Journal of Nuclear and Radiochemical Sciences, Vol. 14, No.1, pp. R1-R9, 2014

Reviews
Radiochemical Verification Technologies for the Detection of Nuclear Explosions– Recent Developments in Radionuclide Monitoring with the Comprehensive
Nuclear-Test-Ban Treaty –Lisa Kokaji*, a and Nobuo ShinoharababSchool of Engineering, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku Tokyo, 113-8656, Japan
Department of Science and Technology for Nuclear Material Management, Japan Atomic Energy Agency, Tokai,
Ibaraki 319-1195, Japan

Received: January 11, 2013; In Final Form: April 10, 2014
The Provisional Technical Secretariat of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) Organization has
established the International Monitoring System (IMS) under its auspices. Through technological advances, the
CTBT has achieved nuclear test detection limits of 1 kiloton by the use of CTBT verification technologies.
However, radionuclides were not observed clearly by the IMS during the first and second nuclear weapon tests
conducted by the Democratic People’s Republic of Korea (DPRK). With the third test carried out by the DPRK,
however, they could observe traces of the event using the IMS, noble gas detection stations, and AtmosphericTransport Modeling (ATM) techniques. After the accident at the Fukushima Dai-ichi Nuclear Power Plant inMarch 2011, the IMS accurately tracked the global movement of radionuclides that originated from this event. In
the following report, the current situation and recent problems regarding environmental monitoring of radionuclides by the CTBT are presented and discussed. Focus will be placed on the detection abilities of the IMS and thesystem used for evaluation of the radionuclides released by nuclear explosions and power plant accidents.

1. Introduction
How does the Comprehensive Nuclear-Test-Ban Treaty(hereafter CTBT) monitoring system detect nuclear explosions
globally? An adequate verification regime by the CTBT is ofvital importance for State Parties in monitoring the peacefuluse of nuclear power. Seventeen years have passed since the
CTBT was adopted by the United Nations (UN) General
Assembly. Eighty seven percent of the CTBT International
Monitoring System (IMS) stations were installed by theProvisional Technical Secretariat of the CTBT organization(CT BTO) a nd t he dat a has been t ra nsm it t i ng to t he
International Data Center (IDC) in Vienna on a provisional
basis.1 The IMS consists of three wave data verification technologies (infrasound, seismological, and hydroacoustic) and aradionuclide detection system. The presence of radionuclides
is the only concrete proof as to whether a nuclear explosion(i.e., the production of radionuclides by a fission event) has
occurred. However, the detection of hidden undergroundnuclear explosions is a challenge as compared to the detectionof atmospheric and underwater explosions. This is due to thefact that most of the fissile components (and thus, the radionuclides required for detection) remains in the subterranean cavity formed by an underground nuclear explosion.
The Democratic People’s Republic of Korea (DPRK)
announced that three nuclear tests had been conducted on
October 9, 2006, May 25, 2009, and February 12, 2013. These
events were anticipated to provide an adequate proficiency test
for the IMS. However, radionuclides were not clearly observedby the IMS in the first and second DPRK tests. During thethird event, the IMS could only detect the associated radionuclides after 55 days. In contrast, the detection abilities of the
IMS during the Fukushima Dai-ichi Power Plant accident were
*Corresponding author. E-mail: kokaji.lisa@jaea.go.jp

found to be more robust. Both during and after this accident,
the capability and usefulness of the IMS radionuclide stations
were noteworthy, even though the IMS stations were originallyestablished to detect clandestine nuclear explosions. At the
Fukushima Dai-chi Nuclear Power Plant accident in March
2011, the quantity of the expanded radionuclides was substantial and some more peculiar radionuclides indicated what had
occurred in the containment vessel.
In this report, the recent situation and problems related toenvironmental monitoring of radionuclides are presented anddiscussed by focusing on IMS detection abilities, with consideration of the systems used to evaluate radionuclides releasedby nuclear explosions and accidents.
2. Monitoring technologies for nuclear explosion detection
There are four types of monitoring stations that support the
CTBT verification regime, as summarized below:
1) Seismic stations: The seismological component of the IMS
can detect seismic events and locate their points of origin. Thissystem comprises 50 primary seismic stations, supplemented
by 120 auxiliary seismic stations.
2) Hydroacoustic stations: Sound waves produced by naturaland man-made phenomena in the world’s oceans can bedetected by these stations, which are extremely sensitive to
explosions and can pick up acoustic waves from underwater
events even over transcontinental distances. Due to the efficient transmission of acoustic energy in ocean water, only afew stations are required by the IMS. A total of 11 stations areincluded in the CTBT global verification regime.
3) Infrasound stations: Sixty of these stations will be built
globally, which can detect low-frequency sound waves produced in the atmosphere. This technique can distinguish© 2014 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 04/23/2014

