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2
TECHNICAL MONITORING CAPABILITIES AND CHALLENGES
Nuclear explosions generate large amounts of energy and radioactive debris in a small
fraction of a second. The rapid release of energy affects the surroundings and is propagated in
ways that can be detected and separated from other phenomena. Monitoring such signals and
radiological sampling are important tools in understanding the capabilities and threats presented
by a State that conducts nuclear-explosion tests. The formal goals for U.S. monitoring are
established by a classified presidential directive based on considerations of available monitoring
technology and risks associated with detected and undetected tests. The requirements,
techniques, and capabilities of the U.S. system are classified and are described only in the
classified version of this report.
This chapter provides an overview of how nuclear-explosion monitoring works, with
different technologies serving as elements of an overall system. U.S. national technical means
(NTM) and the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) International
Monitoring System (IMS) are summarized, followed by an assessment of the capabilities of
monitoring technologies (seismic, radionuclide, hydroacoustic, infrasound, and satellite-based
techniques). Phenomena associated with nuclear explosions, from the subsurface to outer
space, are illustrated in Figure 2-1.The chapter closes with a summary of CTBTO operational
capabilities, on-site inspections, and a discussion of transparency and confidence-building
measures.
OVERVIEW AND 2002 REPORT FINDINGS
To monitor for compliance with the CTBT, it is essential to be able to put potential
violators at risk of detection through vigilant monitoring for nuclear explosions. The possibility of
nuclear-explosion testing must be considered in four environments—underground, underwater,
in the atmosphere, and in space. The Limited Test Ban Treaty of 1963 banned signatories (the
United States, the United Kingdom, and the Soviet Union1) from nuclear-explosion testing
underwater, in the atmosphere, and in space. All known nuclear-explosion tests by other
countries (China, France, India, Pakistan, and the Democratic Peoples’ Republic of Korea) have
been underground since the last Chinese atmospheric explosion in 1980. Underground nuclear-
explosion testing may be attractive to an evader because it offers the possibility of hiding details
of the test and containing radioactive debris.
Each of the four environments requires different monitoring methods, each with different
capabilities. All monitoring methods are based on physical signatures that are associated with
nuclear explosions. These signatures are the basis for (1) concluding that an event has
occurred (detection); (2) determining the location of the event (location); (3) discriminating the
event from non-explosive phenomena (identification); and (4) in the case of a suspected
explosion, evaluating the yield, its nuclear or non-nuclear nature, and the source of the event
(characterization and attribution). A full definition of these terms is given in the glossary in
Appendix K. As described in the 2002 report, and in more detail below, there are three types of
sensor networks used for monitoring purposes: U.S. NTM networks, the international network
1
A number of other states have since signed the LTBT and the Russian Federation is bound by the
Soviet Union’s treaty commitments.
35
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36 The CTBT- Technical Issues for the U.S.
defined in the CTBT, and sensor networks deployed primarily for purposes unrelated to nuclear-
explosion monitoring (for example, to monitor earthquakes). By design, these networks are
separate; in practice, they are often complementary. U.S. NTM provides detection, location,
identification, characterization, and attribution capabilities that are primarily classified. The
CTBT IMS data is available to all Member States, and the CTBTO uses it for detection and
location, but the role of identification and further analysis is defined in the Treaty as one for the
Member States alone. All member states’ NTM, including the U.S. NTM, can make use of other
sensor network data for nuclear explosion monitoring purposes.
Throughout this report IMS network capabilities are given in terms of detection
thresholds. These determine the events the CTBTO reports on and determine the data available
to the States Parties, including data such as IMS auxiliary stations, which are retrieved upon
formation of an event. These thresholds define the IMS data available for States Parties to use
in their role of event identification, characterization and attribution. Differences between
detection and identification threshold are discussed on pages 48-49, and as noted, they can be
the same when calibrated regional discriminants are used.
Figure 2-1 shows the various regions of nuclear explosion phenomenology—subsurface,
atmosphere (low altitude), transition (high altitude), and space (monitored by the satellite
nuclear detonation detection system).
Nuclear Detonation Phenomena
FIGURE 2-1: Phenomenology of nuclear explosions, from the subsurface to outer space.
SOURCE: U.S. DOE, 2004
Table 2-1 summarizes the signals that originate from nuclear explosions in different
media, how they propagate through the environment, and what technologies are used to detect
the signals today.
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Technical Monitoring Capabilities and Challenges
TABLE 2-1: Phenomena Associated with Nuclear Explosions, and Technologies Used for
Monitoring Them.
Phenomena Primary Monitoring Propagation Technology Used
Environments
Seismic Waves Underground, underwater Through the Earth Seismometers
and water
Ground-based and
Radionuclides— Atmospheric, underground, Through air;
airborne collectors;
Particulate and underwater and space through water;
satellite-based
Gases through rock
detectors
fractures; through
space (trapped in
the Earth’s
magnetic field)
Hydroacoustic Underwater Through water Hydrophones—T-
Waves phase seismic
stations
Infrasound Waves Atmospheric Through air Infrasound
detectors
Electromagnetic Atmospheric Through air and Satellites—EMP
Pulse (EMP)2 space burst detectors*
Optical Flash Atmospheric, space Through air and Satellites—Optical
space flash detectors*
Nuclear Radiation Space Through space Satellites—
Radiation
detectors*
* Not included in the IMS but available through NTM.
SOURCE: Committee
In cases where multiple detection technologies complement one another for the same
event, the data from these disparate detection technologies can be brought together or “fused”
in order to improve the capabilities for detection, location, and identification. It is this data fusion
that determines the ultimate monitoring capabilities relevant to the CTBT. Based on
developments over the past decade, each of the technologies that can be used to monitor
compliance with the CTBT is reviewed below and updated from the 2002 Report. For each
technology, the committee considers U.S. NTM, the IMS, and other capabilities. Augmenting the
technologies summarized in Table 2-1 is information derived from confidence-building measures
(CBMs) and on-site inspections (OSI), both of which are also described in this chapter.
The 2002 Report assessed the capabilities proposed under the CTBTO IMS and stated
that, when fully implemented, the IMS would detect and identify explosions with a yield of at
least one kiloton (kt) with high confidence in all environments, assuming no efforts at evasion.
For underground explosions, it said that a yield of 10-100 tons would be detectable, although
explosions of less than a few kilotons might require an on-site inspection to confirm the
explosions as nuclear. Tests above 500-1,000 tons for atmospheric explosions could be
characterized as nuclear, and nuclear-explosion tests as low as one ton would be detected if
2
EMP is an intense pulse of electromagnetic radiation resulting from electric currents produced by
energetic radiation (neutrons, gamma rays and x-rays) from a nuclear explosion. High-altitude EMP
(HEMP) observed at the ground from a space nuclear explosion has a very fast component (E1)
produced by the gamma rays interacting with the atmosphere and a slow component (E3) caused by the
expansion of the debris “bubble” in the Earth’s magnetic field.
