SUMMARY

This committee was asked to review and update the 2002 National Research Council (NRC) report, Technical Issues Related to the Comprehensive Nuclear Test Ban Treaty (NRC, 2002; hereafter referred to as the 2002 Report). That report documented the status of technical issues at the time the Senate declined its advice and consent to ratification of the Comprehensive Nuclear Test Ban Treaty (CTBT) in 1999. The statement of task for the current report requests the committee’s assessment of the following four areas, drawing on the latest evidence:

1) Maintaining the safety and reliability of the U.S. stockpile. The committee will assess, (along with other sources of information) information developed for and produced by the Nuclear Posture Review, the Administration’s plan to manage the risks in ensuring, over the longer term, a safe and reliable nuclear weapons stockpile absent underground nuclear-explosion testing. The experience of the U.S. stockpile stewardship program, particularly in the last decade, will also be taken into account.1 (See Chapter 1)

2) Nuclear explosion detection, location, and identification. The committee will assess present nuclear explosion detection capabilities, taking into account the totality of assets accessible to the United States, including (a) any improvements in U.S. national technical means in the last decade and (b) operating experience of the international monitoring system. The committee might also consider how these capabilities are expected to improve over time. (See Chapter 2)

3) Sustainability. The committee will assess what commitments are required to sustain (a) America’s nuclear stockpile; (b) the U.S. monitoring system; and (c) an adequate international verification regime, including on-site inspection. (See Chapter 3)

4) Technical advances. The committee will assess the potential technical advances to nuclear weapon capabilities for other countries (a) that result from evasive and non-evasive testing at levels below the U.S. detection capability and (b) that result from returning to full-yield testing in a non-test-ban environment. (See Chapter 4)

Maintaining and sustaining U.S. capabilities under the CTBT depends on both political and technical components. This report focuses on technical issues. A clear understanding of what can and cannot be achieved technically is important for informing the policy question of whether the ratification and entry into force (EIF) of the CTBT is in the national interest of the United States. The committee’s findings and recommendations in each of the four topic areas in the statement of task are identified throughout the text and a complete list is given in Chapter 5. Because some of the findings and recommendations relate to specific programs or topics, only a selected subset that is judged to be of interest to the broader community is brought forward into this Summary. However, the numbering of the findings and recommendations as they appear in the text is preserved here, to facilitate the location of the corresponding discussion in the chapters.

SAFETY, SECURITY, AND RELIABILITY OF THE U.S. NUCLEAR WEAPONS STOCKPILE

At the time of the 2002 Report, the Stockpile Stewardship Program (SSP) was in its early stages, and there was uncertainty about maintaining the stockpile in the absence of

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1The committee included safety, security, and reliability in its study of issues.



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SUMMARY This committee was asked to review and update the 2002 National Research Council (NRC) report, Technical Issues Related to the Comprehensive Nuclear Test Ban Treaty (NRC, 2002; hereafter referred to as the 2002 Report). That report documented the status of technical issues at the time the Senate declined its advice and consent to ratification of the Comprehensive Nuclear Test Ban Treaty (CTBT) in 1999. The statement of task for the current report requests the committee’s assessment of the following four areas, drawing on the latest evidence: 1) Maintaining the safety and reliability of the U.S. stockpile. The committee will assess, (along with other sources of information) information developed for and produced by the Nuclear Posture Review, the Administration’s plan to manage the risks in ensuring, over the longer term, a safe and reliable nuclear weapons stockpile absent underground nuclear-explosion testing. The experience of the U.S. stockpile stewardship program, particularly in the last decade, will also be taken into account.1 (See Chapter 1) 2) Nuclear explosion detection, location, and identification. The committee will assess present nuclear explosion detection capabilities, taking into account the totality of assets accessible to the United States, including (a) any improvements in U.S. national technical means in the last decade and (b) operating experience of the international monitoring system. The committee might also consider how these capabilities are expected to improve over time. (See Chapter 2) 3) Sustainability. The committee will assess what commitments are required to sustain (a) America’s nuclear stockpile; (b) the U.S. monitoring system; and (c) an adequate international verification regime, including on-site inspection. (See Chapter 3) 4) Technical advances. The committee will assess the potential technical advances to nuclear weapon capabilities for other countries (a) that result from evasive and non-evasive testing at levels below the U.S. detection capability and (b) that result from returning to full-yield testing in a non-test-ban environment. (See Chapter 4) Maintaining and sustaining U.S. capabilities under the CTBT depends on both political and technical components. This report focuses on technical issues. A clear understanding of what can and cannot be achieved technically is important for informing the policy question of whether the ratification and entry into force (EIF) of the CTBT is in the national interest of the United States. The committee’s findings and recommendations in each of the four topic areas in the statement of task are identified throughout the text and a complete list is given in Chapter 5. Because some of the findings and recommendations relate to specific programs or topics, only a selected subset that is judged to be of interest to the broader community is brought forward into this Summary. However, the numbering of the findings and recommendations as they appear in the text is preserved here, to facilitate the location of the corresponding discussion in the chapters. SAFETY, SECURITY, AND RELIABILITY OF THE U.S. NUCLEAR WEAPONS STOCKPILE At the time of the 2002 Report, the Stockpile Stewardship Program (SSP) was in its early stages, and there was uncertainty about maintaining the stockpile in the absence of 1 The committee included safety, security, and reliability in its study of issues. 3