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KokajiJ. Nucl. Radiochem. Sci., Vol. 14, No. 1, 2014

between natural phenomena such as meteorites, volcanic eruptions, and other events, as well as man-made phenomena suchas re-entering space debris, rocket launches, and atmospheric
explosions.
4) Radionuclide stations: Radioactive particles and gases
released from atmospheric, underground or underwater explosions can be detected by these stations. Some 80 radionuclide
stations were selected for the radionuclide-monitoring network.
These can detect radioactive particles, and half of these stations (40) have radioactive gas detection systems.
In 1963, the Limited Test Ban Treaty banned the testing of
nuclear weapons in outer space. After this treaty was implemented, the P-5 countries (permanent member states of theUnited Nations Security Council: China, France, Russia, theUnited Kingdom, and the United States) ceased atmosphericnuclear weapons tests, and were only able to resume testingunderground. The IMS data derived from the three wave databased technologies cannot detect any absolute evidence of a
nuclear test. However, the radionuclide data is unique and candistinguish with certainty as to whether the observed event is a
nuclear test.
A total of 321 stations and 16 certified radionuclide laboratories are capable of detecting nuclear explosions taking place
underground, within water, and in the atmosphere. The IMS
can detect, locate, and identify nuclear explosions of at least
one kiloton (kt) yield detonated in any environment within 14
days. Data from the stations is authenticated and sent to the
IDC in Vienna, which supports the verification responsibilities
of States Parties by providing raw data and their objective
products while providing the additional services necessary for
effective global monitoring.
The National Academy of Science (NAS) clarified the situa-

TABLE 1: Purposes and plausible achievements for underground testing at various yields in the absence of horizontal proliferation2,*
Yield (tons of TNT equivalent)**

Countries of lesser prior nuclear test
experience

Countries of greater prior nuclear test
experience

Subcritical experiments (permissible • Equation-of-state studies• Limited insights relevant to designs forunder the CTBT)• High-explosive lens tests for implosion boosted fission weapons
weapons• Development & certification of simple,
bulky, relatively inefficient unboosted fission weapons(e.g., gun-type weapon)< 1t(likely to remain undetected)

From 1t to < 100t
(may not be detectable but strongly
location dependent without location)• Building experience and confidence with • One-point safety tests
weapons physics experiments• Validation of some unboosted fission
weapon designs• Address some stockpile and design codeissues• One-point safety tests• Pursue unboosted designs***• Develop low-yield weapons (validation ofsome unboosted fission weapon designs
with yield well below a kiloton)• Possible overrun range for one-point safety
tests• Pursue improved implosion weapon design • Proof tests of compact weapons with yield• Gain confidence in certain small nuclear up to 1kt
designs• Validate some untested implosion weapon
Likely to be detected without evasion,
designs
reduced probability of detection with• Assess stockpile issues and validate someevasiondesign codes(but strong location dependence)
From 100t to < 1kt

From 1kt to < 10kt
Unlikely to be concealable> 10ktNot concealable• begin development of low-yield boosted • Development of low-yield boosted fission
fission weapons
weapons• Eventual development and full testing of • Development and full testing of some
some implosion weapons and low-yield implosion weapons and low yield thermothermonuclear weapons
nuclear weapons• Eventual proof tests of fission weapons • Proof tests of fission weapons with yieldwith yield up to 10kt
up to 10 kt• Eventual development and full testing of • Development and full testing of new conboosted fission weapons and thermonu- figurations of boosted fission weapons andclear weapons or higher-yield unboosted thermonuclear weapons
fission weapons• P u rsue adva nces st rateg ic weapons
concepts* This table is partly revised with table 4-3 in reference 2.
** In this column the committee summarizes the current state of technology for detecting underground nuclear explosions. Thissummary does not represent any particular sensor network, medium, or location. For example, IMS detection capabilities can besubstantially better than what appears in the column for some locations, and detection capability has generally improved over time.
*** Limited improvements of efficiency and weight of unboosted fission weapons compared to 1st generation weapons not needing
testing.