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38 The CTBT- Technical Issues for the U.S.
they were conducted underwater. Technical advances since the 2002 Report are discussed in
sections dealing with specific monitoring technologies later in this chapter.
Several proposed evasion scenarios were also considered in the 2002 Report, which
concluded that the only plausible techniques were cavity decoupling (a potential method for
reducing the size of the seismic signal created by an explosion by muffling the explosion in a
large underground cavity) and mine masking (concealing a nuclear explosion by conducting a
nuclear test in a region that has frequent, large chemical explosions associated with mining
operations). The report also concluded that either of these techniques would be difficult to
employ successfully and that they would have a significant chance of being detected by the IMS
for all but very low yields. Evasion techniques are discussed further in Chapter 4 and Appendix
E.
U.S. NATIONAL TECHNICAL MEANS
The United States developed and maintained the capacity to monitor nuclear-explosion
tests long before any test limitation treaties existed. Today that work is carried out by the Air
Force Technical Applications Center (AFTAC). AFTAC’s mission is “the detection of nuclear
detonations…anywhere in the world: below ground, in water, surface blasts, free-air, and in
space.” It operates and maintains the U.S. Atomic Energy Detection System (USAEDS), which
is used to monitor treaty compliance. AFTAC collects and analyzes data from a variety of
sources and is home to the U.S. National Data Center (U.S. NDC), which engages with the IMS
and the International Data Centre (IDC) of the CTBTO in the exchange of data and data
products as specified in the text and protocols of the CTBT. Other government agencies,
notably NNSA and the DOE national laboratories, carry out research on new and improved
technologies to monitor nuclear explosions and AFTAC draws on the expertise of scientists in
the DOD and DOE laboratories, the U.S. Geological Survey, U.S. academic institutions, and
private contractors. New technologies are transferred to and beta-tested by AFTAC operators.3
In contrast with the CTBTO monitoring system, whose monitoring networks, technologies, and
station placements are defined by the Treaty, U.S. national technical means may take
advantage of technologies not part of the IMS (for example, space-based monitoring) and may
focus its monitoring efforts on areas of particular interest to the United States.
NTM give the United States significant additional information beyond what is available to
other countries that do not have a robust NTM program. U.S. NTM can focus on monitoring
countries of concern to the U.S. The United States global monitoring capabilities are generally
better than those of the CTBTO because they can go beyond data available to the CTBTO with
classified capabilities. However, the inclusion of classified means and data limits the extent to
which analyses and even results may be shared and used openly. Drawing on all available
assets is important because there are CTBTO installations in locations where the United States
cannot readily deploy stations, as well as thousands of stations that operate independently of
U.S. NTM and the IMS.
Finding 2-1: U.S. National Technical Means provide monitoring capability that is superior
to that of the CTBTO, but the use of U.S. NTM for diplomatic purposes may be
constrained due to its largely classified nature.
Finding 2-2: The International Monitoring System provides valuable data to the United
States, both as an augmentation to the U.S. NTM and as a common baseline for
3
See: http://www.tt.aftac.gov/WRT/U.S.IMS/Index.html.
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Technical Monitoring Capabilities and Challenges
international assessment and discussion of potential violations when the United States
does not wish to share NTM data.
THE CTBTO INTERNATIONAL MONITORING SYSTEM
The CTBTO is based in Vienna, Austria. Its key elements are (1) the IMS, which
generates data from its radionuclide, seismic, infrasound, and hydroacoustic networks, and (2)
the International Data Centre, which collects and processes the IMS data. Because the threat of
data manipulation or denial using cyber attacks is a possibility, the IDC takes precautions to
reduce this risk, including the use of a dedicated global communication infrastructure, data
authentication and encryption, station intrusion-detection devices, and regular data back-up.
In addition, if the Treaty enters into force, there will be the possibility for conducting on-
site inspections (OSI). IMS data are transmitted in near-real time to the CTBTO and to those
national data centers (NDC) that request them. Figures 2-2 through 2-6 locate and give the
status of the more than 300 stations (whether certified, installed, under construction, or not yet
started) of the IMS (the 50-station primary seismic network, the 120-station auxiliary seismic
network,4 and 80-, 11-, and 60-station networks for radionuclide, hydroacoustic, and infrasound
technologies, respectively) as of mid-2010. The IMS monitoring station locations were
determined to maximize global coverage without focusing on any particular countries.5
A major advance in the last 10 years is that the number of certified6 IMS stations grew
from three in October 2000 to 264 in February of 2011, with an additional 17 stations installed
and undergoing evaluation. By February of 2011, about 83 percent of the full network was
implemented (certified stations plus those undergoing testing) and will approach 90 percent by
the end of 2011. Different stations address different components of the IMS mission, as shown
in Figures 2-2 through 2-6.
4
Many of the primary stations, which operate continuously for the IMS, are seismic arrays that enable
measurement of the direction to an event of interest and that enhance capability to detect small signals
and features, such as the surface-reflected waves that help determine the depth of the event. The
auxiliary seismic network records continuously but currently its data are used only when deemed
necessary to augment analysis of an event that has already been detected by the primary network
5
In assessing the capabilities of the IMS, it is important to understand that the treaty specifies that event
identification, characterization, and the attribution of a nuclear explosion to a particular country are the
responsibility of the member states, not the CTBTO. Thus the CTBTO provides data on all detected
events, as well as subsets for which events characterized (with high confidence) as earthquake-like have
been screened out, but the United States does not rely on the CTBTO for screening to decide whether a
particular event was a nuclear explosion. CTBTO is not responsible for identifying and characterizing
seismic events as nuclear explosions, earthquakes, or chemical explosions; nor does it deal with
assessing evasive testing such as decoupling (see Chapter 4).
6
A certified station in the context of the CTBT refers to a station that has been judged to substantially
meet a set of technical and operational requirements specified in the operational manuals. Certification is
essentially the last step in the process of establishing a station before data from that station is trusted.
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40 The CTBT- Technical Issues for the U.S.
FIGURE 2-2: Location and status of 49 stations of the IMS Primary Network, as of mid-2010
(the location of an additional station was undecided). Certification is the final step in preparing
to send data in real time via satellite to Vienna, and 42 stations are shown here as having
passed this stage. SOURCE: Modified from CTBTO
FIGURE 2-3: Location and status of 120 stations of the IMS Auxiliary Network, as of mid-2010.
