Available via license: CC BY 4.0
Content may be subject to copyright.
Vol.:(0123456789)
Journal of Radioanalytical and Nuclear Chemistry
https://doi.org/10.1007/s10967-024-09453-2
Establishing thecase foraMay 2010 low‑yield, unannounced nuclear
test inNorth Korea
Lars‑ErikDeGeer1 · ChristopherM.Wright2· LennartRobertson3
Received: 27 June 2023 / Accepted: 1 March 2024
© The Author(s) 2024
Abstract
New data, analyses and modelling are presented concerning the unprecedented mid-May 2010 series of fission product detec-
tions in ground level air on and around the Korean Peninsula. For the first time Ba-140 is revealed at Ussuriysk, for which
only La-140 had been reported. Thus aerosol particles containing the same parent-daughter pair Ba-140/La-140 were detected
at both Ussuriysk and Okinawa, establishing beyond reasonable doubt that their physical, spatial and temporal origins are
the same. Together with Ce-141 and Cs-137, all with short-lived xenon isotope parents, a supercritical fission excursion,
which experienced a near prompt filtered vent, is the only viable scenario for their explanation. New modelling suggests
that the vent occurred around 9s after the excursion and that the CTBT-relevant xenon isotopes Xe-133 and Xe-135 were
‘quenched’ around 25min later and released some 10–20h afterwards. Published corroborating seismic and infrasound data
of an event at the North Korean nuclear test site 8min and 45s past midnight on 12 May 2010 is subsequently reviewed.
These papers adopted a conventional depth of the event although the data suggested a shallower one. Despite arguments in
the seismic community about its exact nature, it is prudent to test how well the waveform signals marry the radionuclide
detection pattern. Thus the location and time are input into a new atmospheric transport model. The advanced software suite
MATCH was used in forward mode with prompt and delayed releases, revealing the presence of plumes at each detection
site at the time of their first detection and extending over the observed timeframe. Thus a very consistent picture of a shal-
low low yield nuclear test is obtained.
Keywords May 2010 nuclear testing in North Korea· Barium-140· Co-added gamma spectra· Xenon-133/135· Seismic
and infrasound detections· Atmospheric transport modelling (MATCH)
Introduction
On 15 May 20101 the Okinawa radionuclide filter station
of the International Monitoring System (IMS) of the Com-
prehensive Nuclear-Test-Ban Treaty Organization (CTBTO)
detected the radioactive fission product (FP) isotopes bar-
ium-140 and lanthanum-140 in aerosol filters. The detec-
tions were to continue daily for the next week. In some sam-
ples sent to laboratories the FP radionuclides cesium-137
and cerium-141 were found as well (see Table1 for a sum-
mary of all radionuclides that are dealt with in this paper).
Also beginning 15 May 2010, the Ussuriysk IMS station
in Russia reported detections of lanthanum-140, on 15, 17
and 18 May. These FP nuclides in isolation were, and to the
authors’ knowledge remain, unprecedented over the period
of operation of the IMS.
* Lars-Erik De Geer
ledg1945@gmail.com
Christopher M. Wright
wrightcm66@gmail.com
Lennart Robertson
lennart.robertson@smhi.se
1 Retired fromtheSwedish Defence Research Agency
andthePreparatory Commission fortheComprehensive
Nuclear-Test-Ban Treaty Organization, Flädervägen 51,
SE-19464UpplandsVäsby, Sweden
2 School ofScience, UNSW Canberra, PO Box7916,
Canberra, BC2610, Australia
3 SMHI, Swedish Meteorological andHydrological Institute,
SE-60176Norrköping, Sweden
1 Unless otherwise noted all time references in this paper are given in
the Universal Time Coordinated (UTC) system.
Journal of Radioanalytical and Nuclear Chemistry
Further, on 13 May 2010 the Republic of Korea’s (South
Korea’s) national noble gas (NG) station at Geojin, just
10km from the border with the Democratic People’s Repub-
lic of Korea (North Korea) in the NE corner had already
detected the FP isotopes xenon-133 and xenon-135. The
presence of the latter was highly unusual and its level, and
ratio to Xe-133, unprecedented at that site. At the IMS
NG station at Takasaki in Japan one or more of Xe-131m,
Xe-133m, Xe-133 and Xe-135 (often referred to as radioxe-
nons) were detected between 15 and 19 May 2010, but these
data carry less weight here due to the Takasaki station’s
proximity to nuclear installations from which radioxenons
are quite frequently detected.
Papers published in 2012 and 2013, hereafter referred
to as DG12 and DG13, postulated that a low yield under-
ground nuclear explosion (UNE) conducted in North Korea
was responsible for the detections, via near-prompt and
delayed vents of radioactive noble gases[1, 2].2 The UNE
hypothesis was based on several facts, but most principally
that all the detected radionuclides were either noble gas iso-
topes or progeny of noble gas isotopes. The fission products
Xe-137, Xe-140 and Xe-141—precursors of Cs-137, Ba/
La-140, and Ce-141 respectively—have the common prop-
erty that they have substantial independent production yields
in fission of uranium and plutonium and that their half-lives
are all very short, a bit shorter than two seconds up to a
few minutes. Further, xenon being a chemically inert and
non-condensable gas in a UNE cavity, it can much easier
than less volatile species escape through an overburden, nar-
row cracks or some less effective filter arrangement.3
The Ba-140 and its daughter La-140 were not in full secu-
lar equilibrium, meaning the fission event must have been
recent, certainly less than a week before the first detections.
Using their ratio with its one sigma uncertainty as a clock
the ‘zero time’ was calculated to be between the midnights
of 10 and 12 May 2010.4 The midpoint was 06:00 on 11
May; an estimate that was soon, inspired by a Finnish group
[3], refined in DG13 and moved eleven hours forward to
around 17:00. Assuming a common origin, the Xe-135
detection at Geojin also established that the fission event
had to be recent, given its relatively short half-life of 9.14h
as compared to the 5.25days half-life of the other detected
xenon isotope Xe-133.
Atmospheric Transport Modelling (ATM) showed that
in all cases the air masses carrying the radionuclides had
passed over northern North Korea, indicating a spatial origin
within that State’s borders and including its nuclear test site
some 20kmNW of the little village Punggye-ri. The implied
source term showed that the fission event could not have
been a mere criticality excursion at a research or develop-
ment laboratory, nor could it emanate from a power, research
or production reactor. The most likely physical mechanism
behind the detections was a UNE.
The DG12 paper, however, experienced a mixed recep-
tion. As subject of reviews in the journals Nature and Sci-
ence, several well-known figures in the nuclear non-prolif-
eration community were interested but non-committal [4, 5].
One authoritative source stated that the “analysis provides
convincing evidence of some kind of nuclear fission explo-
sion”. But the community was in general unconvinced. The
lack of a seismic signature was seen as a weakness, despite
the non-detection being accounted for in DG12’s-and later
others-scenarios due to this UNE’s low yield and potentially
decoupled nature. Due to initial problems in fitting the Geo-
jin radioxenon signature to a single explosion, a scenario of
two UNEs about a month apart was suggested in DG12. The
idea was obviously too much inspired by an official North
Korean statement in the early hours of 12 May 2010 say-
ing that successful fusion experiments had been carried out
nearly a month earlier on 15 April “the Day of the Sun”, the
Table 1 Particulate and xenon radionuclides in the current case with
full names, abbreviated names, half-lives (truncated to three signifi-
cant digits) and roles played
Radionuclide Abbreviation Half-life Role played in the case
Barium-140 Ba-140 12.8days Primary detection
Lanthanum-140 La-140 1.68days Primary detection
Cerium-141 Ce-141 32.5days Lab. detection
Cesium-137 Cs-137 30.1years Lab. detection
Xenon-131m Xe-131m 10.8days Irrelevant detection
Xenon-133m Xe-133m 2.20days Irrelevant detection
Xenon-133 Xe-133 5.25days Delayed vent
Primary detection
Xenon-135 Xe-135 9.14h Delayed vent
Primary detection
Xenon-137 Xe-137 3.82min Near-prompt vent
Cs-137 precursor
Xenon-140 Xe-140 13.6s Near-prompt vent
Ba/La-140 precursor
Xenon-141 Xe-141 1.73s Near-prompt vent
Ce-141 precursor
2 These papers also give details of the original RN data that this
paper’s conclusions are based on.
3 Many radioactive isotopes of the noble gas krypton are also pro-
duced in fission and will also be part of a NG release. There is, how-
ever, no krypton sampling in the CTBT network and there is no par-
ticulate krypton progeny nuclide with long enough half-life combined
with an intensive enough gamma ray to be detected by the IMS sys-
tem.
4 In this paper the term “midnight” refers to “beginning of day” as
prescribed by the International Organization for Standardization in
2019. (ISO 8601-1-2019).
Journal of Radioanalytical and Nuclear Chemistry
official holiday celebrating the birth of the founder of North
Korea Kim Il-Sung. That could explain the observed iso-
tope ratio, but in response to the critics of the two-explosion
hypothesis, a paper was published in 2013, hereafter WR13,
that showed that a more realistic one-explosion model was
enough if xenon fractionation effects were taken into account
[6]. This was independently confirmed in DG13, where the
first version of Xebate, a code written in Mathematica® and
used to show that the Geojin data could be well replicated
by a simplified assumption that all refractive iodine precur-
sors in the 133 and 135 mass chains were trapped in the
condensing and solidifying rock lava at a certain time post
explosion. After that the ingrowth from iodine continued
up till an operational radioxenon release a good number of
hours later. This is further discussed in "Xenon decay chain
dynamics and fractionation" section.