Radiochemical Verification Technologies for the Detection of Nuclear

tion concerning the nuclear tests performed in 2002 and 2012.
Table 1 shows nuclear test explosion yields and intended purposes updated for 2012.2 The NAS has concluded the following:i) The nuclear explosion threshold levels for IMS seismic
detection are now well below a magnitude of 1 kt.
ii) In at least 50% of underground nuclear tests near 1 kt orlarger, the noble gas xenon may be detectable both offsite andonsite above the detection limits of the IMS, via prompt venting of nuclear test sites and from long term seepage of appreciable noble gases. The seismic detection system of the IMS iscomplicated by the non-nuclear phenomena generating seismic
signals every day. The xenon detection system should therefore
be considered a key point for the CTBT verification regime.

J. Nucl. Radiochem. Sci., Vol. 14, No. 1, 2014
3. Radionuclide network

3.1. Particle monitoring. In the 80 radionuclide stations,particles released into the atmosphere are collected on a filter
in order to measure radioactivity by use of a High PurityGermanium detector (HPGe). The relative abundance of different radionuclides in the collected samples can then be
distinguished between those produced by the natural background, a nuclear reactor or a nuclear explosion. A standard
event screening of the γ-ray spectra measured from the samples
is performed by identifying each radionuclide and comparing
the measurements of fission or activation products in a sample.
Each measured spectra is then assigned to a five-level system.
Table 2 gives such a standard list of relevant fission products
and the activation products for event screening. The basic
descriptions of the five assigned levels3 are as follows:

TABLE 2: Standard categorization list for particulate samples3
Fission products

Activation products

Fissionproduct

Half-life

Primary γ
energya)(keV)

Activation
product

Half-life

Primary γ
energy(keV)

91

9.63 h
58.51 d10.18 h64.02 d34.98 d16.91 h65.94 h6.01 h39.26 d35.36 h373.59 d7.45 d21.03 h44.6 d53.46 h9.64 d2.76 y12.46 d3.85 d
9.01 h33.6 d12.36 h30 h8.02 d
3.20 d20.8 h
6.57 h
13.16 d30.07 y12.75 d1.678 d32.50 d33.04 h284.9 d10.98 d53.08 h28.40 h46.27 h4.761 y9.4 h
15.19 d15.18 h1024.31204.8
266.9
756.7
765.8
743.4
140.5 D
140.5
497.1
319.1
621.9
342.1
617.5 D
933.8
336.2
1067.1
427.9
695.0
685.7
743.2
695.9
536.1
773.7
364.5
772.6 D
529.9
1260.41048.1661.7 D
537.3
1596.2
145.4
293.3
133.5
531.0
286.0
340.1
103.2
105.3
203.8
1153.7370.5

2414.96 h12.36 h83.79 d3.349 d27.70 d312.1 d271.8 d70.82 d44.50 d5.271 y244.3 d13.76 h14.1 h
17.77 d1.078 d32.77 d18.63 d106.7 d78.41 h207 d8.28 d418 y249.8 d5.76 d2.724 d60.20 d6.479 d10.52 y2.065 y9.312 h13.54 y93.1 d23.72 h11.78 d73.83 d6.183 d9.7 h
2.695 d51.87 h3.66 d6.75 d2.357 d1368.61524.7
889.3
159.4
320.1
834.8
122.1
810.8
1099.21332.51115.5
438.6
834.1
595.8
559.1
881.6
1076.7
1836.1
909.0
475.1
717.2
722.9
657.8
1171.7
564.2
602.7
667.7
356.0
604.7
841.6
1408
816.0
685.7
186.7
316.5
355.7
147.8
411.8
279.2
241.0
208.0
277.6

Sr
Y
93
Y
95
Zr
95
Nb
97
Zr
99
Mo
99m
Tc
103
Ru
105
Rh
106
Ru
111
Ag
112
Pd
115m
Cd
115
Cd
125
Sn
125
Sb
126
Sb
127
Sb
128
Sb
129m
Te
130
I
131m
Te
131
I
132
Te
133
I
135
I
136
Cs
137
Cs
140
Ba
140
La
141
Ce
143
Ce
144
Ce
147
Nd
149
Pm
151
Pm
153
Sm
155
Eu
156
Sm
156
Eu
157
Eu
91

a)

Na
K
46
Sc
47
Sc
51
Cr
54
Mn
57
Co
58
Co
59
Fe
60
Co
65
Zn
69m
Zn
72
Ga
74
As
76
As
84
Rb
86
Rb
88
Y
89
Zr
102
Rh
106m
Ag
108m
Ag
110m
Ag
120
Sb
122
Sb
124
Sb
132
Cs
133
Ba
134
Cs
152m
Eu
152
Eu
168
Tm
187
W
190
Ir
192
Ir
196
Au
196m
Au
198
Au
203
Pb
224
Rab)
237
U
239
Np
42