Auxiliary stations are supported by the hosting state and not the CTBTO. SOURCE: Modified
from CTBTO
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Technical Monitoring Capabilities and Challenges
FIGURE 2-4: Location and status of 80 stations of the IMS Radionuclide Network, as of mid-
2010. All of these stations will monitor for particulates, with 40 of the stations to monitor for
xenon (gas) isotopes that are diagnostic of nuclear explosions. As of February 2011, 26 of
these noble-gas stations were transmitting data to the IDC in Vienna. SOURCE: Modified from
CTBTO
FIGURE 2-5: Location and status of 11 stations of the IMS Hydroacoustic Network, as of mid-
2010, when damage at two stations was under review for repairs (Crozet Island, shown in
green; and a station off-shore from Chile which was destroyed by a tsunami early in 2010).
SOURCE: Modified from CTBTO
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42 The CTBT- Technical Issues for the U.S.
FIGURE 2-6: Location and status of 60 stations of the IMS Infrasound Network, as of mid-2010.
SOURCE: Modified from CTBTO
The Treaty defines mechanisms for non-IMS data to be used for the purposes of
“consultation and clarification” and for consideration of an OSI request. Member States may
also request particular technical analyses from the CTBTO that can incorporate non-IMS data,
resulting in a special product that is attributed to the requesting State but made available to all
Member States. Finally, States may do their own processing, using any data they wish,
including NTM, and present their results to other Member States for Treaty purposes.
One of the CTBTO’s main products is the IDC Reviewed Event Bulletin (REB) for
waveform-technology-based events (the waveform-based REB merges data from seismic,
hydroacoustic, and infrasound stations). The REB, along with other bulletins, keeps States
informed of events detected by the IMS. The IDC’s event identification role is that of “Assisting
individual States Parties...with expert technical advice...in order to help the State Party
concerned to identify the source of specific events.'' Even though the monitoring capability of
the United States through its NTM is generally superior to that of the IMS, the data collected by
the CTBTO is a trusted, valuable and reliable source of relevant information for U.S. monitoring
purposes. USAEDS and IMS share 22 seismic stations and 3 hydroacoustic stations.
The raw data from the IMS is both protected and authenticated in multiple ways to
decrease the likelihood that data collected and transmitted could be tampered with. In addition,
there are multiple methods used to back up the IMS data so that accidental or intentional
erasure is also very unlikely. Great efforts have been made to provide data that is both well
characterized and secure. Electronic data encryption, intrusion detection, and power backups
protect data at the source; data is then transmitted through the Global Communications
Infrastructure (GCI), a dedicated communications channel designed and enabled specifically for
the CTBT.7 In addition, any state party is free to keep its own back-up of IMS data and/or IDC
products if it chooses to do so.
7
These IMS stations have a satellite link to the IDC in Vienna. Some send data continuously (e.g., the
primary seismic network). Others send data only on request or at regular intervals.
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Technical Monitoring Capabilities and Challenges
The CTBTO plays an important role for the U.S. by providing a common baseline of data
to the world scientific community, as well as providing data from areas that the United States
has previously had difficulty accessing. CTBTO analysis, software, and training/outreach
activities help to create and maintain a level of technical capability and common interpretation of
data. The CTBT provides additional verification tools beyond the IMS through an on-site
inspection mechanism and means to address events of concern through consultation and
clarification with Member States. The IDC event bulletins provide screened information to help
Member States make their own compliance decisions. A common misconception is that the
United States and its allies will rely solely on the IMS for verification of compliance with the
CTBT. “The treaty mandates that each State Party maintain a National Authority to serve as the
national focal point for liaison with the CTBTO and with other signatories” (NRC, 2002, pp. 38).
Recommendation 2-1: The United States should support both the completion of the IMS
and its operations, training, and maintenance, whether or not the CTBT enters into force.
If the CTBT were to enter into force, the resulting expertise would also aid in gaining consensus
on compliance issues, including, for example, authorizing an on-site inspection.
OTHER CAPABILITIES
There are instrument networks that have other primary uses but that have value for
detection of nuclear explosions. Examples are the numerous regional networks of seismometers
used for earthquake detection and the international tsunami warning system. Other networks,
such as those that monitor radionuclides from nuclear power plants, could also provide
potentially relevant data for analysis in the event of a suspected atmospheric detonation.
MONITORING TECHNOLOGIES8
Seismic
Seismology is the most effective technology for monitoring underground nuclear-
explosion testing, the one environment that was not precluded from nuclear-explosion testing by
the Limited Test Ban Treaty of 1963. Seismic monitoring for nuclear explosions is complicated
by the great variety and number of earthquakes, chemical explosions, and other non-nuclear
phenomena generating seismic signals every day. More than 600 earthquakes per day are
regularly documented in an international summary report, and mining operations use several
million tons of chemical explosives each year. Programs to sort out and identify signals from
underground nuclear explosions in the midst of signals from these other phenomena have made
great progress since they commenced in the 1950s, with notable improvements in the past ten
years.
Changes since the 2002 Report
Substantial improvements in the U.S. and international ability to monitor underground
nuclear-explosion testing have been made since the 2002 Report in three areas:
8
In this report, we use the term monitoring to refer to all aspects of detection and characterization of an
event (e.g., nuclear vs. non-nuclear, geolocation, yield estimates, etc.).
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44 The CTBT- Technical Issues for the U.S.
1. Verification research and development efforts have resulted in the implementation of
new regional distance (<1,600 km/1,000 miles) seismic methods to detect, locate
and identify events.
2. Many more high-quality, broader bandwidth, digital seismic stations and arrays have
been deployed globally and many of these stations transmit data in near-real time.
3. Increases in computing power and affordable, online, digital storage of waveforms on
the terabyte scale have led to improvements in all aspects of seismic monitoring.
Two important events since the 2002 Report were the announced underground nuclear-
explosion tests by North Korea in 2006 and 2009. These events were readily detected, located,
and identified as described in Box 2-3,9 demonstrating how the technological advances have
improved both U.S. NTM and CTBTO IMS monitoring. Figure 2-7 indicates the extent of
improvements in seismic monitoring achieved over the last 20 years.
FIGURE 2-7: Improvement in seismic monitoring over the last 20 years. Threshold values
indicate statistically significant confidence of detection. Note that yield and seismic magnitude
scales are logarithmic; each unit of improvement is a factor of ten. Seismic sensitivity to nuclear
explosions has improved significantly due to increased deployment of seismometers and
improved data analysis. For locations of interest, this allows regional monitoring at distances
less than about 1,600 km (1,000 miles), which has a much lower threshold (~ 20 tons or 0.020
kilotons explosive yield) than does global monitoring (~ 200 tons or 0.20 kilotons) recorded at
distances typically greater than 2,000 miles. Monitoring at test sites (e.g., through transparency
9
See, for example, the U.S. Director of National Intelligence (DNI) statements in Box 2-3 and the CTBTO
web site pages on these events. Examples of regional seismic techniques for these events are given in
Appendix D.
