to 2011 have been used for estimating nuclear contamination sources in the
surface air of the Finnish Subarctic over almost 5 decades. Few long time
series of atmospheric radioactivity exist in Subarctic and Arctic regions,
especially of Pu isotopes, and even fewer data have been published about
atmospheric transuranium concentrations in these high northern latitudes
after the Chernobyl and Fukushima accidents. Furthermore, the atmospheric
dispersion of one real and one hypothetical nuclear event has been modeled
for establishing the potential transport of Pu isotopes and effect of these
nuclear events on atmospheric radioactivity levels in Finnish Lapland.
Atmospheric-dispersion modeling completed the experimental data by providing
risk estimates and reference values for future accidental releases of
nuclear material in and close to Arctic regions, as well as indicating the
importance of the accurate source term in calculating the amount of
radioactivity released into the atmosphere after Fukushima.
ExperimentalSampling and procedures for the air filters before any chemical
treatment
The air filter samples were collected at the Finnish Meteorological
Institute's (FMI) Rovaniemi monitoring station at 66∘34′ N,
25∘50′ E with an elevation of 198 m above sea level (a.s.l.). The weekly
volume of sampled air was ∼1000 m3. First, total beta
activity was measured from the filters 5 d after the end of sampling.
Then the filters were combined into suitable sets for the gamma measurement
and determination of 137Cs concentration. The details of air sampling,
combining air filters and measurements for the gamma activity of 137Cs,
and total beta activity have been given by Salminen-Paatero et al. (2019).
Radiochemical separation of Pu, Am and Sr from air filters
A detailed description of the radioanalytical separation procedure and the
radionuclide measurements is given elsewhere (Salminen-Paatero and Paatero,
2020). 238,239,240Pu, 241Am and 90Sr were
separated from dissolved air filter sample sets containing filters from 3 months to 5 years. The separation method included extraction
chromatography and anion exchange steps, and it was modified from the
original method designed for the air filters with a 1–3 d sampling time
presented in Salminen and Paatero (2009). The radiochemical separations were
performed in 2013–2014, i.e., 2–3 years after the last air filter
sample set of 2011 was taken.
Measurement of 238,239,240Pu,
241Am, 90Sr and
240Pu/239Pu in the air filter
samples
The activity concentration of alpha-emitting Pu isotopes 238Pu and
239+240Pu in the air filter samples was determined from the separated
Pu fractions by the Alpha Analyst spectrometer (Canberra). From the separated Am
fractions the activity concentration of 241Am was also measured by
alpha spectrometry to calculate the activity concentration of its mother
nuclide, beta emitter 241Pu, in each air filter sample set from
1965–2011 for the time of sampling. The alpha measurements were performed
soon after the radiochemical separations in 2013–2014.
The activity concentration of 90Sr was measured by a Quantulus 1220
liquid scintillation counter (LSC) via the activity concentration of the
daughter nuclide 90Y. Finally, after an additional purification step of
the Pu alpha-counting samples, the mass ratio of 240Pu/239Pu was
determined by SF-ICP-MS (sector-focusing inductively coupled plasma mass
spectrometry) with an ELEMENT XR (Thermo Scientific). A more detailed description
of the measurements is given in Salminen-Paatero and Paatero (2020).
Results and discussionActivity concentrations of 238Pu,
239+240Pu and 241Pu in the
surface air of Rovaniemi in 1965–2011The activity concentration of 238Pu
In the period studied, 1965–2011, the activity concentration of 238Pu
had the highest value of 259±13 nBq m-3 in 1968 (Table 1 and Fig. 1). The years of the highest concentrations of 238Pu around 1968 are a
consequence of the destruction of the SNAP-9A satellite nuclear power unit
re-entering the atmosphere in 1964. Since 1968, the activity concentration
of 238Pu in the surface air of Rovaniemi has been decreasing and is now
below or close to the detection limit. The concentration of 238Pu was
also under the detection limit in the months after the Chernobyl accident,
April–December 1986.
The activity concentration of 238Pu (thin line; in nBq m-3) and
239+240Pu (thick line; in nBq m-3) in the surface air of Rovaniemi in
1965–2011. Values below the detection limit have been depicted as half the
MDA (minimum-detectable-activity) value (Table 1). The black circles indicate the times of atmospheric
nuclear tests (UNSCEAR, 2000).