D denotes that the radionuclide is detected through the primary line of the short-lived daughter.
Ra needs to be screened with respect to the 224Ra/228Ac ratio.

b) 224R3

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KokajiJ. Nucl. Radiochem. Sci., Vol. 14, No. 1, 2014

Level 1: Typical background. The sample contains only
natural or non-relevant anthropogenic radionuclides
at normal concentrations.
Level 2: Anomalous background. The sample contains natural or non-relevant anthropogenic radionuclides,
but with one or more at abnormal concentrations.
Alternatively, the sample contains a non-relevant
anthropogenic nuclide which is not normallyobserved.
Level 3: Normal anthropogenic. The sample contains one
or more relevant anthropogenic radionuclides which
are often observed at the station, and are at concentrations within normal expected ranges.
Level 4: Anomalous anthropogenic. The sample contains
one relevant anthropogenic radionuclide either not
normally observed at the station, or at concentrations above the normal expected range.
Level 5: Multiple anthropogenic. The sample contains morethan one relevant anthropogenic radionuclide atanomalous concentrations.
Specifications for the IMS stations are based on reports
wr itten in 1995 by the Exper t Groups of the Ad Hoc
Committee.4-6 In the case of the radionuclide stations, the technical requirements for the particulate stations are determinedby the specifications outlined in Table 3.7 Figure 1 shows theglobal locations of the radionuclide stations. Each station iscertified as an IMS station by the CTBTO Provisional
Technical Secretariat (PTS), with satisfaction of technical andoperational requirements.8
A Radionuclide Aerosol Sampler/Analyzer (RASA) is an
automatic high-volume system used to sample and analyze theatmosphere for radioactive debris indicative of nuclear weapons
testing. The RASA meets CTBT requirements for near-realtime, ultra-sensitive field measurements of short-lived fission
products, which permit detection at great distances from anuclear detonation site. The analyzer passes air through alarge-area and low-pressure drop filter at a high flow rate forselectable time periods. It then seals together with bar codesfor sample identification and performs a γ-ray analysis of thefilter. The γ-ray spectrum and auxiliary data, like the meteorological data, are automatically transmitted to the IDC. The filter samples are retained for subsequent analysis.
When the relationship between radioactive decay and ingrowth in a parent-daughter pair such as 95Zr-95Nb and 140Ba140
La are measured in a radionuclide station, the timing of thenuclear explosion event can be determined from the daughter/parent activity ratio. When the number of atoms of the parent
nuclide (Np) at time zero, Np (0), is determined, then Np at any
time t, Np (t), can be expressed by the following equation:
Np (t) = Np (0) · e-λpt

Assuming that the daughter nuclide is produced solely by the

TABLE 3: Specification for the aerosol and particulate monitoring stations7
Characteristics
SystemAir flow
Collection time a)
Decay time b)
Measurement time c)Time before reporting
Reporting frequency
FilterParticulate collection efficiency
Measurement mode
HP Ge relative efficiency
HP Ge resolution
Base line sensitivity e,f)
Calibration range
Data format for gamma spectra andauxiliary data
State of healthCommunication
Auxiliary data
Data availability
Down time h)a)Minimum requirements
Manual or automated
500 m3/h
24 h
24 h
20 h
3d
Daily
Adequate composition for compaction, dissolution and analysis
For filter : > 80 % at Φ = 0.2 µm
Global d) : > 60 % at Φ = 10 µm
HP Ge High resolution gamma spectrometry
40 %< 2.5 keV at 1332 keV
10 to 30 Bq /m3 for 140Ba
88 to 1836 keV
RMS (Radionuclide Monitoring System) format g)
Status data transmitted to IDC
Two-way
Meteorological data
Flow rate measurement every 10 min
95 %7 consecutive days, 15 days annuallyTime specifications allow for an uncertainty of 10 %, except for the reporting time parameter.
This value can be reduced, down to a minimum of 6 hours, if a suspicious event is detected by other stations or techniques.
c)This value allows for authentication measurements for manual systems.
d)This global value includes the 80 % filter efficiency and the collection efficiency of the incoming air circuitry.
e)
The upper limit is intended for high background areas.
f)Certification procedures to be defined for baseline sensitivities (a posteriori MDCs) as well as the efficiency. Sample preparation losses should not affect base line sensitivities.
g)
This format should make provision for auxiliary data, authentication data and state of health data.
h)Provision should be made for spare parts in particular areas where periodicity of transportation facilities
is more than 7 days.
b)(1)Radiochemical Verification Technologies for the Detection of Nuclear

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J. Nucl. Radiochem. Sci., Vol. 14, No. 1, 2014

Figure 1. The IMS radionuclide network. Squares represent stations having the ability to monitor radioactive particulate matter and xenon isotopes in the atmosphere.8 Circles represent stations with the ability to monitor radioactive particulate matter only.