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Technical Monitoring Capabilities and Challenges
measures) can bring the sensitivity down to about 5 tons, or 0.005 kilotons.10 SOURCE:
Seismology Subcommittee
Seismic Signals
Seismic signals can propagate as “body waves” through the solid body of the Earth,
including its deep interior, or as “surface waves,” which travel along the Earth’s surface
(analogous to the ripples on the surface of a pond). Body waves can further be divided into P-
waves (the P standing for “primary”—these waves travel the fastest of all seismic waves and
hence are the first to arrive) and S-waves (S standing for “secondary”—their speed is about 60
percent that of P-waves). P-waves (which entail a longitudinal motion pushing and pulling in the
same direction in which they travel) are efficiently excited by explosions, whereas typical
earthquakes are efficient producers of S-waves (which entail a transverse motion, or shearing,
perpendicular to the direction in which they travel) and surface waves.
Seismic signals are traditionally grouped into categories of “teleseismic” or “regional”
depending upon the distance at which they are observed.11 Teleseismic waves do not greatly
diminish with distance in the range from about 2,500 to 9,000 km (1,563-5,625 mi), so they are
suited to monitoring a large country from stations deployed outside that country's borders.
Teleseismic waves were the basis of most nuclear-explosion test monitoring prior to the 1990s.
For sub-kiloton explosions, teleseismic monitoring can often still help with event detections but
is likely to be inadequate for event identification and therefore monitoring benefits from regional-
distance signals.
Regional waves, though often useful because they can carry more information than
teleseismic waves, are often harder to interpret because they exhibit greater variability. The
characteristics of these waves, and the ways they vary for stable and tectonic regions as well as
for continents and oceans, have been extensively researched, beginning in the late 1980s.
Under the Threshold Test Ban Treaty, the primary monitoring concern was nuclear-explosion
testing over 150 kt. This monitoring requirement could be easily handled at teleseismic
distances; under the CTBT, however, the challenge is to identify and characterize all nuclear-
explosion tests, regardless of size. Stations at closer distances can see smaller-sized events,
but the waves travel through the most complex part of the Earth—the crust and upper mantle—
which can distort the seismic signals. While research into regional seismic methods was
underway in the 1990’s, research products were just beginning to be applied to real-time
monitoring. Today, many regional monitoring methods have proven viable for
detection/association, location, identification and event characterization in real time. For many
continental areas, the use of regional data has improved monitoring sensitivity by as much as a
factor of ten compared to purely teleseismic methods. As more stations and arrays are deployed
10
Approximate fully-coupled yields are shown in Figure 2-7, as described in the 2002 Report (NRC, 2002,
p. 41), with global monitoring sensitive down to magnitudes mb = 3.8 (corresponding to fully-coupled
yields of about 135-250 tons, depending on geology, shown here as 200 tons) and regional monitoring
sensitive down to mb = 2.8 (fully coupled yields of about 6-25 tons, depending on geology, shown here as
20 tons). Sensitivity for monitoring at test sites goes down to mb = 2.2, or ~ 5 tons fully coupled yield.
Yields for Regional and Test Site thresholds refer to unclassified capabilities for monitoring countries of
concern to the U.S. for well-coupled explosions. The mention of a local threshold, lower right in figure,
refers to the future possibility of bilateral monitoring agreements, distinctly separate from the CTBT. The
magnitude estimates listed on this figure are from the IDC and other public sources. See Figure D-1 and
additional detail in Appendix D.
11
Regional and teleseismic refer to event-station distances of less than 1,600 km (1,000 miles) and
greater than 1,600 km, respectively.
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Satellite Event Location and Classification
In the past 10 years, research sponsored by DOE/NNSA at the national labs has
resulted in improved optical detection with better sensitivity and coverage, even in the presence
of obscuration due to clouds, debris, etc. In addition, modern satellite electro-magnetic pulse
(EMP) detectors are able to operate independently of the optical detectors and can now provide
credible confirmation of an atmospheric nuclear explosion. The significance of this is improved
timeliness and confidence in the detection and identification of atmospheric nuclear explosions.
In the case of a high-altitude explosion occurring in the transition region, this information would
also aid civil authorities in assessing the potential damage to ground-based infrastructure
caused by EMP effects. Finally, advances over the past decade due to incorporating modern
data processing and communications allow this vitally important information (i.e., the occurrence
of a nuclear explosion) to be made available to the highest levels of U.S. authorities quickly and
with very high confidence. For example, in the case of a nuclear explosion in space, the results
of satellite detection of nuclear radiation would immediately be available to the military
authorities in assessing the potential effects of an exoatmospheric nuclear explosion on vital
U.S. space assets.
The Future of Satellite-Based Nuclear Detonation Detection Monitoring
Today there is uncertainty regarding the future of the U.S. satellite nuclear detonation
detection capability. Modernization of the detector payloads for both upgraded GPS (GPS Block
IIF and Block III) and the follow-on to the geosynchronous DSP satellites Space-Based Infrared
System (SBIRS) has been accomplished. DOE/NNSA has continued to fund the national labs to
develop the technically complex satellite detonation detection systems, including data
processing and display systems. AFTAC continues to operate the satellite nuclear detonation
detection portion of the USAEDS, including the anticipation of upgrades and improvements in
the detection capabilities. The uncertainty in the future satellite nuclear detonation detection
monitoring capability arises because the lead time for planning future Air Force satellite
programs can be a decade or more and, once the planning and programming decisions are
made, these satellite systems are very expensive to acquire and operate. Changes come
slowly. It is not unusual for a transition to require a decade of time. Thus, during the planning
stages, competition for scarce resources (e.g., size, weight, and power) on future satellites can
be fierce. In this environment, all requirements receive intense scrutiny, including the nuclear
detonation detection mission—in other words, whether the requirement for nuclear detection is
still a sufficiently high priority relative to other military requirements to merit inclusion, or perhaps
the nuclear detonation detection mission might be accommodated on other platforms, become
important issues. Concerns with the latter option are that the alternatives potentially available
are usually far in the future, are more costly, do not meet coverage requirements, or some
combination of these. More detail on the potential loss of capability is given in Appendix G,
which notes that satellite monitoring capability is needed for warfighting and space control
missions, as well as for treaty monitoring.
Finding 2-17: Sustainment of the U.S. satellite monitoring capability to detect any nuclear
explosion in the atmosphere or space, whatever its origin, will continue to be in the
interest of the United States and its allies, regardless of whether the CTBT enters into
force.