The atmospheric-activity concentrations of 238Pu,
239+240Pu and 241Pu in Rovaniemi, Finnish Lapland. The activity
values have been decay-corrected to the middle point of the sampling period. A: activity concentration.
The activity concentration of 239+240Pu in the surface air of Rovaniemi
has been dropping from the highest value 2270±40 nBq m-3 (in
1965) and has been a few nBq m-3 since 1996 (Table 1, Fig. 1). Two
years before the sampling was started, in 1963, saw the deposition maximum
from atmospheric nuclear tests performed before the Partial Test Ban Treaty.
For example, at Sodankylä, Finnish Lapland, 120 km north of Rovaniemi,
the average 239+240Pu activity concentration was 17 000 nBq m-3 in
1963 (Salminen and Paatero, 2009). Slight peaks in 239+240Pu
concentration can be seen in 1974, 1978 and 1981, evidently due to the
atmospheric nuclear tests performed by the People's Republic of China
between 1973 and 1980. The effect of these nuclear tests on the radionuclide
concentration level in Finnish Lapland has been already observed in the
concentration variation of 137Cs (Salminen-Paatero et al., 2019). As
with 238Pu, the concentration of 239+240Pu was below the detection
limit on April-June 1986 following the Chernobyl accident. For comparison,
the concentration of 239+240Pu was 32 µBq m-3 in the
surface air in Nurmijärvi (southern Finland) on 28 April 1986 (Jaakkola
et al., 1986).
Based on the extremely low activity concentrations of both 238Pu and
239+240Pu in the surface air of Rovaniemi in April–December 1986,
hardly any plutonium migrated to Finnish Lapland from the destroyed
Chernobyl nuclear reactor after 26 April 1986. This conclusion is supported
by the high concentration of 137Cs (1294±7µBq m-3)
and the low concentration of 90Sr (5.2±1.1µBq m-3)
in the same air filter samples in April–June 1986 (Salminen-Paatero et al.,
2019). It has been suggested that the initial contamination plume from the
destroyed Chernobyl reactor contained intermediate (90Sr) and
refractory elements (Pu isotopes) and that the plume passed over central and
southern Finland, while the volatile elements such as 137Cs were mostly
in the later contamination plumes which also reached Lapland (Saxén et
al., 1987). However, the observations of 241Pu/239+240Pu activity
ratio discussed in a later paragraph show some possibility of
Chernobyl-derived plutonium in Finnish Lapland.
The activity concentration of 241Pu
The concentration of 241Pu was calculated via ingrowth of 241Am,
and as with 239+240Pu, the activity concentration of 241Pu reached
its highest value in 1965, 38 198 ± 711 nBq m-3, since which its
concentration has been decreasing, except for small peaks in 1974, 1978 and
1981 (Table 1, Fig. 2). In a similar manner to the activity concentration
changes of 239+240Pu, these peaks in the activity concentration of
241Pu are presumably caused by nuclear tests in the People's Republic
of China. The atmospheric-activity concentration of 241Pu was below the
detection limit in April–June 1986, and since July–December 1986, the
amount of 241Pu has returned to the same pre-Chernobyl level in the
surface air of Rovaniemi. Based on the 241Pu concentration alone, there
is no evidence of any Chernobyl-derived 241Pu in Rovaniemi.
The activity concentration of 241Pu (nBq m-3) in the surface
air of Rovaniemi (thick line for 1965–2011 with the left vertical scale; thin line for
1982–2011 with the right vertical scale). Values below the detection limit have been
depicted as half the MDA value (Table 1).
An increase in the activity concentration of 241Pu is seen in 2011,
unlike with 238,239,240Pu. The activity concentration of 241Pu in
2011, 602±131 nBq m-3, is above the concentration level in
Rovaniemi during the last decades before 2011 and probably due to the
Fukushima accident of 11 March 2011. The activity of 241Pu has been
reported as much higher than the activity of 239+240Pu in the emissions
from the destroyed Fukushima Daiichi Nuclear Power Plant (NPP), with the activity ratio
of 241Pu/239+240Pu having a value of 108 in soil and litter samples
(Zheng et al., 2012). The activity concentrations of Pu isotopes were 25 000 nBq m-3 for 241Pu, 130 nBq m-3 for 239Pu and 150 nBq m-3 for 240Pu in the air filters sampled at 120 km from Fukushima
on 15 March 2011 (Shinonaga et al., 2014).