Nd (t) = λp / (λd – λp) · Np (0) · (e

-λpt

-λdt

–e )

(2)

where λ p and λ d are the decay constants of the parent anddaughter nuclides, respectively. Activity ratios of the daughterand parent nuclides at any time t, Rd/p (t), can be obtained from
Eqs. (1) and (2):
Rd/p (t) = (λd · Nd (t)) / (λp · Np (t))= λd / (λd – λp) · (1 – e –(λd – λp)t)

0.1

95Nb/95Zr

0.01

0.001

0.0001(3)

Therefore the time interval, t, can be derived from Eq. (3) as
follows:t = –1 / (λd – λp) · ln (1 – Rd/p (t) · (λd – λp) / λd)

1

Activity ratio

radioactive decay of the parent nuclide, the number of atoms ofthe daughter nuclide at time t, Nd (t), can be calculated as follows:

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Time in days
Figure 2. Activity ratio of a pair of parent 95Zr and daughter 95Nb as
a function of time elapsed from nuclear explosion.

(4)

Figure 2 shows an example of the relationship between activity ratios of 95Nb / 95Zr and time. From the precise measurementof the 95 Nb / 95 Zr value, the time-zero (elapsed time from anuclear fission event) can be estimated radiochemically. The
time-zero has been considered as an IMS standard for the processing of data from IMS radionuclide stations, together withthe five-level assignment applied to the measured radionuclides.
3.2. Noble gas monitoring. Xenon isotopes and their isomers are the radioactive signatures most likely to be observed
following a nuclear weapons test, even in the case of underground explosions.9 About 20 radioactive isotopes of xenon
are formed during fission of heavy elements like uranium or
plutonium. Most of these isotopes decay within seconds, but afew (131mXe, 133mXe, 133Xe, and 135Xe) have half-lives of several
hours to days, and are produced in amounts that can bedetected at great distances from the source of emission. The
isotopes of 131mXe, 133mXe, 133Xe, and 135Xe have been selected
for continuous monitoring by the IMS, which have potential
sources as shown in Table 4. There are 40 noble gas stations
that will be installed within some of the 80 radionuclide stations (see Figure. 1). The minimum requirements for noble gas
monitoring are determined by the CTBTO Preparatory
Commission, as stated in Table 5.10
Four different systems have been developed to sample andmeasure noble gases, namely SPALAX (French system, γ spectrometry), ARSA (American system, β-γ coincidence spec-

TABLE 4: Principal potential sources of the radioxenons
131m

Weapons
testing vent
Reactoroperations
Fuelreprocessing
Medicalisotopes

Xe

133m

Xe

X

133

Xe

X

135

Xe

X

X
X
X

X

trometr y), SAU NA (Swedish system, β-γ coincidence
spectrometry), and ARIX (Russian system, β-gated γ spectrometry). In parallel, specific analytical software is being
developed to examine the spectra produced by these different
systems.11 The SAUNA12,13 system, installed in Takasaki,
Japan, consists of three main parts: sampling, processing, anddetector units. The sampling unit collects atmospheric xenon
over the course of 12 hours via adsorption on activated charcoal. The collected gas is processed further and quantified in
the processing unit, and the β-ray and γ-ray activities are measured in the detector unit. To distinguish the radioxenon produced by a nuclear explosion from other environmental
radioxenon sources, the noble gas background at each station

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KokajiJ. Nucl. Radiochem. Sci., Vol. 14, No. 1, 2014
TABLE 5: Minimum requirements for noble gas monitoring10
Characteristics

Minimum requirements

Airflow
Total volume of sample
Collection time
Measurement time
Time before reporting
Reporting frequency
Isotopes measured
Measurement mode a)
Minimum detectable concentration b)
State of healthCommunication
Data availability c)
Downtime