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Recommendation 2-8: Enhanced satellite nuclear detonation detection systems should
be deployed in upgrades to GPS (GPS Block IIF and Block III) and the follow-on to DSP,
the Space-Based Infrared System (SBIRS). Provision for adequate ground-based data
processing is also essential. Decisions regarding whether and at what level to maintain
the satellite nuclear detonation detection capability should be made as part of high-level
national security policy and acquisition assessments.
OPERATIONAL CAPABILITIES OF THE CTBTO
The committee was asked to assess what commitments are required to sustain an
adequate international verification regime, including on-site inspection. The views below were
developed following interactions with the staff of the Preparatory Commission of the CTBTO and
a visit by a sub-group of the committee to the CTBTO in Vienna, Austria.
The budget available to the CTBTO is determined by the Member States to the Treaty.
In principle, the States could agree to any level of budget needed to establish and operate the
CTBTO monitoring network. The total annual operating budget for the CTBTO for 2009 was
$100M, which funded all of the activities of the CTBTO such as installation and certification of
the stations in the IMS, funding of the Global Communications Infrastructure (GCI), preparation
activities for on-site inspections, post certification activities of the stations such as operations
and maintenance of stations, reporting, analysis of data from the IMS, preparedness exercises,
management of the organization, etc. (The U.S. contribution to the annual operating budget was
$25M in 2009 and $30M in 2010.) In addition to the agreed budget allocated to the CTBTO,
States also can make voluntary contributions to the budget as well as provide support to the
verification regime, via contributions-in-kind. Although the States hosting primary and/or
auxiliary stations may and typically do make voluntary contributions, including contributions-in-
kind to defray these costs, the CTBTO must be in a position to cover the costs fully, if
necessary, because there is no requirement for States that host monitoring stations to
contribute. In addition, the CTBTO bears the cost of preparing and transmitting reports
requested by the States and must train inspectors for on-site inspection.
In 2004–2005, the CTBTO performed a System-Wide Performance Test 1 (SPT1) to
establish a performance baseline for the IDC and IMS. “Drawing upon SPT1, narrower focused
exercises to test individual components of the system were conducted in 2007 and 2008. These
exercises might lead to another system-wide performance test at a later time” (Dahlman et al.,
2009; Zerbo, 2006).
The CTBTO has a competent and dedicated staff that is operating well. It has managed
the significant increase in the number of certified IMS stations from three in October 2000 to 264
in February 2011, with an additional 17 stations installed and undergoing evaluation prior to
certification.20 These installed stations represent about 83 percent of the full network of 337
stations (321 plus 16 laboratories) and should approach 90 percent by the end of 2011. The
CTBTO staff level, however, has changed very little historically, despite the very large increase
in operating stations, data flow, and analysis. As a result, there are some signs of strain,
primarily due to the long hours worked. To move to full-time operations after entry into force, the
CTBTO will need a significant increase in staff.
In addition to collecting raw data, the CTBTO also generates a series of waveform-
based (seismic, hydroacoustic, and infrasound) events lists (REB) and a series of radionuclide
detections lists (RREB). The CTBTO also provides software, training to Member States and the
potential for State-directed additional analysis, which could include additional non-Treaty station
data.
20
See map at: http://www.ctbto.org/map.
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To meet Treaty obligations at entry into force, all of these components will have to be
fully operational. Sustaining the monitoring capabilities into the future will require further steps.
For example, the CTBTO is subject to a number of political and funding restrictions outside its
control. Examples of these include:
• a cap on the total number of employees (about 280), now limited by the provisional
status of the organization;
• a slightly lower pay scale than their neighboring agency, the IAEA due to the
CTBTO’s provisional status;
• a number of limitations tied to the Treaty itself (e.g., confidentiality of data, auxiliary
station data “by request” only, and the need to operate by consensus); and
• CTBTO employees’ are currently being limited to a 7-year tenure. Exceptions are
granted, but only for a few years. This is a potential problem for maintaining a core
group of competent analysts, because it takes several years for analysts to become
proficient.
Finding 2-18: Although the IMS is operating effectively, meeting the needs of CTBT entry
into force will require more staff and easing of budgetary constraints.
Recommendation 2-9: The United States and others should ensure that priorities and
funds are sufficient for IMS to meet ongoing needs, including after entry into force.
Table 2-3 summarizes the CTBTO’s current provisional operations compared with what
will be required when the Treaty enters into force. In many areas, the CTBTO already meets its
Treaty requirements, but this is not true in all cases.
TABLE 2-3: Comparison of the CTBTO’s Current Provisional Operations with Those That Will
Be Required When the CTBT Enters into Force.
Function Provisional Operation Entry into Force
Constant operation
Hours of operation People on duty only for business
hours (Monday–Friday) as
directed by Working Group B
Analyst training Up to a year of training needed to
reach high proficiency. The
CTBTO conducts training classes
in Vienna using the software to
create a pool of potential analysts
for future hires and to help staff
Member States National Data
Centers (NDCs).
Reviewed Event Bulletins ~ 10 days ~ 48 hours*
Standard Event List (SEL) Automated bulletins are generated Level of automation and
in 1, 4, and 6 hours after an event timelines currently meets EIF
requirement
Screened Event Bulletin Lists events that have earthquake- Screening algorithms may need
(SEB) like signals and that therefore updating as experience with the
should not be of concern as full IMS is gained.**
potential Treaty violations.
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Radionuclide processing 80-station particulate network, 40- A plan for an 80-station noble-
station noble-gas network gas network will be considered.
Sensor Operation 24 hours a day 24 hours a day
Notes: * Automatic reviewed event bulletins (REBs) will be produced less than 48 hours from the end of
the day on which the event occurred, 98 percent of the time. This will necessitate a significant increase in
staff, particularly analysts.
** The majority of events not processed have mb < 4.5. Because the 2006 and 2009 North Korean
declared nuclear-explosion tests came close to being screened out by the Ms:mb criteria, the Waveform
Expert Group under Work Group B recommended modifying the rules, and the new Ms:Mb screening
criteria were adopted in 2010.
SOURCE: Committee
IMS Construction and Operations
The CTBTO has done a good job of building and operating the IMS. The CTBTO
appears to be on a solid path to have more than 90 percent of the network built, certified and
close to meeting entry-into-force operational objectives by the end of 2010. Budgetary
constraints have caused slower than optimal construction of the IMS, and fewer analysts are
currently employed than are needed for 24-hour operation. The CTBTO has started to build
maintenance and replacement costs into its budget estimates. The seismic and infrasound
stations seem to have well-understood failure rates and sustainment budgets. The radionuclide
stations have higher failure rates than desired and may need further failure analysis and
hardware development both to meet entry-into-force operational objectives and reliable
sustainment cost estimates.