The signal strength was lower at Ussuriysk than at
Okinawa and barium-140 was not reported by the CTBTO
from the former site. That caused some doubts whether the
lanthanum-140 detected at both sites had a common origin.
We now show in "Barium-140 in the Ussuriysk IMS spec-
tra" section, by co-adding three spectra with lanthanum-140,
that actually also barium-140 was present at Ussuriysk.
A couple of renowned seismologists published a paper
that argued against an unannounced UNE in North Korea,
based on that no relevant seismic signal had been observed.
In 2015, however, there was a breakthrough when a Sino-
American group actually found a clear seismic signal 8min
and 45s past midnight on 12 May 2010 at a station in China
just about 100km from NKTS. The signal was then also
confirmed by two other well-reputed seismic groups that
analysed data from stations some 200km away, with lower
signal-to-noise ratios. One group then claimed these signals
were typical for quakes and not explosions. This and other
waveform observations are dealt with in "Comments on the
potential seismic and infrasound evidence" section.
Many backward and forward ATM studies by multiple
authors employed different software to test the proposed
North Korean origin. In one of those, WR13, the publicly
available HYSPLIT code was used and with two different
meteorological data sets it was indicated that the North
Korean nuclear test site (NKTS) near Punggye-ri was the
most likely origin. Several other publications and conference
poster presentations by other ATM experts using independ-
ent techniques gave results that in general were consistent
with those of DG12 and WR13 [7–10]. In detail, however,
all failed to replicate the combined detection patterns of
both Okinawa and Ussuriysk. But no forward analysis had
before been based on the seismic time stamp at midnight
12 May. That became our next step and the results are pre-
sented in "ATM based on the NKTS waveform time stamp"
section.
Although we argue that the detected radionuclide signa-
tures were clear signs of a filtered nuclear explosion WR13
set out to meet some critics´ suggestions that it could have
been leaks from a regional reactor. But candidate nuclear
reactor sites spanning the length and breadth of Japan, along
with those in South Korea, China, Taiwan and Russia, were
found to be unsuitable from an ATM point of view. This was
especially so if all the detections, but particularly those at
Okinawa and Ussuriysk, were related to a common single
event, something the new spectrum analyses reported in this
paper show to be all but indisputable. In "Anything else than
an explosive fission event?" section we also look shortly at a
quite far-fetched idea of what could have caused the detected
signatures.
Xenon decay chain dynamics
andfractionation
The interest in xenon isotopes gained momentum when sen-
sitive radioxenon surveillance became part of the CTBT ver-
ification regime in the mid-1990s, but also by an experiment
in 1993 with a simulated 1kt5 UNE 400m underground
at the US Nevada Test Site. Noble gas transport through
faults and fractures were studied with helium-3 and sulphur
hexafluoride as substitute gases [11, 12]. In recent years
many theoretical and experimental studies have been pub-
lished that have dealt with subsurface transport of radioac-
tive debris and interactions between different compartments
such as cavity, melt puddle and host rock. The goal has been
to understand the processes and be able to make realistic
interpretations of especially radioxenon isotope ratios. These
have applied to vents, operational releases as well as On Site
Inspections [13–20].
Back in 2013 the Mathematica® notebook Xebate was
written to model xenon decay chain dynamics and then
from the measured Xe-135/Xe-133 activity ratio estimate a
quenching (or rainout) time during early cooling when the
refractive iodine precursors condense and thus stop feeding
the volatile iodine and xenon isotopes (DG13, WR13). As
the containment and other setup parameters of the May 2010
event were, and still are, basically unknown this is necessar-
ily a simplification, so the result should more be seen as an
equivalent quenching time that provides a reasonable cause
for the observed fractionation between the two isotopes.
The Mathematica® notebook Xebate‑3
Xebate has been extended and improved and is now named
Xebate-3. The code can be found in Online Resource 1. It
5 1.3 kt chemical explosives.
Journal of Radioanalytical and Nuclear Chemistry
covers all decay chains that include a xenon isotope relevant
for CTBT verification. Those are masses 131, 133 and 135
prone for delayed releases and 137, 140 and 141, which
need a very prompt release to set a mark via their respec-
tive particulate progenies Cs-137, Ba/La-140 and Ce-141.
Xebate-3 is preloaded with nuclear data from most current
sources available, cumulative and independent fast neutron
fission yields from JEFF-3.3, half-lives and branching fac-
tors from metastable states from NuDat-3.0, branching ratios
to metastable states from ICRP publication 107 and some
data on delayed neutrons from IAEA Live Chart of Nuclides
[21–24]. All six chains with all data noted are shown in
Online Resource 1, with one of them, 133, also here as
Fig.1.
In the collected data, chain members with half-lives less
than 1s are truncated and their independent fission yields
are added to the next member in the chain. In the current
calculation the limit was extended to one minute for the
radioxenon chains 131, 133 and 135. The notebook uses the
Bateman equation [25] that gives the number of nuclides as
a function of time for the parent and all daughter nuclides in
a straight chain starting with the number of parent nuclides
(here the fission yield in percent). The radioxenon chains
are a bit complex with many routes of straight decay, 14
for the 131 chain, followed by 20, 4, 5, 4 and 4 for the 133,
135, 137, 140 and 141 chains. The partial results are then
multiplied by all branching ratios along the partial chain, and
finally all partial results are added for each nuclide.
Some illustrative plots fromXebate‑3
Xebate-3 is easy to modify to make calculations and produce
all kinds of plots based on the preloaded six xenon chain
data. In Fig.2 the evolution of all seven relevant xenon iso-
topes and metastable states are plotted between 3.6s and
nearly 6weeks (0.001–1000h). There one can clearly see the
separation in time between the short-lived xenons and the
radioxenons with a delineation line around 15min.
Figure3 shows the evolution of the seven xenon related
detectable nuclides expressed inμBq per fission. Here we
can see that Ba/La-140 and Ce-141 are good messengers for
the IMS filter stations up to weeks or longer while the time
Fig. 1 Mass 133 decay chain. Half-lives just below the nuclide sym-
bols, fast neutron induced fission yields in % just above, with blue on
top for Pu-239 and red below for U-235 fission. The box on the right
side gives the chain yields in the same format. Branching ratios in %
are shown on the decay lines when not 100%. The nuclide symbols
are colour coded for half-life: Yellow 1–10s, red 10 s–1 m, purple
1–10m, blue 10 m–1 h, green 1 h–6 h, black 6h–1000 y and white
1000 y–stable. The five other chains are shown in Online Resource 1
Fig. 2 The evolution of CTBT xenon isotopes with time after fission,
blue lines for plutonium-239 and red for uranium-235
Fig. 3 A loglog illustration of the evolution of the CTBT xenon
related and IMS detectable nuclides with time after fission (Pu-239
blue and U-235 red)
Journal of Radioanalytical and Nuclear Chemistry
window for the Xe-135/Xe-133 ratio has the much shorter
time window of a few days after fission.
Note that one has to be careful when comparing the two
groups as their release fractions reasonably differ and the
detection limits for them differ typically three orders of
magnitude in favour of the particulates. To illustrate this
more clearly Fig.4 shows the data from Fig.3 on a linear
scale and with the xenon progeny particulates and theradi-
oxenonsseparated in parts a and b. We can see that the figure
is totally dominated by the nuclides that built the May 2010
signature.
Xebate‑3, theBa‑140/Ce‑141 ratio andthetime
ofthenearly prompt release
In DG13 a release of short-lived xenon-isotopes at one
instant shortly after fission was assumed and then the
Ba-140/Ce-141 ratio in the 15 May Okinawa sample implied
that it happened at 7.8s (if Pu) or 9.4 s (ifU) after fis-
sion (both with 1 σ uncertainties of ± 0.9 s). Xebate-3
gives almost the same results, 8.9s (if Pu) or 10.3s (ifU),
both with 1σ uncertainties of ± 1.5s. We now also note in
Xebate-3 that the data does not comply with any scenario
where the leak actually starts immediately after detonation
as a strictly prompt leak and then goes on for several sec-
onds. Irrespectively of this we mostly use the conventional
terminology and refer to a quite early leak as a prompt
one.See also Online Resource 1.
Cs‑137 inthe15 May sample fromOkinawa
In Fig.3 one can see that in an unfractionated mass of fis-
sion products the Cs-137 activity is some 800 times smaller
than the Ba-140 one between 15min and one week after
fission. That time span covers the 3.5days Mid Of Sampling
(MOS) time of the strongest fresh particulate fission product
sample of the episode, the 15 May one at Okinawa, which
yielded 81.9μBq/m3 of Ba-140. That implies a Cs-137 sig-
nal of about 81.9/800 = 0.1μBq/m3, well below the detec-
tion limit requirements for the IMS stations. But samples
that show at least two CTBT-relevant radionuclides are
split and sent to two of the 16 CTBT certified labs to look
for longer-lived nuclides with higher sensitivity gained by
reduced background and primarily longer counting times.
The laboratories that analysed the 15 May sample then
reported 0.45 ± 0.03 and 0.44 ± 0.24μBq/m3 of Cs-137, very
well agreeing with the Xebate-3 result for a xenon release
at 9s post fission of 0.45μBq/m3 if plutonium fission is
assumed. Due to a significantly different yield population
in the early mass 137 chain the value is nearly three times
lower, 0.17μBq/m3, for uranium fission.