0.4 m3/h
10 m3≤ 24 h≤ 24 h≤ 48 hDaily
131mXe, 133mXe, 133Xe, and 135Xe
Beta-gamma coincidence or high resolution gamma spectrometry
1 mBq/m3 for 133Xe
Status data transmitted to IDC
Two-way95%≤ 7 consecutive days, ≤ 15 days annuallya)Calibrations need to be defined.
MDCs for the other isotopes are not defined here since they critically depend on the detection system used.
c)This is a goal to be reached.
b)

has to be well understood and studied carefully.
These systems are now undergoing trials at several locations
worldwide as a part of the so-called International Noble Gas
Experiment (INGE).14 The INGE has found that the global
radioxenon background is strongly dominated by radiopharmaceutical production facilities. A few major production plants
for 99Mo and other medical isotopes currently release an orderof magnitude larger amount of 133Xe than the combined output
from all of approximately 450 nuclear power reactors that areoperational worldwide. Stations in the southern hemisphere orin Arctic regions rarely detect radioxenon, if at all.
3.3. Atmospheric Transport Modeling. AtmosphericTransport Modeling (ATM) uses meteorological data to calculate the travel path of a given radionuclide. This calculation
can be used for back tracking, in order to identify the area
where a radionuclide may have been released. Forward ATM
also predicts where radionuclides may travel from their known
point of release, also through the use of meteorological data.15
Therefore, ATM can be used in conjunction with IMS radionuclide measurements to trace and locate the original radionuclide release point.

ATM technology played an important role during early
CTBT negotiations in estimating the performance of various
designs for the global radionuclide monitoring network used
for treaty verification.16 The models FLEXPART, used by theEuropean Centre for Medium Range Weather Forecasts(ECMWF), and HYSPLIT, used by NOAA (National Oceanicand Atmospheric Administration, United States Department ofCommerce) are used operationally for CTBT analyses.9 Thesemodels also help to determine the coverage and threshold of
IMS network monitoring.
4. Application of the IMS network
4.1. DPRK nuclear test in 2006. On October 9, 2006, theDPRK government declared that they had conducted a nuclear
test. The closest IMS radionuclide particulate stations werelocated at Takasaki and Okinawa in Japan, and at Sand Point in
Alaska. Following the event, they were unable to measure any
relevant radionuclides.17 At that time, a noble gas station was
not installed at the Takasaki station. The activity of 133Xe wasonly measured at Yellowknife, Canada, as shown in Figure. 3.18

Figure 3. Daily activity concentrations of 133Xe measured at Yellowknife, Canada, during three years from 15 August 2003 to 31 October 2006
given in the upper figure (a).18 The data measured on October 2006 are enlarged in the middle figure (b), where the continuous grey pattern represents the minimum detectable concentration of 133Xe. The FLEXPART model was applied to the event on 9 October 2006 by assuming theemission of 1 PBq of 133Xe from the DPRK test site and the calculated results are shown in the lower figure (c), where the small grey histogramshows the contribution of the facility in Chalk River Laboratory of Atomic Energy of the Canada Limited.