Opportunities for Technical Improvements to CTBTO Capabilities
The CTBTO seismic monitoring system could be improved if auxiliary station data were
available and incorporated into the automated system on a continuous basis. This would be
straightforward to do technically, but because the Treaty states that auxiliary data is “upon
request” and auxiliary stations are supported by the hosting state and not the CTBTO, making
such a change is a political issue and does not seem to be under consideration at present.21
Finding 2-19: A technical exercise that tests the advantages of incorporating auxiliary
seismic station data into the CTBTO’s automated system would be useful to demonstrate
the feasibility of this proposed improvement.
Finding 2-20: Location accuracy of events identified with waveform signals (seismic,
hydroacoustic or infrasound) can be improved technically by better calibration to reduce
the size of error ellipses and to improve detection and location accuracy. A technical
review that evaluates calibration efforts, such as station tuning, phase labeling, and
location accuracy, could identify ways to improve absolute location accuracy.
The CTBTO’s International Scientific Studies Conference (ISS), which occurred in 2009,
provided a way to bring CTBTO staff into contact with the broader community. These contacts
21
Note, however, that two points need to be appreciated:
(1) Lowering the magnitude level at which events are detected has the potential to increase the
number of events that remain unidentified.
(2) If the auxiliary network is incorporated into a detection network, then the CTBTO could have a
different type of obligation for maintenance of auxiliary stations.
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should be encouraged and continued as a way to provide ideas, resources, and peer review
needed to improve the CTBTO’s performance. In addition, the CTBTO could make greater use
of scientific community catalogs and bulletins (e.g., International Seismological Center [ISC],
National Earthquake Information Center [NEIC], local bulletins) to evaluate its REB.
Finding 2-21: The CTBTO benefits from systematic, sustained interaction with the
broader scientific communities involved in areas relevant to its mission.
On-Site Inspections
Article IV.D. of the CTBT provides that each State Party has the right to request an on-
site inspection (OSI) for “the sole purpose . . . to clarify whether a nuclear weapon test explosion
or any other nuclear explosion has been carried out in violation of Article I and, to the extent
possible, to gather any facts which might assist in identifying any possible violator” (CTBT
Art.IV.D.35). Inspection requests must be based on data collected by the IMS, National
Technical Means, or both.22 The OSI Request must be approved by the CTBTO Executive
Council.
After entry into force, once an inspection request is submitted, which could occur after a
lengthy political process, the location where an OSI may occur is determined from information
from either the IMS or Member States NTM. The inspection area is limited by the Treaty to 1000
km2 (390 mi2), a location uncertainty that can usually be achieved with data from the IMS. Within
the inspection area the Inspected State Party can declare restricted-access sites that can be no
larger than 4 km2 (1.6 mi2) each. The Inspected State Party can declare up to a total of 50 km2
(20 mi2) of restricted-access sites. The restricted-access sites must be separated by a minimum
of 20 m (66 ft). Unless stopped by the CTBTO Executive Council following the required 25-day
report from the Inspection Team Leader, the OSI continues up to the allowed 60 days. A
request for continuation to perform specified activities can extend the OSI for an additional 70
days. A request to conduct drilling to obtain radioactive samples can be submitted at any time
during the OSI.
The conduct of an OSI consists of a continuing process to focus the search on the
location of a possible nuclear-explosion test. For underground nuclear-explosion tests, the goal
is to ultimately find the location of the explosion test and recover radiological evidence of a
recent nuclear-explosion test by means of gas, liquid, or solid sampling. Technologies are used
in three ways in this process: (1) to narrow the search area to one or more subareas, (2) to
identify specific sites within these subareas for application of very localized OSI measures such
as the detection of relevant radionuclides emitted by the detonation, and (3) to find direct
evidence of the nuclear character of the suspect event.
The Treaty’s OSI provisions provide for the use of a broad range of technologies,
including visual inspection from the ground and during overflight, seismic aftershock monitoring,
radionuclide measurements, multispectral imaging, environmental sampling and analysis,
geophysical technologies, and drilling. Within the list of inspection technologies, drilling requires
a special request to the CTBTO Executive Council, a majority of which must approve the
request.
22
Based on experience with other treaties, some doubt it will prove possible to garner the necessary 30
votes to authorize an inspection from the 51-member Executive Council. The 2002 Report discussed this
point and concluded that on-site inspection “constitutes a deterrent to treaty violation whether or not an
inspection actually takes place, and it provides a mechanism for the innocent to clear the record” (pp. 55-
56). The committee agrees with this conclusion; therefore this section is limited to assessing what could
be gained from an OSI if it were to occur.
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When the ratification of the CTBT was considered by the U.S. Senate in 1999, there
was limited experience with how the OSI process would work. Since then, the CTBTO mounted
a significant Integrated Field Exercise in 2008 (IFE08), during which a mock inspection took
place at the former Soviet nuclear-explosion test site at Semipalatinsk, Kazakhstan. Planning
and performance of the exercise did not mimic the timeline of a real inspection, but the CTBTO
was able to work through the challenges of fielding large amounts of technical equipment;
negotiating access by the mock inspection team with the mock inspected State; dealing with
Treaty constraints; and identifying a number of logistical, scientific, and technical areas that
need further development, such as communications, deep penetration geophysics, noble gas
sampling and analysis, multi-spectral imaging, and active seismic surveys (CTBTO, 2008).
The Effectiveness of an OSI
Several factors can influence the effectiveness of an OSI: the size and placement of
the supposed nuclear-explosion test; constraints such as Treaty-allowed managed access
areas; the expertise of inspectors; time since the detonation; evasive actions that are taken
before, during or after the presumed nuclear-explosion test, such as containment or shrouding
of equipment; and the precision and accuracy of location by monitoring.
Given the difficulty and uncertainty associated with the containment of nuclear
explosions, a nuclear-explosion test of sufficient size to be detectable is likely to result in
radionuclide evidence (depending on the time since detonation) that could be detected during
an OSI, and in fact radionuclide evidence detected remotely in one or more IMS stations may be
one of the factors on which an OSI request is made. The extent of radionuclide evidence will
depend on whether the explosion is successfully contained, and it may be necessary to obtain a
fairly accurate event location (within a few km for the case of gas migration and within ~100 m
for the case of drilling) in order to find such evidence. In addition to radionuclide evidence, other
evidence associated with nuclear-explosion testing, such as detection of a borehole casing or
other nuclear-explosion test artifacts, evidence of containment measures, disturbed ground,
and/or location of the explosion cavity or rubble zone, may be considered sufficient.