The existence of only one combined air filter sample of Rovaniemi for 2011 is
unfortunate: the annual concentration is an average of the weekly
concentrations in 2011, and the signal from the Fukushima accident has been
diluted under the excess effect of global fallout in the air filters.
Analysis of plutonium isotopes in weekly filters separately from March 2011,
in order to determine Fukushima-derived 241Pu concentrations and isotope ratios
in Finnish Lapland, would have been of interest.
The activity ratios of
238Pu/239+240Pu,
241Pu/239+240Pu,
239+240Pu/137Cs and total beta activity to 239+240Pu and mass ratio of
240Pu/239Pu in the air filters238Pu/239+240Pu activity ratio
The activity ratio of 238Pu/239+240Pu was 0.022±0.003–0.444±0.023 in Rovaniemi in 1965–2011, with values below the
detection limit excluded (Table 2, Fig. 3). The variation in the activity
ratio values is 200-fold. The activity ratio of 238Pu/239+240Pu in
the surface air can vary greatly even in a short time, for example due to
stratospheric–tropospheric exchange, resuspension and an introduction of
several contamination sources. For example, the activity ratio of
238Pu/239+240Pu varied from 0.014±0.003 to 0.32±0.11
in Sodankylä in 1963 alone; still, the most typical value was
∼0.03, which represents the activity ratio for global
fallout (Salminen and Paatero, 2009). The ratio started to increase in 1966
in Rovaniemi, reaching a maximum in 1967 due to the aforementioned SNAP-9A
satellite accident in 1964. Previously, an increased
238Pu/239+240Pu activity ratio due to the SNAP-9A accident has
been found in lichens both in Subarctic Finland (Jaakkola et al., 1978) and
Sweden (Holm and Persson, 1975) a couple of years after 1964. This delay of
over 2 years after the accident indicates the slowness of the
interhemispheric transport of stratospheric radionuclides (Fabian et al.,
1968).
The activity ratio of 238Pu/239+240Pu in the surface air of
Rovaniemi as a function of time.
The activity ratios of 238Pu/239+240Pu,
241Pu/239+240Pu and 239+240Pu/137Cs and the mass ratio of
240Pu/239Pu in the air filters collected in Rovaniemi. The
uncertainty is 1σ error for the activity ratios and 2σ error for
the mass ratio. The symbol “–” means that one or both isotopes had a concentration below
the detection limit.
The activity ratio of 238Pu/239+240Pu cannot be determined for the
period immediately after the Chernobyl accident because the activity
concentrations of 238Pu and 239+240Pu were below the detection
limit in April–December 1986. This finding is in agreement with the
previous assumptions about there being barely any Chernobyl-derived refractory elements
in Finnish Lapland (Salminen-Paatero et al., 2019). Because the activity
concentrations of 238Pu and 239+240Pu were below the detection
limit, the activity ratio of 238Pu/239+240Pu cannot be determined for
the year of the Fukushima accident, 2011, either. For comparison, both
238Pu and 239+240Pu were detected in Lithuania, ∼1300 km south of Rovaniemi, soon after the Fukushima accident
(Lujanienė et al., 2012). The combined air filter sample set in the
Lithuanian study contained the sampled air volume of ∼2×106 m3 from 23 March to 15 April 2011, with the activity
concentration of 239+240Pu being 44.5±2.5 nBq m-3 and the
activity concentration of 238Pu being 1.2 times higher than that of
239+240Pu. The resulting activity ratio of 238Pu/239+240Pu in
Lithuania was 1.2, clearly deviating from the activity ratio values of the
Chernobyl fallout and global fallout from nuclear-weapon testing.
241Pu/239+240Pu activity ratio
The activity ratio of 241Pu/239+240Pu varied between 8.2±0.7
and 79±17 in the surface air of Rovaniemi in 1965–2011, except for
April–December 1986 and 2011, when the concentration of one or both
isotopes (either 239+240Pu or 241Pu) was below the detection limit
(Table 2, Fig. 4). These two periods following the accidents of Chernobyl
and Fukushima would have interesting 241Pu/239+240Pu activity
ratio values for determining the Pu contamination source in Rovaniemi.
Unfortunately, the concentration of 239+240Pu in the surface air of
Finnish Lapland was extremely low during those periods.
The activity ratio of 241Pu/239+240Pu in the surface air of
Rovaniemi as a function of time.