If we assume there was no contamination, e.g. from
24years old Chernobyl fallout via resuspension or wildfires
this analysis suggests that the May 2010 device used pluto-
nium fuel. The detection limit for Cs-137 at the Okinawa
station is around 2μBq/m3 so many concentrations below
that could have escaped detection during the years. But if
resuspension/wildfires were a common source of Cs-137
at Okinawa below the detection limit there should reason-
ably have been more cases also above. But among the more
than 1500 samples measured since the certification of the
Okinawa station in 2006 up to the Fukushima accident in
2011, Cs-137 was only detected in 20 cases, mostly at levels
of a fewμBq/m3. Two of the 20 detections occurred, how-
ever, at levels of 2–3μBq/m3 two weeks before the 12 May
event. Pu-fuel is thus hinted but not guaranteed.See also
Online Resource 1.
Xebate‑3, theXe‑135/Xe‑133 activity ratio andthetime
ofequivalent quenching
As mentioned above, Xebate-3 has now also revis-
ited the analysis in DG13 where the measured Xe-135/
Xe-133 activity ratio at Geojin on 13 May (10.01 ± 0.6)/
Fig. 4 Linlog version of Fig.3 with the xenon progenypaerticulateand theradioxenon groups separated in a and b
Journal of Radioanalytical and Nuclear Chemistry
(2.45 ± 0.2) = 4.1 ± 0.3 was used to estimate a postulated
quenching (or rainout) time, in a cooling underground cav-
ity, when the refractive iodine precursors condensate and
stop feeding the iodine and xenon isotopes. The two isotopes
were obviously fractionated, which in a UNE scenario can
be explained by quenching or an equivalent quenching.
Xebate-3 calculated the xenon isotope ratio as a function
of time and fissile material in two versions, one that assumed
full filtering of iodines during the operational release and
another that assumed no iodine filtering at all. The results
as shown in Fig.5 tell that neither fuel nor iodine-filtering-
at-release has any great impact on the estimated equivalent
quenching time. Considering this spread and the uncertainty
in the measured xenon isotope ratio the equivalent quench-
ing time is 25 ± 5min.6
In the calculations to compare the results depending
on full or nil iodine filtration at the operational release an
assumed release time is used. It doesn’t have to be very
exactly known. The atmospheric transport modelling
described in "ATM based on the NKTS waveform time
stamp" section several scenarios during 12 May are tested
for how the cloud hits Geojin during the sampling between
11 hand 23 hon 13 May. We select a release centred at 15h
on 12 May.
This section has dealt with a few points in time that are
relevant for the May 2010 event. A couple are just noted
others are derived from measurements and ATM. They are
summarized in Table2.
Barium‑140 intheUssuriysk IMS spectra
Since it has not been elsewhere described it is worth not-
ing the extremely high confidence with which Ba-140 and
La-140 are identified in all eight Okinawa aerosol samples,
and La-140 identified in at least three Ussuriysk samples. At
Okinawa 5 and 13 gamma ray lines of Ba-140 and La-140
respectively can be seen in the raw spectra. All are definite
detections because they appear with good statistical signifi-
cance above noise and are not present on the four days before
15 May and the week after 22 May. At Ussuriysk at least the
two strongest lines of La-140 are observed on 15, 17 and 18
May, and one or two others on one or another of those dates.
Fig. 5 Solving the equivalent quenching time assuming Pu-239 (blue)
and U-235 fuel (red) as well as including all (lower curves) or no
(upper curves) iodines in the release
Table 2 Significant time marks in the analysis of the 12 May 2010 event
Time UTC Occurrence Defined by
12 May 00:08:45 A low-yield fission explosion at NKTS Waveform detections discussed in "ATM based on the
NKTS waveform time stamp" section + ATM presented
in "Anything else than an explosive fission event?" section.
12 May 00:08:54 A prompt release of short-lived noble gases Measured Ba-140/Ce-141 activity ratio at Okinawa 15 May
gives 9.2 ± 2.2s after fission
12 May 00:34 Equivalent quenching Measured Xe-135/Xe-133 activity ratio at Geojin 13 May
gives 25 ± 5min after fission
12 May 15:00 A delayed operational release/ventilation of noble gases Best fit to ATM but given with a fairly long uncertainty as
15 ± 6h after fission
13 May 17:00 Mid Of Sampling (MOS) radioxenons at Geojin Regular sampling program 41h after fission
15 May 12:00 MOS particulates in the first and strongest sample at
Okinawa
Regular sampling program 84h (= 3.5days) after fission
6 If the 25 min equivalent quenching time is not just equivalent but
the actual surprsingly late rainout time it could reasonably tell some-
thing about the experimental design and setup.
Journal of Radioanalytical and Nuclear Chemistry
However, the CTBTO did not report Ba-140 in the
Ussuriysk IMS spectra, only La-140. This raises the quite
far-fetched possibility that the La-140 detected there was not
a decay product of Ba-140, and thus not necessarily a fission
product and not originating from a UNE. Instead it has been
implied that the La-140 could have been made by neutron
irradiation of natural lanthanum (99.9% La-139) in a nuclear
reactor [26]. On multiple instances in Central Europe and
Scandinavia during the 1980s, 1990s and 2000s, such sole
La-140 was detected in the air shortly after it had been used
in exercises at the Division Decontamination et Etudes de
Protection (DEP) at the Etablissement Technique de Bourges
(ETBS), a French military facility near the city of Bourges
[27, 28]. The maximum allowed La-140 activity dispersed
at DEP is reported to be 370GBq.7 The detected aerosol
activity concentrations 500–2000km away were of a similar
magnitude to those seen at Ussuriysk in May 2010.
To the authors’ knowledge sole La-140 had never previ-
ously been detected at Ussuriysk, nor since. So if nothing
else it would have been a one-off event, unlike the French
field exercises, which may even, have occurred or occur
annually. Thus, given the extreme rarity of sole La-140
detections worldwide, and the unprecedented nature of dual
Ba/La-140 detections outside of major nuclear reactor acci-
dents or known vented UNEs, it would be an extraordinary
coincidence for such to occur at the very same time in the
very same part of the world. Nevertheless, although highly
unlikely to be relevant to May 2010, the possibility was suf-
ficiently motivating to check the Ussuriysk spectra in more
detail.
Doing so it was seen that especially one sample, collected
during 17 May, showed the primary sign of Ba-140, being
its gamma-ray peak at 537.3keV. Two samples before and
after—on 15 and 18 May—also showed evidence for the
line. All three cases however had insufficient counts—com-
pared to channels on either side of the peak—to be identified
by the automated routine of the CTBTO International Data
Centre (IDC). Co-adding the three data sets would feasibly
enhance the signal-to-noise and provide more convincing
evidence of the detections.
The channel-to-energy conversion (calibration) of IMS
spectra drift on a day-to-day basis, indicated by the peak
position of ‘background’ lines changing by one or more
channels. This would degrade the quality of the co-add, so
the spectra had first to be aligned to a common energy scale.
Approximately 30 gamma-ray lines—spanning the energy
range 40–2700keV—could be confidently identified, arising
from the thorium or uranium series or other natural atmos-
pheric nuclear reaction processes. A separate calibration for
every date was constructed from these lines, and then each
data set interpolated onto a common energy scale before
being co-added. The resulting spectrum is displayed in
Fig.6a. It shows a clear signal, for which Gaussian fits using
three different baselines (constant, linear and quadratic) all
give a peak position of 537.3keV and a full width at half
maximum (FWHM) of 1.3keV.8 These parameters are in
very good agreement with expectation for the Ba-140 line.
Fig. 6 Ussuriysk spectra—counts versus energy in keV—with the
Ba-140 537.3keV primary gamma ray in the centre. a Aligned and
co-added spectrum from 15, 17 to 18 May 2010. b 17 May 2010 raw
data with the optimum SCA window and an estimated background
line marked. The vertical axes are counts per channel (cpc)
7 La-140 is well suited to such experiments as it has a relatively short
half-life of 1.68days, decaying to its stable daughter Ce-140 within
a week or so. Moreover, high purity La-139 (with an isotopic abun-
dance of 99.91%) can be irradiated in a reactor without producing
unwanted and potentially long-lived radioactive by-products of other
elements.
8 The FWHM is a measure of the spectral resolution of the equip-
ment.
Journal of Radioanalytical and Nuclear Chemistry
The peak area of around 340 counts on top of a background
of some 5800 counts yields an estimated signal-to-noise of
340∕√5800 ≈4.5
.
The formal (quantitative) decision-making is however
performed on data containing the strongest signal, i.e.
the data from the sample collected on 17 May. Figure6b
shows the optimum Single Channel Analyser (SCA) bin.
Its width is 2.0keV, the approximate expected spectral
resolution (FWHM) at 537keV of 1.60keV multiplied by
1.25 [29]. Expressed in channels it is 2.0 times the channel
tokeV ratio of 2.88, equalling 5.76 channels. The SCA thus
encompasses close to 6 channels in the spectrum. With the
background estimate in the figure that gives a gross signal
of 2247 counts and a background of 2006 counts, yielding
a net of 241 counts. This implies that accepting a 5% risk
for a false detection there is a critical9 (or decision-) level
of
1.645 ∗√2∗2006 =104
counts, whilst for 1% risk the
critical level is
2.326 ∗√2∗2006 =147
counts, which
indicates a detection even at a 1% risk. One can argue about
the background line estimate in Fig.6b, but if it is increased
by 10cpc (counts per channel) the peak is still detected at
1% risk. If it is increased by 20cpc the peak is detected at
5% risk, but not at 1%.
