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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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37 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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39 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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41 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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43 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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45 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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66 The CTBT- Technical Issues for the U.S. 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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67 Technical Monitoring Capabilities and Challenges 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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68 The CTBT- Technical Issues for the U.S. 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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69 Technical Monitoring Capabilities and Challenges 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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70 The CTBT- Technical Issues for the U.S. 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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71 Technical Monitoring Capabilities and Challenges 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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72 The CTBT- Technical Issues for the U.S. 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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73 Technical Monitoring Capabilities and Challenges 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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74 The CTBT- Technical Issues for the U.S. 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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75 Technical Monitoring Capabilities and Challenges 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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76 The CTBT- Technical Issues for the U.S. • 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.