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4 The CTBT- Technical Issues for the U.S. nuclear-explosion testing. The intervening 10 years have seen the SSP discover and resolve significant stockpile issues, but notable concerns have also arisen about maintaining the physical and human infrastructure needed for the SSP. Finding 1-1: The technical capabilities for maintaining the U.S. stockpile absent nuclear- explosion testing are better now than anticipated by the 2002 Report. Finding 1-2: Future assessments of aging effects and other issues will require quantities and types of data that have not been provided by the surveillance program in recent years. Finding 1-3: The committee judges that Life-Extension Programs (LEPs) have been, and continue to be, satisfactorily carried out to extend the lifetime of existing warheads without the need for nuclear-explosion tests. In addition to the original LEP approach of refurbishment, sufficient technical progress has been made since the 2002 Report that re-use or replacement of nuclear components can be considered as options for improving safety and security of the warheads. Finding 1-4: Provided that sufficient resources and a national commitment to stockpile stewardship are in place, the committee judges that the United States has the technical capabilities to maintain a safe, secure, and reliable stockpile of nuclear weapons into the foreseeable future without nuclear-explosion testing. Sustaining these technical capabilities will require at least the following:  A Strong Scientific and Engineering Base. There must be continued adherence to the principle that the ability to assess and certify weapons rests on technical understanding of weapons phenomena, data from past nuclear-explosion tests, computations, and data from past and ongoing experiments. Maintaining both a strategic computing capability and modern non-nuclear-explosion testing facilities (for hydrodynamic testing, radiography, material equation-of-state measurements, high explosives testing, and fusion testing) is essential for this purpose.  A Vigorous Surveillance Program. An intensive surveillance program aimed at discovering warhead problems is crucial to the health of the stockpile.  Adequate Ratio of Margin to Uncertainty. Performance margins that are sufficiently high, relative to uncertainties, are key ingredients of confidence in weapons performance.  Modernized Production Facilities. Most of the nuclear weapons production facilities are old (50 years in some cases) and are both difficult and costly to operate in accordance with modern standards of safety and security.  A Competent and Capable Workforce. Nuclear weapons work (e.g., the SSP) is key to meeting a range of challenges in the broader national security landscape. Exploration of these broader areas (nonproliferation programs, render safe, etc.) can provide opportunities for intellectual stimulation and professional development that will attract a diverse, capable workforce. It is equally important to ensure that the Department of Defense, particularly the Defense Threat Reduction Agency, the Navy’s Strategic Systems Project Office, and the Air Force’s Ballistic Missile Organization, maintain a technically competent workforce. Recommendation 1-1: To address each of the essential elements of stockpile stewardship listed in Finding 1-4, NNSA, working with the Administration and Congress as appropriate, should:

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5 Summary  Maintain a continuing dynamic of experiments linked with analysis. Both are essential to maintaining the capability to render judgments about stockpile issues.  Maintain a vigorous surveillance program that is systematic; is statistically based where possible; and continuously reflects lessons learned from annual surveillance, LEPs, fixing problems, and science-based analysis. Nondestructive tools and experimentally validated computational analysis should be developed and applied to introduce more predictive capability into the surveillance system.  As part of each LEP, explore options for achieving adequate margins through reuse or replacement scenarios in addition to refurbishment, to determine how best to meet military, technical, and policy objectives. Assess uncertainties associated with each scenario.  Develop and implement a long-term production facility modernization plan. This should include maintaining a plutonium science and production facility, including the ability to produce various types of pits for weapons in the stockpile.  Broaden the base of its nuclear expertise by involving nuclear-capable personnel in related national security projects (nuclear forensics, intelligence, threat reduction programs, basic science applications of stewardship activities, etc.). TECHNICAL MONITORING CAPABILITIES AND CHALLENGES The possibility of nuclear-explosion testing must be considered in four environments– underground, underwater, in the atmosphere, and in space. Each of the four environments requires different monitoring methods, each with different capabilities. Three streams of data for nuclear-explosion test monitoring are available: national technical means (NTM),2 the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO),3 and complementary sources (e.g., the scientific research community). 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 United States. The U.S. global monitoring capabilities are generally better than those of the CTBTO because they can enhance 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. 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 international assessment and discussion of potential violations when the United States does not wish to share NTM data. 2 In the United States, the responsibility for monitoring nuclear-explosion tests rests with the Air Force, which supports NTM for detection, location and identification through the U.S. Atomic Energy Detection System (USAEDS). 3 Under the CTBT, the international monitoring effort consists of the IMS, which generates data from its radionuclide, seismic, infrasound, and hydroacoustic networks; the International Data Centre, which collects and processes the IMS data; and a secure communications system, all managed by the CTBTO. The identification and attribution of detected signals is not carried out by the CTBTO but is the responsibility of the member states to the treaty. Throughout the text, the acronym CTBTO refers to the Preparatory Commission of the Comprehensive Nuclear Test Ban Treaty.