The 241 counts correspond to an air concentration
of 11.9 ± 4.3 (1σ) μBq/m3 of Ba-140 on 17 May 2010 at
Ussuriysk. Besides being a confirmed detection, this com-
pares very well numerically with the value 12.2 ± 2.3 (1σ)
in DG12 that was based on the detected La-140 concentra-
tions and assuming near equilibrium in the decay chain. The
data thus shows with very high confidence that Ba-140 was
present in the air at Ussuriysk during several days around
17 May 2010.
For absolute clarity it could, however, be useful to con-
sider other possible origins for the 537keV feature before
ruling them out. There is actually one isotope capable to add
counts to the 537keV bin in typical lead-shielded IMS par-
ticulate sample measurements. That is stable Pb-206, which
comprises around 24% of natural lead. Its first two states at
803.06 and 1340.53 keV can be excited by the ground-level
cosmic neutron background and then de-excite by emitting
an 803.06 keV gamma respectively a cascade of 537.47 ±
0.03 and 803.06 keV gammas [30].
The first thing to note is that the Pb-206 537.47keV
line is 0.21keV higher in energy than the Ba-140 line at
537.26 ± 0.01keV, whereas the co-add energy calibra-
tion scale is as good as ± 0.05keV. The peak position of
the observed feature in the co-added spectrum is defi-
nitely 0.15 ±0.03keV lower than 537.45keV, so on that
basis alone the Pb-206 line is unlikely to give a significant
contribution.
The most decisive argument, however, for the 537keV
gamma not being to a significant degree due to cosmic inter-
actions, is the one used in DG13, where a Finnish group was
cited that had co-added 285 Okinawa spectra from 2010 with
no detected anthropogenic content. There was a faint signal
around 537keV with a count rate of 18 counts in three days,
just 5% of the 340 counts in the May 2010 co-add. And there
is no reason that the cosmic neutron generated contribution
should be significantly different at Ussuriysk.
The fact that the co-add anyway has a clear 803keV peak
with about 400 counts is explained by Pb-210 and Po-210 in
the lead shield and in the sampled aerosols. They are at the
very end of the natural uranium-238 series where Po-210
finally decays by alpha emission basically directly to the
ground state of Pb-206. A tiny bit, however, takes a detour
via the 803keV first excited state in Pb-206 and renders
counts to a noticable peak in IMS spectra.
It is thus conclusively proven that also the La-140 at
Ussuriysk was a fission product—or in fact a decay product
of the ‘original’ fission fragment Xe-140. The scenario of
two unrelated events causing the Okinawa and Ussuriysk
signatures is thus excluded at a very high level of confi-
dence. Since the Ussuriysk and Okinawa sampling stations
are some 2000km apart it also strongly negates the possibil-
ity of a Ba-104/La source local to either IMS station.
Comments onthepotential seismic
andinfrasound evidence
The publication of the May 2010 radionuclide findings in
DG12, WR13 and DG13 inspired at least four groups to
carefully look for waveform traces of an event that could
potentially corroborate the clear radionuclide evidence of a
small nuclear explosion. Three groups published papers in
the 2015–2017 time frame that focused on seismic signals.
They were from the University of Science and Technol-
ogy of China in Hefei, Anhui, China cooperating with the
State University at Stony Brook in New York, USA, from
Lawrence Livermore National Laboratory in California,
USA and from Lamont-Doherty Earth Observatory in New
York, USA. In 2019 a fourth study focusing on possible
infrasound signals from the May 2010 event was published
by a German group at the Federal Institute for Geosciences
and Natural Resources in Hannover. We hereafter refer to
these four papers as ZW15, FW15, KR17 and KP19 respec-
tively [31–34]. In 2023 the authors of ZW15 presented new
results of a study, below referred to as ZW23, based on data
from the same network as used in KR17 [35]. Searches for
a seismic signal were done at stations that are/were parts of
five networks, a regional one in NE China, the IMS seismic
network, the GSN (Global Seismographic Network) and
two temporary networks, NECESS, the North East China
9 A net signal at the critical limit implies a detection decision with
the given risk of making an error.
Journal of Radioanalytical and Nuclear Chemistry
Extended SeiSmic array, which was operated between Sep-
tember 2009 and August 2011and DBSN, the Dongbei
Broadband Seismic Network, which was operated between
June 2004 and September 2010. In broad terms we can say
that the first was about 100km from NKTS, the two tempo-
rary ones at about 200km and the IMS and GSN stations at
about 400km.
First out was the Sino-American research group that had
access to the regional non-open Chinese network. They
concluded, inter alia from a station with code name SMT
105km from NKTS, that there was a signal from an event
that occurred at 00:08:45 on 12 May 2010 UTC with an
Lg-wave-derived magnitude ofm(Lg) = 1.44 ± 0.13.10 By
simultaneously cross-correlating its fairly clear phases with
those of the 2009 and 2013 announced nuclear tests, the
origin was estimated to be within NKTS, less than one km
roughly south of the locations of the 2009 and 2013 ones.
They inferred an explosive yield of 2.9 ± 0.8 toneq,11 assum-
ing a fully tamped explosion—i.e. perfectly coupled to the
surrounding rock—at a depth of 230m.
Soon the Livermore group replied that when they scru-
tinized the data from IMS and GSN, no signals in the time
frame defined by ZW15 were found. They also showed that
the detection thresholds should indeed have allowed such
detections in the ZW15 scenario. Two caveats were, how-
ever, given, a misslocation of several km or an overestimated
yield by a factor of at least three could actually explain
the IMS/GSN non-detections. We note then that ZW15´s
2.9 ± 0.8 toneq is not cut in stone as it is based on a burial
depth of 230m that was just taken as the elevation difference
between their localization point on the slope of Mount Man-
tap and the assumed north portal tunnel entrance (originally
named west portal but re-named the north portal in recent
literature after a new portal to its west was opened). Little is
actually known about the burial depth and it can easily have
been much less than what ZW15 assumed. With the formula
employed by ZW15 the explosive yield is calculated from
burial depth and Lg magnitude to be:
With m(Lg) = 1.44 ± 0.13 the FW15 upper limit of one
ton gives upper limits of the overburden of about 40, 60 or
90m (Fig.7. According to Google Earth these depths occur
about 130, 180 and 260m into the north portal tunnel that
is slowly running up to the workpoint of the 3 September
2017 test. That line is the one shown by North Korean offi-
cials to represent the tunnel in front of international press in
May 2018. Note that if the yield was e.g. 0.5 ton the over-
burden interval creeps down to 20–40m, corresponding to
60–130m from the tunnel entrance. This agrees quite well
with the relocation reported in ZW23 to a point “close to”
the north portal.
FW15 reported that they actually found a clear signal
from at least one NECESS station, NE3C, at the time given
by ZW15, but as NECESS had operated for just two years
and could not provide any template data from announced
nuclear tests at NKTS, it carried less information. A similar
signal was, however, found in the NE3C data for 6 June
2010. That raises of course a warning flag for false alarms
from e.g. construction or mining works in the area. The risk
is, however, softened by the fact that ZW15 detected no
other signal above their cross-correlation threshold (ZW15,
Fig.1b) during April and May than the midnight 12 May
one.12
After ZW15 the Lamont-Doherty group extended their
search to data from the Dongbei and NECESS networks.
Both covered the 12 May 2010 event but only Dongbei cov-
ered any of the nuclear tests announced by North Korea,
which gave or would have given signals that are prerequi-
sites for the cross-correlation techniques applied. KR17 then
reported detections, compatible with ZW15, at five elements
of the Dongbei network, where the best signal appeared at
a station 202km north of NKTS. The analysis using the
2009 announced explosion as a template gave a magnitude
of around 1.5, quite similar to the magnitude estimated by
yield in
ton
eq
=1000 ∙10
(m(Lg)+0.7875∙log
10
(burial dept h in m)−5.887)∕1.0125
Fig. 7 The burial depth/yield relation according to ZW15. The three
curves refer to the estimated m(Lg)-magnitude including its uncer-
tainty limits. The yield scale ends at one ton, which is the highest
possible value according to the FW15 analysis
10 Lg-waves are a particular type of seismic surface waves. Also Pg
waves will be referred to in this paper and that stands for body waves
that are longitudinal waves that travel in all directions. The g-index
in Lg and Pg stands for “granite” and marks waves that travel in the
crust of solid rocks.
11 The size of a nuclear explosion is most often expressed as the
mass of the conventional explosive trinitrotoluene or TNT that
releases the same amount of energy (1.162TWh/ton). In this paper
the units tonequivalent, toneq and just ton or t are used alternately. 12 Data gap 16:00 15 May + 24 h.
Journal of Radioanalytical and Nuclear Chemistry
ZW15. The preferred location in KR17 was, however, some
3 to 8km essentially southwest of the ZW15 one.
KR17 very much focused on whether the detected phases
were due to an earthquake or an explosion. For that purpose
they first developed a new, so called, Linear Discrimina-
tion Function (LDF), but they also used the same method as
ZW15. Both methodsplot the Pg/Lg spectral amplitude ratio
(expressed as Log10(Pg/Lg)), the former against the LDF-
value for a well-defined frequency band (6–9Hz in KR17)
and the latter against several discrete frequencies up to 18Hz
or more. Explosions generally show positive LDF and in the
May 2010 case it was negative, but less so than all (vertical-
component data) or three quarters (three-component data)
of the 12 quakes in the training set. In the Log10(Pg/Lg)/
frequency plots explosion curves run higher than the earth-
quake curves at higher frequencies. KR17 and ZW15 both
show plots of the 12 May event based on vertical-component
data from two Dongbei stations and SMT respectively. Com-
paring them it is obvious how they both run in the “channel”
above 9Hz between their explosion and earthquake train-
ing sets. From that ZW15 and KR17 reached almost dia-
metrically opposed conclusions, ZW15 that the 12 May 2010
event was an explosion and KR17 wrote”From our work,
there is at present still no explosive seismic event to associ-
ate with the known radionuclide anomalies reported by De
Geer (2012)”. The latter conclusion carried a heavy weight
in the community even though there is a somewhat mitigat-
ing formulation in their discussion section which reads “We
raise these questions to make clear that in this article we are
not stating there is rigorous evidence that the seismic event
of 12 May 2010 was an earthquake”.