Some of the radionuclides associated with a nuclear-explosion test are long-lived
radioactive noble gases, which are known to seep slowly from a nuclear explosion site (Carrigan
et al., 1996), and can be present at the surface at detectable levels for hundreds of days
following a nuclear-explosion test, even at yields of 1 kiloton and less. As Figure 2-14 shows,
detections under an OSI are probable with noble gases alone if conducted within 150 days of
the event, and longer if detection thresholds are improved.
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Noble gas activity at the surface after a 1 kT underground nuclear test
1.E+08 1.E+08
Xe-131m
Xe-133
1.E+07 1.E+07
Ar-37 (carbonate)
1.E+06 1.E+06
Ar-37 (halite)
1.E+05 1.E+05
1.E+04 1.E+04
1.E+03 1.E+03
mBq/SCM
Expected Ar-37
1.E+02 1.E+02
detection limit
1.E+01 1.E+01
1.E+00 1.E+00
1.E-01 1.E-01
Expected Xe-131m & Xe-133
1.E-02 1.E-02
Detection Limits
1.E-03 1.E-03
1.E-04 1.E-04
0 100 200 300 400 500 600 700
Decay Time (days)
FIGURE 2-14: Plot of the approximate surface concentrations of the noble gases expected
after a 1-kt nuclear detonation. The horizontal lines represent the detection limits that are
expected from instrumentation that will be used during an OSI. The plot shows two curves for
xenon (Xe-131m and Xe-133) and a range of values (shaded in grey) for argon (Ar-37). The
latter corresponds to the concentration of Ar-37 that would be expected, over the range of
values of calcium content that could be encountered during an OSI. SCM: Standard cubic meter
of air. SOURCE: Adapted from Carrigan et al., 1996
In the case where an Inspected State Party (ISP) has violated the CTBT and attempts
to thwart an OSI through the use of restricted-access areas allowed in the Treaty, it could
actually make the job of the inspectors easier because the restricted-access areas may provide
a map of the most interesting locations to perform environmental sampling. Even with
restricted-access areas, inspectors could still operate equipment as close as 1,000 m to ground
zero. This distance is expected to be close enough for both gaseous noble gas radionuclide
measurements and geophysical techniques to be effective, as well as for slant drilling to reach
the cavity.
Although it is somewhat scenario dependent, it is likely that in the 0.1 to 1 kiloton range
of yields, there will be significant aftershocks; possible surface disturbances, such as visible and
detectable cratering and fracture zones; human-made artifacts; and/or radionuclides at the
surface that have vented from the nuclear explosion. Also, the Treaty allows the use of
magnetic, electrical, and gravimetric techniques to assist in the detection of borehole casings
and other underground structures and artifacts that will aid in pinpointing the location of a
supposed nuclear-explosion test.
Unless other factors not guaranteed were used or detected, such as human intelligence
or other evidence associated with nuclear-explosion testing as discussed on the previous page,
it is likely that the effectiveness of an OSI to detect a test much below about 0.1 kilotons of yield
would be low.
The requirements on evidence needed for the Executive Council to call an OSI may be
so high that an OSI is never called. Moreover, if an OSI is called, some believe that treaty
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provisions for managing access to the site could limit the effectiveness of an OSI. However, our
assessment is that if the United States had enough evidence to call for an OSI, it would likely
have enough information from NTM and other sources to determine whether a nuclear
explosion had taken place.
Finding 2-22: A CTBTO on-site inspection (OSI) would have a high likelihood of
detecting evidence of a nuclear explosion with yield greater than about 0.1 kilotons,
provided that the event could be located with sufficient precision in advance and that the
OSI was conducted without hindrance.
Transparency/Confidence Building Measures
Any monitoring system intended to support verification of Treaty compliance—whether
the CTBT’s IMS, National Technical Means or other systems—has to be considered potentially
imperfect. The system will have sensitivity, as well as resolution in space and time, below which
events are unreliably detected; or parts of the system may occasionally break down. Such
eventualities call for additional mechanisms, including confidence-building measures, to
enhance Treaty verification.
There are many opportunities for confidence-building measures to support CTBT
monitoring. For example, China, Russia, the U.S., France and the UK all claim to sustain
nuclear arsenals through stockpile stewardship programs involving no nuclear explosions, and
there are many aspects of these programs that can be discussed openly and in a manner
enhancing mutual confidence.
Several areas that could be considered are:
1. Plutonium Science: There is an ongoing series of conferences at which
researchers from the United States, the United Kingdom, France, Russia and
(most recently) China discuss the basic science of plutonium. Though sensitive
topics are avoided, the exchange of information is useful for these nations’
technical communities. In addition to the scientific information that is
communicated, simply evaluating the level of expertise available in each country
provides significant insight about competencies and capabilities.
2. Subcritical Tests: Advanced nuclear-weapons states conduct subcritical tests,
often at past nuclear-explosion test sites; these are by definition compliant with
the CTBT and are below the sensitivity of any remote-monitoring system. How
does the testing nation assure sub-criticality, however? What procedures are in
place, and what is the combination of computational simulation and experimental
measurement used to document subcriticality? Although details are likely to be
too sensitive to be discussed, many aspects of the procedures used can be
shared among individual nations. As demonstrated during the U.S.-Soviet Joint
Verification Experiments of the late 1980s, one could even allow close-in
monitoring of one nation’s subcritical tests by another nation, with appropriate
protection of classified information.
Other opportunities involve collaboration in closely related scientific disciplines, such as
inertial confinement fusion (ICF) for producing energy or the study of warm dense matter (WDM)
at conditions existing deep inside planets. These involve major experimental facilities that can
be made available to users worldwide, with appropriate controls, including non-nuclear as well
as nuclear-weapons states.
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In addition to such activities in basic research, cooperative efforts in monitoring can
significantly enhance confidence by engaging scientists and engineers from all nations around
the world. This is already well documented in seismology, a field largely devoted to mitigating
the hazards of earthquakes, as well as documenting the structure and dynamics of our planet.
Similarly, atmospheric monitoring—and global environmental monitoring more generally—could
be greatly enhanced through cooperative measurement and modeling efforts if, for instance, it is
applied to data sharing on issues related to radiological incidents and emergency response, and
documenting spatial-temporal patterns in abundances of carbon dioxide, other greenhouse
gases, dust, or industrial pollutants.
Confidence-building measures are best established on a bilateral (or, possibly, limited-
multilateral) basis in order to ensure protection of sensitive information. For example, certain
technical details that can be shared between advanced nuclear-weapons States should not be
revealed more widely, for fear of proliferating weapons-relevant knowledge. Also, there are
many examples of information considered classified by one nation but not by another, thereby
limiting the extent of cooperation. This means that confidence building should be pursued
distinct from and in parallel with the CTBT, thereby complementing the monitoring and other
capabilities associated with the Treaty regime. The cooperative measurement of radionuclides
in the atmosphere is an example of an area that could lead to confidence-building and data
exchange in the future.