The 241Pu/239+240Pu activity ratio values of Rovaniemi were mainly
due to atmospheric nuclear-weapon testing in 1965–March 1986 and
1987–2005. The influence of the Chernobyl accident can be seen as elevated
ratio values. The 241Pu/239+240Pu activity ratio was determined to
be 15 in fresh nuclear fallout in 1963–1972 (Perkins and Thomas, 1980), and
the corresponding ratio values in the fallout from the Chernobyl accident
have been 85 in Sweden and Poland (Holm et al., 1992; Mietelski et al., 1999)
and 95 in Finland (Paatero et al., 1994). The published
241Pu/239+240Pu activity ratio values for the Fukushima-derived
contamination are also high, e.g., 89 in air filters (calculated from the
individual isotope concentrations in Shinonaga et al., 2014) and 108 in
soil and litter samples (Zheng et al., 2012).
240Pu/239Pu mass ratio
The mass ratio of 240Pu/239Pu was 0.117±0.009–0.278±0.093 in 1965–2011 (Table 2 and Fig. 5), and the majority of ratio values
corresponds to the value ∼0.18 for global fallout from
atmospheric nuclear-weapon testing in the Northern Hemisphere (Beasley et
al., 1998), taking into account the relative measurement uncertainties. The
highest mass ratio value occurred in April–June 1986, while the activity
concentrations of 238Pu, 239+240Pu and 241Pu were under the
detection limit by alpha spectrometry. Therefore, it was possible to
determine 239Pu and 240Pu by mass spectrometry even from the
samples with very low Pu concentrations (April–December 1986, 2011, etc.),
although the relative measurement uncertainties of ICP-MS are much higher
for these samples with very low Pu concentrations compared to the measurement
uncertainties of samples with a higher Pu concentration level.
The mass ratio of 240Pu/239Pu in the surface air of Rovaniemi as a
function of time.
The mass ratio of 240Pu/239Pu is higher in the emissions from the
destroyed Chernobyl reactor than the global-fallout value. For example, a
mass ratio value of 0.408±0.003 has been determined from samples of the
Chernobyl-contaminated soil layer (Muramatsu et al., 2000), and two hot
particles that migrated to Finland from Chernobyl had the mass ratios
0.33±0.07 and 0.53±0.03 (Salminen-Paatero et al., 2012). The
air filters sampled in Rovaniemi in April–June and July–December 1986 seem
to have elevated mass ratios, 0.278±0.093 and 0.254±0.073,
respectively, but with consideration of their high measurement
uncertainties, these post-Chernobyl ratio values might be close to the
global-fallout ratio of 0.18 after all.
In a similar manner to the refractory-element emissions from the Chernobyl
accident, the fuel particles released from the Fukushima accident have a
significantly higher mass ratio of 240Pu/239Pu than the global-fallout value of 0.18. Dunne et al. (2018) have compared the mass ratios of
240Pu/239Pu in soil, sediment and vegetation samples collected at
the surroundings of Fukushima with the known mass ratios in global fallout
and in the destroyed nuclear reactors of the Fukushima Daiichi NPP. The mass ratio of
240Pu/239Pu for the Fukushima reactor units was obtained using
ORIGEN (Oak Ridge Isotope GENeration) code, being 0.344 for Reactor 1, 0.320 for Reactor 2 and 0.356 for
Reactor 3, respectively (Nishihara et al., 2012). All investigated
environmental samples from the proximity of Fukushima had
240Pu/239Pu atom ratios between the global-fallout value and the
value for the Reactor 3 calculated by ORIGEN, with the exception of one
deviating value (Dunne et al., 2018).
The same study highlighted that the concentration level of Pu isotopes and
the mass ratio of 240Pu/239Pu varies greatly in the environment of
Fukushima and that they do not necessarily correlate with each other. The
lowest mass ratio values in Fukushima have also been at the global-fallout
level. Other Fukushima-related investigations have also noted this variety
of isotope concentrations and isotope ratios. In a litter and soil sample
set collected 20–32 km from Fukushima, three samples had high 241Pu
concentrations and mass ratios of 0.303–0.330 that can be considered as
representing contamination from the destroyed reactors of Fukushima (Zheng
et al., 2012). The rest of the soil and litter samples from the proximity of
Fukushima in Zheng et al. (2012) had low 241Pu concentrations, and the
240Pu/239Pu mass ratios were at the Northern Hemisphere global-fallout level. In another study, the air filter samples collected at 120 km
from Fukushima formed two groups: one having a low 239Pu concentration
and a fairly similar mass ratio to global fallout (0.141±0.002) and
another having a high 239Pu concentration and a mass ratio clearly
deviating from global fallout (≥0.3) (Shinonaga et al., 2014).