As the seismic laboratories do not agree we cannot do
anything else at this stage than to judge a draw. But it moti-
vates us to check in the following section how well a forward
atmospheric dispersion model based on the seismic midnight
12 May time stamp replicates the radionuclide detection pat-
tern both in space and time.
We note that neither ZW15 nor KR17 considered that the
burial depth by no means was locked to 230m. For a very
small nuclear test the arrangements can be very different
than for a “standard” kt-sized UNE. One possibility could
e.g. be to use a heavy and strong steel chamber at ground
level or buried in an underground alcove of a tunnel or a
borehole like those quite frequently used in the past by both
the US and the USSR. In Soviet Union such a chamber was
called a KOLBA (Fig.8). A testing chamber of a realistic
size can withstand explosions of up to several tons13 and
it would reasonably have a filtered exhaust pipe ending at
ground level to rapidly release the pressure created by the
explosion. That could explain the infrasound signal that we
describe in the next paragraph. One good reason for North
Korea to do KOLBA-type nuclear testing in 2010 would
have been that their stockpile of fissile material at the time
was quite small, while they had imminent needs to test
nuclear explosion principles to climb the ladder of increas-
ing yields [36]. A solution could then have been to do very
low yield tests in a chamber where, after the test, plutonium
and/or uranium could fairly easily be recovered and recycled.
Note that a nuclear fission charge needs a minimum of sev-
eral kilogram of fissile material to reach explosive criticality,
while a very small explosion only consumes a tiny bit of that
(around 0.1g per toneq). Great resources can thus be saved
by doing very low-yield tests in steel chambers, especially
in the early stages of a nuclear weapons R&D program. The
May 2010 test could very well have been such a test and it
could also have been one of several more that were never
detected.
The debate about the 12 May 2010 event inspired KP19
to analyse available infrasound detections of this event and
all six announced nuclear tests in North Korea. Data were
collected from three regional stations, two being components
of the CTBT/IMS and one a national station in South Korea.
The IMS stations were IS45 at Ussuriysk, around 400km
northeast of NKTS and IS30 at Isumi nearly 1200km away
on the Pacific coast of Japan and some 65km southeast of
Fig. 8 A KOLBA chamber outside its storage bunker at the former
Semipalatinsk Test Site. The diameter is 2.4m and the length is 7m.
It was made of steel reinforced with Kevlar and fiberglass. Source:
Siegfried Hecker. Page 27 in “Plutonium Mountain” published by
Belfer Center at Harvard Kennedy School. (2013)
13 Cf. the Jumbo steel container prepared for the Trinity test in 1945.
It was designed to withstand an explosion of the 2.4 ton high explo-
sives in the”gadget”, should the nuclear package fail and valuable and
unhealthy plutonium be scattered around locally.
Journal of Radioanalytical and Nuclear Chemistry
Tokyo. The South Korean station (KSGAR) is located in
the north-eastern corner of the country, some 300km from
NKTS and incidentally very close to Geojin where the radi-
oxenon detections were first made on 13 May 2010.
All six nuclear tests that have been announced by North
Korea were shown to have produced infrasound signals
that were detected by one or more of the three mentioned
arrays. Further parameters of azimuth, slowness and celerity
were consistent with the directions and distances to NKTS.
But weak signals were also detected at all three stations on
12 May 2010 with parameters that agreed well with those
inferred from the announced tests. The IMS arrays indicated
an infrasound source in the NKTS area within a minute or so
close to the seismic detections at 8min 45s past midnight
on 12 May 2010. Could that be just a coincidence? To help
assess the credence of a common source hypothesis KP19
scanned the IMS station records for the six years 2009–2014.
They found then that at the Ussuriysk array a signal like the
one associated with the 12 May 2010 event had occurred due
to background noise artefacts was not very likely; just once
or twice a week. That gives an error risk of around 0.01%
(one minute divided by one week).14 At Isumi the corre-
sponding risk was some ten times higher due to its proximity
to metropolitan Tokyo.
But now we face the problem that if a burial depth is
assumed that is similar to the depths of several hundred
meters as employed at the announced nuclear tests, it is hard
to imagine how a regionally detectable infrasound signal
could be generated via uplift (‘piston effect’) of the surface
acting on the air by an explosion that is some 1000–100,000
times more feeble [37]. One can however, imagine other
mechanisms whereby an infrasound signal from a low yield
explosion could be generated, e.g. related to the venting of
high-pressure hot gases through a small orifice, via the test
tunnel entrance or a dedicated pipe. This is an increasingly
realistic mechanism in the shallow test scenario that is sup-
ported by the FW15 observations and the ZW23 analysis
that put the event within the north portal area with a much
reduced overburden compared to the 230m previously
assumed.
It is important to recognise that three different teams
using three different seismic networks and presumably three
different correlation detection algorithms all found the event.
Its reality is firm. The major disagreement between the ZW
and KR teams is with respect to its discrimination as an
explosion or an earthquake. That cannot be decided here. But
it is stressed that the ultimate diagnostic of what occurred in
mid-May 2010 are the radionuclides, particularly Ba/La-140
and Ce-141. Emissions of their short-lived xenon precur-
sors in their decay chains are uniquely characteristic of an
impulsive release from a supercritical fission chain reaction.
Nevertheless, and regardless of detailed technicalities a
few related remarks are also in order. Firstly, the waveform
phases revealed a local origin time of 09:09 that was con-
sistent with the six announced North Korean nuclear tests,
all between 09:00 and 12:00 local time [38]. Secondly, it
occurred during a time of extreme quietness—spanning
almost a decade—in the natural seismicity at NKTS. Thirdly,
it occurred south of the announced tests whereas all other
“natural” tremors which have been accurately geo-located—
e.g. aftershocks following the approximately 250 kiloton
2017 explosion —have occurred to the north [39–47].
ATM based ontheNKTS waveform time
stamp
No one could doubt that the radionuclides detected in mid-
May 2010 originated in a very rapid fission event, most
likely a nuclear explosion. Early atmospheric backtrack-
ing calculations combined with the Ba-140/La-140 nuclear
clock in the debris suggested that an explosion took place
in North Korea. The nuclear clock pointed, first in DG12,
on an explosion time between the midnights of 10 and 12
May (based on one-sigma uncertainty in the nuclide ratio
and a maximum at 06:00 on 11 May). This was later refined
in DG13 to an interval of nearly the same width but with a
maximum 11h later, around 17:00 on 11 May.
As the waveform data has given very good reasons to
believe that the actual time zero was a few minutes past mid-
night on 12 May (which is within the nuclear clock window),
we decided to test how well an advanced forward ATM dis-
persion calculation based on a release of xenon-137/140/141
at NKTS at that time would replicate the observed detection
pattern. Such a study also has the potential to help resolve
the current disagreement between two renowned seismology
laboratories on the interpretation of the event as an explo-
sion or an earthquake.
The MATCH model
With good experience from a recent study on details of the
dispersion of the Chernobyl cloud, the dispersion model
MATCH was selected [48]. This is a comprehensive hybrid
Lagrangian−Eulerian atmospheric transport and chemistry
model, developed at the Swedish Meteorological and Hydro-
logical Institute (SMHI) [49–51]. It has been extensively
benchmarked, scoring very well in international comparative
studies [52, 53]. The model covers transport, deposition,
atmospheric chemistry and radioactive decay and is thus
made to be a complete tool for a wide range of applications,
14 The standard deviation of the “Window start after OT (Origin
Time)” numbers in KP19’s Table 2 for the 2010, 2013, 2016 (Jan)
and 2017 event detections at Ussuriysk was 42s.
Journal of Radioanalytical and Nuclear Chemistry
with nuclear accidents/explosions, chemistry health stud-
ies and volcano eruptions as examples. The model is also
equipped with backtracking capabilities [54].
The hybrid approach is to describe the first part of the
transport by a Lagrangian model (up to 12h) after which
the model Lagrangian particles are merged into a model
grid where the Eulerian transport equations take over. This
enables initial sub-grid transport with diffusion closer to the
expected physical one. For this study the meteorological
data is taken from the European Centre for Medium Range
Weather Forecasts (ECMWF) operational model (Inte-
grated Forecasting System,IFS) with 0.1-degree resolution
(5 × 10km) and fed into the MATCH model in 3-h intervals,
but internally interpolated into 1-h steps.15 The output from
the transport calculations is given in 1-h time resolution.
The prompt and delayedsources are described as pulses in
a 50m deep layer just above the ground level.16 We chose
to leave radioactive decay outside the MATCH runs as that
limits the number of calculated dilution maps and can easily
be taken care of afterwards.