Finding 2-23: There are many opportunities for confidence-building measures to support
nuclear explosion monitoring, particularly through engaging scientists and engineers in
cooperative efforts.
Recommendation 2-10: The United States should pursue bilateral (and, to the extent
justified and politically feasible, limited multilateral) programs of scientific cooperation
for purposes of confidence building in support of monitoring nuclear explosions. These
programs should be periodically reviewed for effectiveness and for appropriate controls
on information.
Test Site Transparency
Because of the detection limits at extremely low yields and the lack of a clear definition
of "nuclear test explosion," transparency measures at known test sites could become an
important adjunct to the Treaty and merit special consideration.
One of the actions that a country, and especially a nuclear weapon state, can take to
show that it is acting in good faith in complying with the CTBT is to grant access to its nuclear-
explosion test site, including allowing some types of non-sensitive measurements to take place
at the test site. Allowing continuous measurements at a test site would also decrease the
detection threshold significantly for that location.
The U.S. Department of Energy/National Nuclear Security Administration (DOE/NNSA)
recently conducted a series of experiments to test technologies that could be used under a test
site transparency regime. Technologies evaluated included geophysical, ground-based visual,
remote monitoring, overflight, and radiological technologies. In addition, a technical evaluation
matrix was introduced to demonstrate how a set of criteria could be used to prioritize possible
monitoring technologies. Those criteria included relevance, intrusiveness, detection sensitivity,
measure confidence, equipment factors, personnel factors, and composite (an overall
assessment).
The subcritical experiment carried out at the test site contained on the order of a few
kilograms of explosive, and this amount was easily detected by seismometers approximately a
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kilometer away from ground zero. In addition, radioactivity sensors were placed near the
detonation point to verify that no radioactivity was emitted, such as xenon noble gases. These
measurements showed that it is possible to conduct a series of null-measurements at a nuclear-
explosion test site and give some level of confidence that the activities were not consistent with
a nuclear-explosion test. NNSA concluded that, among other technologies, passive seismic,
acoustic, radioactive xenon noble gases, and video would be effective for test site transparency
applications.
The technologies demonstrated during this experiment could have applications to a long-
term monitoring strategy at a test site, which might be an option for policy makers to have
confidence that activities below the detection limits of the IMS or NTM are consistent with the
CTBT.
Although in principle it would be desirable to improve test site transparency with all
States possessing nuclear weapons, the United States should give priority to Russia and China.
They are the States whose weapons programs are of the greatest strategic concern. They are
also the States most capable of benefiting from very-low-yield testing (yields up to about 1 ton,
fully coupled; see Chapter 4). Transparency measures with nuclear-capable States outside the
NPT would be more difficult to agree on politically and less important strategically.
Finding 2-24: Test-site transparency agreements can provide a mechanism for mitigating
concerns about very-low-yield testing (yields up to about 1 ton).
MONITORING AND THE NORTH KOREA NUCLEAR-EXPLOSION TESTS
The two announced Democratic People’s Republic of Korea (DPRK) nuclear-explosion
tests of 2006 and 2009 provided practical opportunities to exercise the monitoring capabilities of
the technologies discussed in this chapter. The results are discussed in Box 2-3.
BOX 2-3 The 2006 and 2009 DPRK Nuclear-explosion Tests
DPRK Status: The DPRK has not signed the CTBT; it withdrew from the NPT in 2003. In 2005,
it declared that it possessed nuclear weapons.
2006 Test: On October 9, 2006, the DPRK declared that it had conducted a nuclear-explosion
test.
• Seismic signals: registered magnitude 4.1 as reported by the CTBTO IDC Reviewed
Event Bulletin (REB) (22 stations reported—14 primary; 8 auxiliary).
• Radionuclides: Xe-133 was detected approximately 14 days following the detonation by
the IMS station in Yellowknife, Canada; Xe-133 and Xe-133m were detected by Swedish
researchers working in South Korea.
• Infrasound: none reported.
• U.S. Assessment: On October 16, 2006 the U.S. Director of National Intelligence (DNI)
released a statement: “Analysis of air samples collected on October 11, 2006, detected
radioactive debris which confirms that North Korea conducted an underground nuclear
explosion in the vicinity of P’unggye on October 9, 2006. The explosion yield was less
than a kiloton” (DNI news release, 2006).
2009 Test: On May 25, 2009, North Korea declared it had conducted a second nuclear-
explosion test.
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• Seismic signals: Registered a magnitude 4.5 as reported by the CTBTO IDC REB (61
stations reported—31 primary; 30 auxiliary)
• Radionuclides: none reported.
• Infrasound: Very small signal detected (e.g., Che et al., 2009).
• U.S. Assessment: On June 15th, the U.S. DNI released a statement that “North Korea
probably conducted an underground nuclear-explosion test… (t)he explosion yield was
approximately a few kilotons” (DNI news release, 2009).
Background: The DPRK has relatively few natural earthquakes per year compared to other
countries in the region (e.g., China, Japan and South Korea). Natural earthquakes in the DPRK
above magnitude 4 occur only once every few years, so the seismic signal stood out as unusual
even before the DPRK government made its announcements. There are several IMS primary
and auxiliary seismic stations within 1,200 km (750 mi) of the DPRK. Within the same distance
there are additional non-IMS seismic stations that produce publicly available data and that were
used by seismic researchers to analyze the tests in the days immediately afterward. In addition,
China, Japan and South Korea each maintain dense networks of seismic stations to monitor
earthquake hazards in the region. Although the data from these stations are not all publicly
available, researchers in these countries have used them in analysis of the DPRK tests (e.g.,
Hong et al., 2008). In the region, earthquakes are routinely reported at levels of magnitude 3
and below; mine blasts and industrial chemical explosions are usually less than magnitude 3.5.
Observations:
• The seismic signals from the two events clearly indicate that they were explosions and
not earthquakes. The 2006 nuclear explosion was an excellent real world test of
empirical seismic methods for a sub-kiloton explosion in a new region and the
discrimination methods worked very well.
• A question surrounding the two DPRK tests is why radionuclides were reported following
the 2006 test but not following the 2009 test. Many, if not most, tests in the range of 1 kt
have resulted in the release of detectable levels of radioactive noble gases. Containment
of radionuclides following a test is complex, but it is thought that containment may be
harder for a smaller test than for a larger one because it may be more straightforward to
establish a “stress containment cage” for larger tests (see Appendix D). No system is
perfect and the failure to detect in one case does not invalidate the utility of such a
detection network.
The two DPRK tests made it clear that if multiple nuclear-explosion tests occur in the same
region, then relative (“differential”) methods of detection, location, discrimination and yield
estimation can be brought to bear on the verification problem.