The 240Pu/239Pu mass ratio was only 0.145±0.091 in the
surface air of Rovaniemi in the year of the Fukushima accident, 2011. Again,
the activity concentrations of both 239Pu and 240Pu were extremely
low in Rovaniemi in that year, and the uncertainty of the mass ratio is
therefore high, suggesting that the ratio value in 2011 is probably due to
global fallout.
239+240Pu/137Cs activity ratio
The activity ratio of 239+240Pu/137Cs varied between 0.0005±0.0001 and 0.0393±0.0038 in the surface air of Rovaniemi in
1965–2011, excluding the samples of April–December 1986 and 2011, when the
concentration of 239+240Pu fell below the detection limit (Table 2).
The lowest value for the activity ratio occurred in 2006–2010, when the
activity concentration of both radionuclides (239+240Pu and 137Cs)
in the surface air had been constantly decreasing for decades. The range of
the values in Rovaniemi is in agreement with the previous studies of surface
air in Finland. The activity ratio of 239+240Pu/137Cs was
0.0020±0.0008–0.029±0.010 in Sodankylä in 1963
(Salminen-Paatero and Paatero, 2012) and 0.005±0.002–0.012±0.004 (range of annual mean values) in Helsinki (southern Finland) in
1962–1977 (Jaakkola et al., 1979).
Bossew et al. (2007) have calculated the reference values for the
239+240Pu/137Cs activity ratio in global fallout and the Chernobyl
accident, obtaining 0.0180±0.0024 (data from Bunzl and Kracke, 1988)
and 6.6×10-6 (data from Irlweck and Khademi, 1993), respectively. The
values for Rovaniemi are higher than those for Chernobyl contamination, and
some values for Rovaniemi are even higher than the value for global fallout.
In contrast with high 239+240Pu/137Cs ratio values in the surface
air of Rovaniemi and in global fallout, very low 239+240Pu/137Cs
activity ratios have been observed in the Fukushima environment. Among all
litter and soil samples of Fukushima in the study by Zheng et al. (2012),
the three samples that represent the Fukushima-derived contamination, i.e.,
have both a high 241Pu concentration and high 240Pu/239Pu mass
ratio, had the 137Cs/239+240Pu activity ratios of 4×10-8, 2×10-7 and 5×10-6 in 2011.
Activity ratio of total beta activity to 239+240Pu
The ratio between total beta activity (Salminen-Paatero et al., 2019) and
239+240Pu remains rather constant during the atmospheric-nuclear-testing era (Fig. 6). The ratio reflects the produced nuclide composition
after fission and activation reactions in the detonating devices. Following
the Chernobyl accident, the ratio increases by almost 3 orders of
magnitude. After the initial explosion plume, the emissions from the burning
reactor were dominated by volatile fission products, which explains the high activity ratio of
total beta activity to 239+240Pu. After the decay of
short-lived fission products, the ratio soon returns to near the
pre-Chernobyl level. Towards the end of the 20th century, the ratio
starts to gradually increase. This is explained by the decreasing amount of
plutonium in the atmosphere, while the total beta activity remains on a
constant level due to natural atmospheric radioactivity, mainly 210Pb.
The ratio of total beta activity (Salminen-Paatero et al., 2019) and
239+240Pu activity content in the surface air in Rovaniemi in
1965–2011. 239+240Pu values below the detection limit have been
replaced with half the MDA values (Table 1).
Effect of actual and hypothetic nuclear detonations on the
surface air of Subarctic Finland
At least two new nuclear facilities in or close to the European Arctic
region are under preparation. The construction of infrastructure for a new
nuclear power plant at Pyhäjoki, western Finland, has begun. The
Shtokman natural gas field is located in the Barents Sea between northern
Finland and Novaya Zemlya. The plans indicate that future gas extraction
production facility will be powered by a floating nuclear power plant. The
atmospheric dispersion of plutonium contamination in the event of accidents
in these future plants was assessed with atmospheric transport modeling. In
this study, ADM (atmospheric-dispersion modeling) provided risk estimates
and reference contamination levels related to future nuclear activities in
and close to Arctic regions, which can be compared with earlier actual
releases.