The complete results of the MATCH runs are pro-
vided in Online Resource 2. They primarily take the form
of sequences of dilution maps where, assuming relevant
release times, they show time slice maps at the four detection
points—Okinawa and Ussuriysk for particulates and Geojin
and Takasaki for inert gases—for every 3h between 12 May
06:00 and 20 May 00:00. All dilution maps are put in time-
slice order in two folders. There is one for a prompt release
at 12 May 00:00 that aims at particulate radionuclides (like
Ba-140) detected at Okinawa and Ussuriysk. These maps
come in two versions, one assuming no rain and one that
does take rain into account based on stored forecasts. There
is also a map of the forecasted precipitation rates. Then there
is one folder with six sub-folders of closer-view sequences
for potential release times of 00, 03, 09, 15, 21 and 24h on
12 May that aims at the radioxenon detected at Geojin and
Fig. 9 The dilution factor (times 1015) in 1/m3 as a function of time
during 12–20 May 2010 at the four detecting stations. The two upper
graphs refer to the prompt release time at midnight 12 May and the
lower two to a delayed release 15h later. In the Okinawa and Ussuri-
ysk panes the dotted parts of the curves mark the impact of precipita-
tion. The grey fields mark collection periods in which the reported
activity concentrations were greater than 10% of the largest Ba-140
and Xe-133 values measured at the respective station. These col-
lections extended over a 24-h period from midnight to midnight for
Okinawa and Ussuriysk, and a 12-h period for Geojin and Takasaki.
The reported values are given below the scales inμBq/m3 of Ba-140
at Okinawa and Ussuriysk and inmBq/m3 of Xe-133 at Geojin and
Takasaki. It is important to realize that these would be representative
of the total activity collected over the relevant period, i.e. the inte-
grated counts—or area under the curves—rather than the peak counts
15 The basis for numerical weather prediction is assimilation of
observed data. Such analyses are the basic meteorological input for
the transport-dispersion calculations here performed. These analyses
docome in 6 h intervals and we increase the temporal resolution by
intermediate 3 h forecasts. Precipitation is however not part of any
observation assimilation and the first forecasted hours are then used
for this rather important parameter.
16 In this section the relevant sampling periods from DG12 have sev-
eral times been slightly trimmed to the full or half-full UTC day to
adapt to the atmospheric transport calculations.
Journal of Radioanalytical and Nuclear Chemistry
Takasaki. There are six of them to facilitate an estimate of
the delay of the radioxenon release.17
All maps can be studied individually but also as “videos”
covering 12–20 May. These “videos” appear as stacks of
maps that can be rolled in time order by the keyboard arrows
and/or the mouse wheel. The dilution factors were actually
calculated for every hour between the midnights of 12 and
20 May. To further facilitate judgments on the connectivity
between the two releases and the four detecting radionuclide
stations, the dilution factors were also plotted for relevant
releases as functions of time in Fig.9.
For Okinawa and Ussuriysk the release time of inter alia
xenon-140 was nearly prompt—at about 10s—and it was
in the model set to midnight 12 May, just some 9min before
the potential seismic event time. But here, as the rapidly
formed (via 1min half-life cesium-140) decay product
barium-140 attaches to natural aerosol particles, the dilu-
tion will be affected by precipitation along the track. For
barium the dilution calculations were therefore also done
taking precipitation into account, and both types of maps
are shown in Online Resource 2 together with corresponding
precipitation maps.
For the Geojin and Takasaki stations six different release
times during 12 May (0, 3, 9, 15, 21 and 24h) were investi-
gated as the release time of the more long-lived xenon iso-
topes is not known or suggested by any other technology.
Such a second release could of course be accidental, but it
could reasonably also be due to operational ventilation of the
explosion chamber before entry to collect experimental data.
Results forparticulates (basically Ba/La‑140)
The new ATM results provide multiple and significant
advances over forward plume concentration models previ-
ously published. For instance, in contrast to the study of
WR13, which was based on an emission at 06:00 on 11 May
(i.e.18h earlier than here), the new dilution maps replicate
the essentially simultaneous detections at Okinawa and
Ussuriysk on 15 May. The HYSPLIT concentration model
of WR13 with the earlier release time produced no plume at
Ussuriysk (WR13, Fig.9).
Also, as seen in DG12, the detection series of Ba-140
at Okinawa showed an abrupt increase, from zero to about
80μBq/m3 on 15 May, followed by three days of lower
detections (by a factor of 3), and then a second increase to
about 50μBq/m3 on 19 May. Whilst WR13 could reproduce
the abrupt increase on 15 May (WR13, Fig.9), the ensuing
temporal behaviour—i.e., the overall longevity of the plume
over Okinawa, the ‘plateau’ and the ‘second peak’ on19
May could not be replicated. But this behaviour is reason-
ably well reproduced by the MATCH dispersion analyses.
Whilst the model with precipitation also succeeds in rep-
licating the initial Ba/La-140 ‘hit’ at Okinawa on 15 May
(middle panel of Fig.10 as taken from Online Resource 2 and
the dotted line in the upper left panel of Fig.9), the dilution
factor is about a factor of ten lower. In other words, and with
all else being equal, a Ba-140 activity concentration of less
than 10µBq/m3 would be expected. Moreover, on 17 May it
is predicted to increase, in contrast to the observations. But
it must be borne in mind that the long travel distance from
Fig. 10 Dilution maps of a release at ground level within the NKTS
at 00:08:45 on 12 May 2010. This slice refers to 21:00 on 15 May
towards the end of sampling at Okinawa where the first and strongest
detection of Ba-140 was made that day. The leftmost map a ignores
precipitation, the next b does not, and assumes wet deposition of par-
ticulate matter with diameters less than 2.5 μm (PM2.5). The right-
most pane shows the precipitation rate. The color-coding indicates the
dilution factor in m−3 and the rainfall in mm per 3-h interval
17 Double-click one of the folders and push Command-A and then
Command-O on a Mac or open the first map in one of the folders on a
PC. Then use the keyboard arrows or the mouse wheel to browse the
map sequences between 12 May 06:00 and 20 May 00:00.
Journal of Radioanalytical and Nuclear Chemistry
NKTS, plus the fact that Okinawa during the relevant times
is close to the edge of the plume—for which the dilution
contours perpendicular to the direction of approach are quite
dense—makes the analysis prone to uncertainty.18 Small
temporal or spatial perturbations in the input meteorological
data, or in interpolation between the model grid cells, may
produce substantial variation in the output. Also, the rain
area in the model is quite small and concentrated (rightmost
panel of Fig.10) with predicted heavy rainfalls that are some
ten times heavier than the officially observed precipitation
reported for Afuso, where the CTBT station is located (less
than 4mm per day on 15 and 16 May) [55].
At Ussuriysk, with a more than five times shorter travel
distance and very little impact of forecasted precipitation
(upper right panel of Fig.9), the dispersion results are quite
uniform and quantitatively more reliable.
It is interesting to note in Online Resource 2 that the
prompt release dilution factor at Beijing was quite sub-
stantial (up to 0.17 10−15 m−3) during the night between 16
and 17 May 2010. A CTBT particulate radionuclide station
(RN20) has been operating in Beijing since 2006, but with
the caveat that data for many years was only sent in delayed
mode to the Data Centre in Vienna on compact discs. As
that did not fulfil Treaty requirements these data were not
analysed in Vienna. But had they been, it wouldn’t anyway
helped the May 2010 case as the Beijing station was down
during the first half-year of 2010, reportedly due to a failing
multichannel analyser.
Results forinert gases (basically Xe‑133 andXe‑135)
The six hypothetical release times for the xenon plumes were
analysed in detail for both the Geojin and Takasaki stations
in an attempt to obtain an estimate of the time delay follow-
ing the near-prompt Xe-140 release. But there was little dif-
ference between the model runs for delays of 9, 15 and 21h
on 12 May, so the middle one was selected for display in
Fig.11 as taken from Online Resource 2. It shows the most
relevant dilution map for radioxenon detections at Geojin
for a release on 12 May 15:00, a snapshot on 13 May 18:00,
close to the midpoint of the second 12h Geojin sampling
that day. The corresponding map for Takasaki is shown in
Fig.12 as taken from Online Resource 2, being the snap-
shot at the midpoint of the three strongest (and consecutive)
detections at noon on 17 May.
The same distance effect mentioned for Okinawa is
also valid for the radioxenon data from Takasaki, and thus
Fig. 11 Dilution map of an inert gas release at ground level within
the NKTS at 15:00 on 12 May 2010. This slice refers to 13 May
18:00, which is close to the middle of the sampling period at Geojin
during the second half of that day
Fig. 12 Dilution map of an inert gas release at ground level within
the NKTS at 15:00 on 12 May 2010. Here time has passed nearly four
days further from the previous figure to 12:00 on 17 May, which is
close to the middle of the 3 × 12h sampling period starting in Taka-
saki at 18:46 on 16 May
18 The dense isolines, or equivalently the steep concentration (dilu-
tion factor) gradient at the edges of the plume, are a feature common
to other forward models for the May 2010 event, e.g. in WR13.
Journal of Radioanalytical and Nuclear Chemistry
makes the Geojin data set the more reliable one.19 Further,
the Takasaki observations are less certain due to the den-
sity of nuclear reactors in Japan—which occasionally leak
long-lived xenon isotopes—and the station’s location on the
premises of a nuclear laboratory. For example, there was a
Xe-135 detection reported for 18 May, which is too late to be
related to a 12 or 13 May release at NKTS given the passage
of more than ten half-lives.
It is interesting to note that a more precise estimate of the
delay time could have been made if South Korea had pub-
lished the Geojin data for the next 12-h sample, scheduled
to start collection at 23:00 on13 May, given the calculations
displayed in the lower left panel of Fig.9.Having said that,
a radioxenon release at 15:00 on 12 May corresponds well
with earlier estimates—i.e. between 06:00 and 18:00 on 12
May—of a delayed release of 1 to 1.5days after the origi-
nally estimated fission zero time of 06:00 on 11 May. In the
case of Takasaki the arrival and duration of the plume over
the station is reproduced quite well by the model, as shown
in the lower right panel of Fig.9.