Radiochemical Verification Technologies for the Detection of Nuclear

The FLEXPART model was applied as an ATM model basedon wind data provided by the ECMWF.18 According to
FLEXPART calculations, the radionuclides would havereached the nearest operating noble gas station in Yellowknife,
Northern Canada, on October 22, 2006, resulting in a strong
peak on the 23rd and a relatively weak peak on the 27th (seeFigure. 3 c). However, the observations of 133Xe in the
Yellowknife station were two days ahead (see Figure. 3 b). It
can be seen from Figure. 3 a that there have been 5 peaks ofeven greater radionuclide concentrations since 2003.
Regardless, the CTBTO concluded that the detection of the
DPRK test by the Yellowknife station was an excellent demonstration for the IMS and showed that noble gas stations arecapable of providing evidence of a nuclear explosion.17
Since the 2006 test, it is clear that radionuclide stations,
including those tracking noble gases, can provide unique proofof a nuclear event. However, the observation data from October
2006 was not definitive, as it did not show the highest radionucl ide pea ks as obser ved by t he Yel lowk n ife st at ion.
Furthermore, there are many operational nuclear power plants
and radioisotope production facilities in the northern hemisphere. In fact, high 133 Xe levels have been occasionallyobserved and traced back to emissions from the Chalk River
Laboratory, built in Ontario, Canada for the purposes of radiomedical radioisotope production.18 If the two day time lag as
seen in Figure. 3 can be predicted by the global dispersion
model, a more detailed evaluation is necessary to detect clandestine nuclear tests.
4.2. DPRK nuclear test in 2009. The government of the
DPRK claimed that another nuclear test had been conducted on
May 25, 2009. However, none of the radionuclide stations
detected a visible signal that could be attributed to a nuclear
explosion. Figure 4 shows the evolution of the detectableradioactive plume, as calculated for an immediate venting scenario and plotted at the times when the greatest concentrationof the radioactive plume arrived at each station.19 Three of theradionuclide stations shown in Figure. 4 were operating continuously at full performance. Their overall detection capability(Minimum Detectable Concentration, MDC) was 0.2 mBq/m3
or better throughout the relevant time period.19 By utilizing
calculations based on the assumption that the containment rate
corresponds to the full release of radioactive 133Xe, as generated by a 4 kt TNT equivalent explosion (4×1016 Bq), the explosion magnitude scale should measure 4.52 rather than 4.1, as
measured in 2006.20
In the case of 90% radionuclide containment, the detectable
plume would cover the shaded area in Figure. 4. For a containment of 99.9%, the detectable plume would cover only theareas inside of the shaded region. Since the stations located inthis zone did not record signals at corresponding times, it was
concluded that the containment of any generated xenon wasabove 99.9% 19, assuming that this was indeed a nuclear test.
The IMS coverage was intended to detect a nuclear explosionof 1 kt magnitude or greater, and the 2009 nuclear explosion at
the DPRK test site was evaluated to be more than 4 kt.
However, this event also highlights the difficulties involved
with using radionuclide stations to detect nuclear explosions
with absolute certainty.
4.3. DPRK nuclear test in 2013. On February 12, 2013,
the DPRK announced that a third nuclear test was conducted.
After 55 days following that announcement, i.e., April 8 and 9,
2013, 131m Xe and 133Xe were detected at Takasaki station andlower levels were picked up by another station in Ussuriysk,
Russia. The CTBTO press center stated that the CTBTO has
detected radioactivity consistent with the February 12
announcement of a North Korean nuclear test. According tothis article, the ratio of the detected xenon isotopes is consistent with a nuclear fission event occurring more than 50 days
prior to the detection. Using ATM on the basis of weather

J. Nucl. Radiochem. Sci., Vol. 14, No. 1, 2014

R7(1 )RN58(2) RN38(3) RN22
Figure 4. Evolution of the detectable radioactive plume as calculated
for an immediate venting scenario, plotted at the times of the strongest arrivals at (1) RN58, USSURIYSK Russian Federation at 30 May
2009, 12 UTC, (2) RN38, TAKASAKI Japan at 31 May 2009, 21
UTC, and (3) RN22, GUANZHOU China at 2 June 2009, 0 UTC.19
Mark shows each station location.

data, the DPRK test site was identified as a possible source forthe emissions.21
4.4. Fukushima Dai-ichi Nuclear Power Plant disaster.
The accident at Fukushima Daiichi Nuclear Power Station wascaused by the loss of reactor core cooling functions and theprolonged simultaneous loss of all power, due to the 13 meterhigh tsunami brought about by the massive M9.0 earthquake
on March 11, 2011. What followed after the tsunami was aserious accident in which a number of explosions occurred at
the reactor buildings, releasing radioactive materials into theatmosphere and ocean.22
The first analytical results of the IMS monitoring data wereavailable a few days after the accident. Initial detection ofradioactive materials were made on March 12 by the Takasaki
monitoring station in Japan, located about 200 km from thenuclear power plant. The dispersion of the radioactive isotopes

KokajiJ. Nucl. Radiochem. Sci., Vol. 14, No. 1, 2014

Concentration (mBq/m3 air)

R8

Collection date
Figure 5. Monitoring results of the radionuclide particles measured at Takasaki station from 12 March to 31 May 2011.24