241Pu dispersion in the atmosphere was simulated with the SILAM (System for Integrated modeLling of Atmospheric composition) model
(Sofiev et al., 2006, 2008). The model runs were based on the meteorological
forecast data of the European Centre for Medium-Range Weather Forecasts
(https://www.ecmwf.int, last access: 6 April 2020) with a horizontal resolution of 0.25∘ and with nine
vertical levels up to a height of 7700 m. Transport and dispersion
calculations for both sites were made for each day in the year 2010. The
average activity concentrations of 241Pu in the surface air during the
first 48 h after the release were calculated.
The following accident conditions, previously listed in Paatero et al. (2014), for the Pyhäjoki reactor at 64∘32′ N, 24∘15′ E were used:
a pressurized water reactor with thermal power of 4000 MW;
the end of the refueling interval;
an immediate release after shutdown with an effective release height of
200 m a.s.l.; and
a 241Pu inventory of 6.2×1017 Bq, a release fraction of 0.1 %
and a release of 6.2×1014 Bq.
The following accident conditions for the case of the Shtokman gas field in the
Barents Sea at 73∘ N, 44∘ E were used (previously used by
Paatero et al., 2014):
an ice breaker reactor with a fuel burn-up rate of 466 000 MW d T-1 HM (heavy metal);
an immediate release 2 h after shutdown;
a radionuclide inventory according to Reistad and Ølgaard (2006);
an effective release height of 100 m a.s.l.; and
a 241Pu inventory of 3.2×1014 Bq, a release fraction of 0.2 %
and a release of 6.4×1011 Bq.
Varying meteorological situations have a decisive effect on atmospheric
plutonium transport following accidental emissions from a nuclear reactor.
The wind direction determines the path of the emission plume. The wind speed
sets how quickly the emission plume is advected. However, the wind speed
also affects the turbulence that disperses the plume vertically and
horizontally. This influences the plutonium concentrations in the air.
Precipitation, for its part, efficiently brings plutonium-bearing particles
from the atmosphere to the surface, which affects the deposition of
plutonium and furthermore its transfer to food webs.
From the Rovaniemi region perspective, the worst of the calculated 365
dispersion cases would have caused an average 241Pu activity
concentration of less than 1 kBq m-3 in ground-level air in the first
48 h after the release (Fig. 7). This equals an annual average
241Pu exposure of 5 Bq m-3. For comparison, the atmospheric
nuclear tests caused the 241Pu activity concentration to vary between a
few dozen and some 1700 µBq m-3 in 1963 in
northern Finland or in other words several orders of magnitude lower
(Salminen and Paatero, 2009). In practice, the human exposure to 241Pu
via inhalation would remain on a clearly lower level because the civil-defense
authorities would order the population to stay indoors with ventilation
systems turned off and doors and windows sealed.
The average activity concentration of 241Pu in the surface air
during the first 48 h after a hypothetical reactor accident at
Pyhäjoki, with an assumed release of 20 January 2010.
Compared with the Pyhäjoki accident scenario, the consequences after a
hypothetical accident in a floating nuclear reactor in the Barents Sea would
be much less significant from the northern Finnish perspective. This would
be due to smaller emissions, a greater distance and favorable climatic
conditions, namely prevailing westerly and southwesterly winds. Only one
dispersion calculation of 365 produced an atmospheric transport pattern that
reached the northernmost part of Finland (Fig. 8). The ground-level
241Pu activity concentrations would have been less than 0.01 Bq m-3 in the first 48 hours, corresponding to an annual average
concentration of 55 µBq m-3. This is similar
to the activity concentrations occurring in the early 1960s.
The average activity concentration of 241Pu in the surface air
during the first 48 h after a hypothetical accident in a floating
reactor at the Shtokman natural gas field, in the Barents Sea, with an assumed release 5 May 2010.
Case of Fukushima 2011 and 241Pu
An earlier work by Paatero et al. (2012) observed that the SILAM model
simulates the temporal behavior of the Fukushima emission plume in the High
Arctic well. The calculated activity concentration levels, however, were an
order of magnitude lower than the observed ones. This deviation was
attributed to inaccuracies in the source term. From the same model dataset,
the 137Cs activity concentration in the surface of Rovaniemi was
extracted. The level of these values was then corrected by adjusting them to
the observed weekly 137Cs activity concentration of 170 µBq m-3 between 28 March and 4 April 2011
(Salminen-Paatero et al., 2019). From these values, the 241Pu activity
concentrations were obtained by multiplication with the 241Pu/137Cs
activity ratio of 7.81×10-6. This activity ratio was found in hot
particles close to the Fukushima Daiichi NPP by Igarashi et al. (2019). The
calculated hourly 241Pu activity concentration reaches a maximum level
of 0.01 µBq m-3 for two short periods (Fig. 9).