Uncertainties inthetransport‑dispersion modelling
In order to bring some insight into the uncertainty of the
transport-dispersion modelling the footprints from three sites
with the peak measured values; Okinawa (Ba-140, 15–16
May 00:00–00:00 UTC), Ussuriysk (Ba-140, 17–18 May
02:00–02:00 UTC) and Geojin (Xe133, 13 May 11:00- 23:00
UTC) were calculated. The footprint is the result of a unit
response at the location and sampled time period follow-
ing this backward in time and thus upstream. A footprint
indicates the potential release sites upstream. By nature, the
footprint gets larger backwards in time due to the diffusive
nature of the transport. The footprint also gets enlarged by
the feeding in the response over a longer period as here up
to 24h. The 12 footprints from these measurements for the
dates 13, 12 and 11 (all at00:00 UTC) are shown in Online
Resource 3. The footprint from Okinawa and Ussuriysk
show as expected larger footprints than the footprint from
Geojin, given the time period shown. A more distinct signal
from the Okinawa measurement does not reach the Korean
peninsula before 12 May while the Geojin site has the most
distinct footprint given its location. It is the combined foot-
prints that narrow in the time and location of the potential
source. A way to refine the footprints is to take the multipli-
cative result that is a mutual-exclusive operation. Figure13
shows the mutual-exclusive footprints of the footprints in
Online Resource 3 for midnights 13, 12 and 11 May and the
estimated time of the delayed release at 15:00 on 12 May.
The proposed release site is clearly indicated in the first two
maps while the potential source area enlarges backward in
time. The potential site for a release at midnight 12 May is,
however, concentratedto the eastern side of North Korea
with a focus close to NKTS.20
Fig. 13 The normalized product of the footprints in Online Resource
3 yielding a mutual-exclusive footprint that is supported by all the
footprints. The footprints are masked to yield only land areas and
refer to potential releases at the midnights of 13, 12, 11 May and
the estimated time of the delayed release. Circles for Ussuriysk and
Okinawa, diamond for Geojin and a triangle for NKTS
19 Note that precipitation is essentially irrelevant for Xe-133 and
Xe-135 given that xenon is a noble gas and therefore neither attach to
aerosol particles nor dissolve in rain drops.
20 Note here that the uncertainty discussion here does not cover the
magnitude of the potential releases.
Journal of Radioanalytical and Nuclear Chemistry
Using MATCH toput aconstraint ontheexplosive
yield
Finally we use the dilution factors and the specific decay
dynamics of the radionuclides involved to estimate the sizes
of the prompt and delayed releases. Such estimates are done
in Online Resource 4. The Excel file has two sheets, one
dealing with plutonium-239 fission and the other with ura-
nium-235 fission. The Ussuriysk and Geojin release esti-
mates are selected as being more reliable than the Okinawa
and Takasaki ones, due to their much shorter atmospheric
transport. According to Online Resource 2 the prompt
(Ussuriysk) and delayed (Geojin) releases are then estimated
to be:
Prompt release 12 May 00:00 For Pu-239
fastfission
For U-235
fastfission
0.2 tons
0.1 tons
Delayed release21 12 May 15:00 ForPu-239
fastfission
ForU-235
fastfission
1.2 tons
1.4 tons
Recognising that the average dilution factors across the
sampling times carry uncertainties of the order of tens of
percent these estimates anyway indicate that the delayed
release was close to 100%. This is quite reasonable for a
planned ventilation of radioactive gases before re-entry.
Accepting that and the dilution uncertainty estimates imply
a yield in the range of 0.5–1 toneq and a prompt release of
some 20–40 and 10–20% respectively. Obviously this analy-
sis neither provide exact numbers nor details on arrange-
ments that could possibly lead to some decoupling,21 but
it does illustrate a set of data that is very consistent with
a low-yield meant-to-be fully contained nuclear explosion.
Anything else thananexplosive fastssion
event?
The present authors believe that the clear and unambiguous
detections of particularly Ba-140 and La-140 at Okinawa
and Ussuriysk, but also Ce-141 and Cs-137 at Okinawa—all
progeny of very short-lived noble gases—can only have had
their origin in an explosive fission event, which occurred
within a medium or a technical device capable of containing
most fission products but with a certain risk of releasing the
noble gas fraction.
The diagnostic power of what was observed—when con-
sidered in tandem with what was not observed—is so great
that these are not just necessary evidence to draw such a con-
clusion, but also sufficient. A small nuclear explosive device
is thus the most obvious source of the detected nuclides. An
explosive fission event could, however, also theoretically refer
to a compact nuclear reactor rapidly undergoing a destruc-
tive supercritical excursion. Actually, on 8 August 2019, an
experimental Russian nuclear propulsion reactor accidentally
exploded when it was being recovered from the seabed near
Nyonoksa some 30km west of Severodvinsk on the White
Sea. A few weeks later the Russian Meteorological Service,
Roshydromet, reported that strontium-91, barium-139 and
barium/lanthanum-140 had been detected in the area. These
are all daughters of noble gas isotopes (Kr-91, Xe-139 and
Xe-140) [56, 57]. We do, however, consider a Nyonoksa type
nuclear explosive excursion in the Mantap Mountains in 2010
to be an extremely unlikely event. At the time North Korea
was fully focused on its nuclear weapons program.
Conclusions
This paper presents new data and new analyses of the
extraordinary series of radionuclide detections on and sur-
rounding the Korean Peninsula in mid-May 2010. It supports
the original suggestion in DG12/13 of a low yield under-
ground nuclear explosion around 12 May 2010 in North
Korea. The new analyses focus on three subjects:
Firstly the Mathematica® notebook Xebate-3 was devel-
oped and used to present the evolution of the relevant
radionuclides in time and then also to, based on the meas-
urements, estimate the time for the nearly prompt release
and the time for what we call an equivalent quenching that
explains the observed xenon isotopic fractionation. An anal-
ysis of the faint Cs-137 detection and its potential to indicate
the fissile fuel of the detonated device was also done.
Secondly, it is shown by co-adding three spectra that
Ba-140, not just La-140, actually was detected at the CTBTO
station at Ussuriysk. This establishes beyond doubt that the
Okinawa and Ussuriysk detections had a common origin,
something that had been questioned.
Thirdly, we note the seismic signals, with an origin in
the NKTS area at 8min and 45s past midnight on 12 May
2010, detected by three well-known groups. Their interpre-
tations, however, differed. Based on analyses of the ratio of
two seismic phases, two labs using different networks and
methods characterised the event differently, the first as an
explosion and the third as an earthquake (numbering based
on chronology). In a recently published abstract the first
group stands by the explosion characterisation also when they
apply their own technique on the data previously used by the
third group. They also reported on an explosion point close
21 If we believe in a delayed release being near 100% the decoupling
factor cannot be very high.
Journal of Radioanalytical and Nuclear Chemistry
to the north portal, which indicates a test in a shaft inside that
tunnel fairly close to the entrance. That scenario supports
our hypothesis of a shallower explosion than was previously
assumed. That idea stemmed from the observation by the sec-
ond lab that a 2.9 toneq explosion should have been detected
at the PS37, PS31 and MDJ stations at more than twice the
distance from NKTS than the detecting ones. “Moving” the
test point upwards from the guessed depth of 230m reduced
the estimated yield to less than one ton. Moreover a shallow
explosion also opens for more modes of generating the infra-
sound signals that were detected with space and time source
parameters very close to those of the seismic signal.
Fourthly, as the waveform data cannot tell whether a
small explosion is conventional or nuclear, there is a need for
a “bridge” to connect the seismic/infrasound source param-
eters with the observed pattern of nuclear debris detections.
So, with an advanced ATM tool run in forward mode with a
prompt release of very short-lived xenon nuclides at NKTS
at the seismic event time, a very good resemblance between
calculations and observations resulted. That was also the
case for a 15h delayed release of the longer-lived radioxenon
nuclides. The dilution values were then used to estimate the
sizes of the prompt and delayed releases, which turned out
to be quite consistent with UNEs in the general range of
a half to a full toneq with reasonable release fractions of
some 10–20% for the prompt and close to 100 percent for
the delayed release.
Finally we note that the May 2010 UNE stands out as
being coherently defined by fusion of information from as
many as six elements of the CTBTO verification regime;
Radionuclide Aerosols, Radionuclide Noble Gases, Atmos-
pheric Transport Modelling, Seismology, Infrasound and
National Technical Means.