were then tracked to eastern Russia on March 14, and to thewest coast of the United States two days later.23 Isotopes of 131I
and 137Cs were detected continuously at the Takasaki station
and reported to the IDC (see Figure. 5).24 Filters from March
13 and 14 were contaminated by the radioactive plume which
arrived on March 15. On March 16, data from the previous day
had been expected, but nothing was received due to a scheduled blackout. For this reason, data from March 12 to 15 does
not appear on Figure. 5. The early detection of 95Nb and 103Ru
was a timely indicator of a meltdown inside one or more of thereactors at Fukushima.25 By March 15, traces from the accident in Fukushima were detectable all across the northernhemisphere. By April 13, the associated radioactivity had
spread to the southern hemisphere of the Asia-Pacific region
and was clearly detectable at CTBT IMS stations located inAustralia, Fiji, Malaysia, and Papua New Guinea.
5. Discussion
The wave detection capability of the IMS using the infrasound method will provide 95% global coverage and a better
detection capability for nuclear explosions of yields equal to, or
greater than, 0.9 kt.26 For example, a 0.02 kt TNT explosion
occurring on the continental shelf off the coast of Japan was
detected more than 16,000 km across the Pacific Ocean by ahydroacoustic station.27 Following the DPRK test in 2006, the
CTBTO announced that they could detect the nuclear explosion
mainly through seismology stations. The CTBTO also reported
that nuclear test traces of the radioactive noble gas 133Xe had
been detected by the Canadian station in Yellowknife, as mentioned in Section. 4.1. However, it seems that the Yellowknife
station data could not be used as absolute evidence of the
DPRK nuclear test, since the environmental xenon concentrations are high in that area due to the presence of nuclear facilities that produce large quantities of noble gas. After the second
test by the DPRK in 2009, the 39 seismic stations registered asignal measuring 4.52 on the Richter scale at 00:54 GMT.28
However, radioxenon gas or particles were not detected byradionuclide stations in Japan, although the Takasaki noble gas
station had been installed after the first nuclear test in 2006.
There are two possibilities as to why no radionuclide signals
were detected: (i) the DPRK did not conduct a nuclear test, or(ii) all fission products were mostly sealed up by the collapseof heated sand and rock, and therefore more than 99.9% of theradionuclides were contained underground. The radionuclide

observations will strongly depend on three environmental factors: i) how much radioactive material is released, ii) how thegases or particles are carried by wind, and iii) the sensitivity ofthe detection system itself.
In the case of the Fukushima accident, worldwide attention
has focused on, and recognized, the importance of ATM. Stohl
et al.29 determined that atmospheric emissions of two isotopes,
the noble gas 133Xe and the aerosol particle 137Cs, have very different release characteristics and behavior in the atmosphere.
By using the results of the atmospheric transport model
FLEXPART28 and measurement data from several dozen IMS
stations in Japan, North America, and other regions, a total
release of 15.3 (uncertainty range of 12.2–18.3) EBq of 133Xe
was estimated to be derived from structural damage to reactor
components and/or leaks due to overpressure. For 137Cs, theresults give a total emission of 36.6 (20.1–53.1) PBq, which
may have also originated in part from the spent-fuel pool ofunit 4 of the Fukushima Dai-ichi Nuclear Power Plant, in addition to the damaged reactor cores. These evaluations can bepicked out by the ATM model, which has been used as a toolfor CTBT verification. Terada et al.30 analyzed the regionalscale atmospheric dispersion and surface deposition of 131I and137
Cs during the Fukushima accident using the WSPEEDI-II
simulation. The results suggest that the present surface deposition distribution of 137Cs over eastern Japan was produced
mainly by wet and/or dry deposition in several areas of thecountry.
6. Conclusion
In order to improve the present detection abilities of the
CTBT IMS, the ATM models mentioned above must be put topractical use. The simulation models of FLEXPART29 used for
CTBT global environmental modeling and WSPEEDI-II30 will
give more specific and detailed information. The DPRK
nuclear test in 2013 demonstrated the importance of ATM
information. The CTBT radionuclide stations also revealed anability to monitor the worldwide effects of the Fukushima
nuclear disaster. Furthermore, to improve radionuclide detection abilities and for immediate detection following a nuclear
explosion, increasing the number of noble gas stations will benecessary. Furthermore, some radionuclides are of importancefor the clarification of emergency situations, as they provide aradiological perspective that can help ascertain the internal
damage of a compromised nuclear power plant. Even thoughRadiochemical Verification Technologies for the Detection of Nuclear

the CTBT has not yet entered into force, the IMS technology isvital for maintenance of the present moratorium by State
Parties, and for nuclear accident preparation. The way in which
each State Party utilizes these IMS framework technologies
will be important in the future, depending on each country’srole and contributions.
Acknowledgements
The authors would like to thank Prof. Satoru Tanaka for hiscomprehensive advice on conducting this research and Prof.
Yusuke Kuno for his instructive comments on the manuscript.
They also extend thanks to Mr. Yoshiharu Kagawa for his valuable advice, Mr. Tetsuzo Oda for his technical advice, and Dr.
Chushiro Yonezawa for his practical help on the description of
the paper.
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