The calculated peak activity concentrations are very low, 6 orders of
magnitude lower than daily 241Pu activity concentrations observed in
northern Finland in 1963 (Salminen and Paatero, 2009). However, there is a
discrepancy between this assessment and the annual observed 241Pu
activity concentration of 0.6 µBq m-3 (Fig. 2).
If we assume that the background 241Pu activity concentration due to
the atmospheric nuclear tests and the Chernobyl accident were 0.03 µBq m-3, then the average activity concentration should
be 9.3 µBq m-3 between 27 March and 17 April: in
other words, 1000 times higher. An obvious explanation is that the
241Pu/137Cs activity ratio we used (7.81×10-6) is not valid.
The value may not be representative of the bulk emission mixture of the
destroyed reactors. Zheng et al. (2012) found out that the
137Cs/239,240Pu activity ratio in environmental samples varied
over 4 orders of magnitude. In addition, the hot particles were found
close to the source, and fractionation processes were possible during the
atmospheric transport of over 10 000 km.
Modeled hourly 241Pu activity concentration (µBq m-3) in the surface air of Rovaniemi in March–April 2011.
Conclusions
Based on the activity concentrations of 238,239,240,241Pu, hardly any
refractory elements from the exploded Chernobyl reactor reached Finnish
Lapland in 1986. Previously Chernobyl-derived 137Cs, a more volatile
isotope, has been detected from the same air filter samples, whereas there
was no increased concentration of 90Sr in the samples after March 1986.
The influence from the Fukushima Daiichi accident has seen an increased
concentration of 241Pu in the air filters. Nuclear-weapon testing in
the 1950s and 1960s, later nuclear tests on 1973–1980, the SNAP-9A satellite
accident in 1964, and the Fukushima accident in 2011 have been the main
sources of Pu in the surface air in Finnish Lapland during 1965–2011.
Overall, the mass ratio of 240Pu/239Pu is a more sensitive
contamination source indicator than the activity ratios of
238Pu/239+240Pu or 241Pu/239+240Pu because of the lower
detection limit of ICP-MS, compared with alpha spectrometry and LSC.
However, it is always useful to analyze more than one isotope ratio or
activity ratio and single isotope concentrations when characterizing the
origin of Pu contamination. In this case, the contribution of the Fukushima
accident in Rovaniemi would not have been observed without analyzing the
concentration of 241Pu in the air filter samples.
Dispersion modeling results with the atmospheric-dispersion model SILAM
indicate that Pu contamination in northern Finland would be negligible due
to a hypothetical accident in a floating nuclear reactor at the Shtokman
natural gas field in the Barents Sea. The Pu contamination risk would be
higher in the event of a severe accident at the intended nuclear power plant
at Pyhäjoki, western Finland, due to the larger, closer reactor. The
modeling of the Fukushima case demonstrated the importance of accurate
source term data for predicting the activity concentrations of the
radionuclides in the air following an atmospheric release of radioactivity.
Data availability
Data will be available in the University of Helsinki open-data system (https://www.helsinki.fi/en/researchgroups/radioecology, last access: 12 May 2020).
Author contributions
SSP performed radiochemical analysis and data analysis.
JV produced SILAM calculations. JP provided the air
filter sampling and sampling data and planned the accident scenarios. All
authors contributed to the writing of the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We acknowledge Emil Pesonen's help in cutting the air filter samples before
ashing and Ilia Rodushkin's (ALS Scandinavia Luleå laboratory) help in
measuring the Pu samples with ICP-MS. This is associated with the “Collaboration
Network on EuroArctic Environmental Radiation Protection and Research
(CEEPRA)”. The project was funded by the EU Kolarctic ENPI CBC 2007–2013 program
managed by the Regional Council of Lapland. The authors would like to thank
the EU project “TOXI Triage” (project ID 653409) for additional support.
Financial support
This research has been supported by the H2020 Future and Emerging Technologies (TOXI-triage; grant no. 653409) and the EU Kolarctic ENPI CBC 2007–2013 program (CEEPRA).Open access funding provided by Helsinki University Library.
Review statement
This paper was edited by Manvendra K. Dubey and reviewed by two anonymous referees.
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