Supplementary Information The online version contains supplemen-
tary material available at https:// doi. org/ 10. 1007/ s10967- 024- 09453-2.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
References
1. De Geer L-E (2012) Radionuclide evidence for low-yield nuclear
testing in North Korea in April/May 2010. Sci Glob Secur
20:1–29
2. De Geer L-E (2013) Reinforced evidence of a low-yield nuclear
test in North Korea on 11 May 2010. J Radioanal Nucl Chem
298:2075–2083
3. Ihantola S, Toivonen H, Moring M (2013) La-140/Ba-140
ratio dating of a nuclear release. J Radioanal Nucl Chem
298:1283–1291
4. Brumfiel G (2012) Isotopes hint at North Korean nuclear test.
Nature https:// doi. org/ 10. 1038/ nature. 2012. 9972. Accessed 11
Apr 2024
5. Unsigned (2012) Telltale Isotopes Hint At North Korea Weapons
Test. Science 335, No 6069:643
6. Wright CM (2013) Low-yield nuclear testing by North Korea in
May 2010: assessing the evidence with atmospheric transport
models and xenon activity calculations. Sci Glob Secur 21:3–52
7. Wotawa G (2013) Meteorological analysis of the detection of
xenon and barium/lanthanum isotopes in May 2010 in eastern
Asia. J Radioanal Nucl Chem 296:339–347
8. Suh K-S, Park K, Min B-il, Kim S (2017) Atmospheric transport
modelling for the radionuclides detection of northeast Asia in
2010. In: CTBT Science and Technology Conference poster, pp
26–30 T1.3-P3
9. Ross O, Zähringer M, Schlosser C, Ceranna L, Bönnemann C
(2017) Atmospheric transport modelling confining possible ori-
gin of east-Asian radionuclide detections in May 2010. CTBT
Science and Technology Conference poster pp T2.2-P5
10. Krupcale M (2021) Modelling dispersion of radionuclides in
the turbulent atmosphere. PhD diss, University of Michigan
11. Carrigan CR, Heinle RA, Hudson GB, Nitao JJ, Zucca JJ (1996)
Trace gas emissions on geological faults as indicators of under-
ground nucleaar testing. Nature 382:528–533
12. De Geer L-E (1996) Sniffing out clandestine tests. Nature
382:491–492
13. Carrigan CR etal (2016) Delayed signatures of underground
nuclear explosions. Sci Rep 6:1–9
14. Yamba K, Kalinowski MB, Sanago O (2018) Nuclear event
zero time determination using analytical solutions of radi-
oxenon activities under in-growth condition. Appl Rad Isot
139:217–223
15. Carrigan CR, Sun Y, Simpson D (2019) The characteristic release
of noble gases from an underground nuclear explosion. J Environ
Radioact 196:91–97
16. Carrigan CR, Sun Y, Pili E, Neuville DR, Antoun T (2020) Cav-
ity-melt partitioning of refractory radionuclides and implications
for detecting underground nuclear explosions. J Environ Radioact
219:106269
17. Bourret SM, Kwicklis EM, Stauffer PH (2021) Evaluation of sev-
eral relevant fractionation processes as possible explanation for
radioxenon isotopic activity ratios in samples taken near under-
ground nuclear explosions in shafts and tunnels. J Environ Radio-
act 237:106698
18 Sun Y etal (2021) A closed-form solution for source-term emis-
sion of xenon isotopes from underground nuclear explosions.
Transp Por Media 139:131–153
19. Carrigan CR, Sun Y, Antoun T (2022) Evaluation of subsurface
transport processes of delayed gas signatures applicable to under-
ground nuclear explosions. Sci Rep 12:1–17
20. Sun Y, Carrigan CR, Pili E, Antoun T (2023) Implications of
underground nuclear explosion cavity evolution for radioxenon
isotopic composition. Pure Appl Geophys 180:1395–1406
21. JEFF 3.3 (2020) The joint evaluated fission and fusion nuclear
data library. Available via NEA JANIS
22. NuDat-3.0 (2024) National nuclear data center, brookhaven
national laboratory Available at https:// www. nndc. bnl. gov/ nudat3/
23. ICRP-107 (2007) Nuclear decay data for dosimetric calculations
Journal of Radioanalytical and Nuclear Chemistry
24. IAEA (2024) Live chart of nuclides, nuclear structure and decay
data. Avaiable at www-nds.iaea.org/relnsd/vcharthtml/VChartH-
TML.html
25. Bateman, H. (1910) Proceedings of the london mathematical soci-
ety, vol s2–8, pp 223–264
26. Lewis J (2015) The great 2010 DPRK nuclear test debate, 38
North web site https:// www. 38nor th. org/ 2015/ 04/ jlewi s0428 15.
Accessed 23 Jun 2023
27. Zähringer M, Bieringer J, Schlosser C (2008) Three years of oper-
ational experience from schauinsland CTBT monitoring station. J
Environ Radioact 99:596–606
28. Kolb W, Weiss W (1999) Occurrence of lanthanum-140 in
ground-level air. J Environ Radioact 13:79–83
29. De Geer L-E (2004) Currie detection limits in gamma-ray spec-
troscopy. Appl Radiat Isot 61:151–160
30. Negret A etal (2015) Cross section measurements for neutron
inelastic scattering and the (n, 2nγ ) reaction on Pb-206. Phys Rev
C 91:064618
31. Zhang M, Wen L (2015) Seismological evidence for a low yield
nuclear test on 12 May 2010 in North Korea. Seismol Res Lett
86:138–145
32. Ford S, Walter W (2015) International monitoring system correla-
tion detection at the Norh Korean nucleartest site at Punggye-ri
with insights from the Source Physics Experiment. Seismol Res
Lett 86:1160–1170
33. Kim W-Y, Richards PG, Schaff DP, Koch K (2017) Evaluation of
a seismic event, 12 May 2010, in North Korea. Bull Seismol Soc
Am 107:1–21
34. Koch K, Pilger C (2019) Infrasound observations from the site of
past underground nuclear explosions in North Korea. Geophys J
Int 216:182
35. Zhang M, Wen L (2023) Examination of the debate on the 12
May 2010 low-yield nuclear test. Poster at AGU23, American
Geophysical Union. Seismol Res Lett 94:1187
36. Pabian FV, Hecker SH (2012) Contemplating a third nuclear test
in North Korea. Bull Atomic Sci, August 6. https:// thebu lletin.
org/ 2012/ 08/ conte mplat ing-a- third- nucle ar- test- in- north- korea/
Accessed 7 Apr 2024
37. Yang G, Zhao LF, Xie XB, Zhang L, Yao ZX (2021) High-preci-
sion relocation with the burial depths of the North Korean under-
ground nuclear explosions by combining Pn and Pg differential
traveltimes. J Geophys Res Solid Earth 126:1–20
38. (2022) List of nuclear weapons tests of North Korea. Wikipedia
39. Dodge DA (2018) Searching for induced seismicity at Punggye-
ri nuclear test site using subspace detectors. Seismol Res Lett
89:2094–2112
40. Gibbons SJ, Kværna T, Näsholm SP, Mykkeltveit S (2018) Prob-
ing the DPRK nuclear test site down to low-seismic magnitude.
Seismol Res Lett 89:2034–2041
41. He X, Zhao L-F, Xie X-B, Yao Z-X (2018) High-precision relo-
cation and event discrimination for the 3 September 2017 under-
ground nuclear explosion and subsequent seismic events at the
North Korean test site. Seismol Res Lett 89:2042–2048
42. Kim W-Y, Richards PG, Schaff D, Jo E, Ryoo Y (2018) Identi-
fication of seismic events on and near the North Korean test site
after the underground nuclear test explosion of 3 September 2017.
Seismol Res Lett 89:2120–2130
43. Myers SC, Ford SR, Mellors RJ, Baker S, Ichinose G (2018)
Absolute locations of the North Korean nuclear tests based on
differential seismic arrival times and InSAR. Seismol Res Lett
89:2049–2058
44. Schaff DP, Kim W-Y, Richards PG, Jo E, Ryoo Y (2018) Using
waveform cross correlation for detection, location, and identifi-
cation of aftershocks of the 2017 nuclear explosion at the North
Korea test site. Seismol Res Lett 89:2113–2119
45. Tian D, Yao J, Wen L (2018) Collapse and earthquake swarm after
North Korea’s 3 September 2017 nuclear test. Geophys Res Lett
45:3976–3983
46. Yao J, Tian D, Lu Z, Sun L, Wen L (2018) Triggered seismicity
after North Korea’s 3 September 2017 nuclear test. Seismol Res
Lett 89:2085–2093
47. Kitov IO, Sanina IA (2022) Analysis of sequences of aftershocks
initiated by underground nuclear tests conducted in North Korea
on September 9, 2016 and September 3, 2017. Seismic Instr
58:567–580
48. De Geer L-E, Persson C, Rodhe H (2018) A nuclear jet at Cher-
nobyl around 21:23:45 UTC on April 25, 1986. Nucl Techn
201:11–22
49. Robertson L, Langner J, Engardt M (1999) An Eulerian limited-
area atmospheric transport model. J Appl Meteorol Climeteorol
38:190–210
50. Andersson C, Bergström R, Bennet C, Robertson L, Thomas M,
Korhonen H, Lehtinen KEJ, Kokkola H (2015) MATCH-SALSA−
multi-scale atmospheric transport and chemistry model coupled to
the SALSA aerosol microphysics model—Part 1: model descrip-
tion and evaluation. Geosci Model Dev 8:171–189
51. Andersson C, Langner J, Bergström R (2007) Interannual varia-
tion and trends in air pollution over Europe due to climate vari-
ability during 1958–2001 simulated with a regional CTM coupled
to the ERA40 reanalysis. Tellus 59B:77–98
52. Colette A etal (2017) Eurodelta-trends, a multi-model experiment
of air quality hindcast in Europe over 1990–2010. Geosci Model
Dev 10:3255–3276
53. Vivanco MG etal (2018) Modeled deposition of nitrogen and
sulfur in Europe estimated by 14 air quality model-systems: evalu-
ation, effects of changes in emissions and implications for habitat
protection. Atmos Chem Phys 18:10199–10218
54. Robertson L, Langner J (1998) Source function estimate by means
of variational data assimilation applied to the ETEX-I tracer
experiment. Atmos Environ 32:4219–4225
55. Afuso Historical Weather, https:// www. world weath eronl ine. com/
afuso- weath er- histo ry/ okina wa/ jp. aspx, Accessed 12 Jun 2023
56. Gertz B (2019) Russia covered up explosion of Skyfall nuclear
superweapon. The Washington Times
57. Bodner M (2019) Russia says radioactive isotopes released by
mystery blast: State agency identifies four substances in samples
taken from city of Severodvinsk, The Guardian
Publisher's Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