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6 The CTBT- Technical Issues for the U.S. 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. Seismic Monitoring4 Seismology is the most effective technology for monitoring underground nuclear- explosion testing. Seismic monitoring for nuclear explosions is complicated by the great variety of geologic media and the variety and number of earthquakes, chemical explosions, and other non-nuclear phenomena generating seismic signals every day. Finding 2-4: Technical capabilities for seismic monitoring have improved substantially in the past decade, allowing much more sensitive detection, identification, and location of nuclear events. More work is needed to better quantify regional monitoring identification thresholds5, particularly in regions where seismic waves are strongly attenuated. Recommendation 2-3: To meet its national security needs, the United States should continue to enhance and sustain its NTM seismic monitoring capabilities. Finding 2-5: One of the major advances in monitoring in the last 10 years is that most of the IMS seismic stations are operating now, and most of those have been certified for data quality (including calibration) and integrity (with respect to tampering and data authenticity). The threshold levels for IMS seismic detection are now well below 1 kt worldwide for fully coupled explosions. (See Chapters 2 and 4 for further discussion) Finding 2-6: Seismic technologies for nuclear monitoring have the potential to improve event detection, location, and identification substantially over the next years to decades. Recommendation 2-4: The United States should renew and sustain investment in seismic R&D efforts to reap the rewards of new methodologies, source models, Earth models, and data streams to enhance underground nuclear explosion monitoring, regardless of the status of CTBT ratification. Radionuclide Monitoring Nuclear explosions produce radionuclides from fission and from nuclear reactions in the materials in and around the device. Such radionuclides are produced in large quantities and high concentrations relative to natural processes and can be detected when they are either released (or vented) into the atmosphere or deposited on the ground near the detonation point. Finding 2-8: AFTAC has demonstrated notable achievements over the past decade, including major enhancements in all aspects of radionuclide monitoring. 4 In this report, the committee uses the term monitoring to refer to all aspects of detection and characterization of an event (e.g., nuclear vs. non-nuclear, geolocation, and yield estimates). 5 Throughout this report (unless otherwise qualified), by “detection threshold” the committee means detection at 90 percent confidence and at enough stations to provide a location estimate.

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7 Summary Recommendation 2-5: The United States should continue to actively support radionuclide collection, including R&D activities to better discriminate nuclear-test signature radionuclides from background, thus improving the ability to detect well- contained and lower-yield nuclear-explosion tests. Finding 2-9: In the past 10 years, the IMS radionuclide network has gone from being essentially non-existent to a nearly fully functional and robust network with new technology that has surpassed most expectations. Finding 2-11: Ongoing measurement and understanding of global backgrounds of radionuclides relevant to nuclear-explosion monitoring are critical for improving radionuclide detection. Hydroacoustic Monitoring Hydroacoustic monitoring for nuclear explosions in or near bodies of water has been utilized by the United States for many decades. Finding 2-13: The IMS detection threshold for in-water explosions is 10 tons (0.01 kt) or below worldwide and below 1 ton (<0.001 kt) throughout the majority of the world’s oceans. Infrasound Monitoring Infrasound waves are sound waves with frequencies between 0.01 and 10 Hz, below the sensitivity range of the human ear. They are produced by explosions in the atmosphere and can be detected at great distances. They are also produced by ground motion from underground explosions, and provide a complementary source of data to detect and discriminate underground nuclear-explosion tests. Finding 2-16: Integration of infrasound with seismic data and analysis will provide better detection, location, and identification of explosions. Satellite-Based Monitoring The United States has produced and maintained an impressive satellite nuclear detonation detection capability for continuous coverage of Earth and space. Modern coverage is provided by nuclear detonation detection payloads carried on multi-mission satellites (Global Positioning System, [GPS] and, at geosynchronous altitudes, the Defense Support Program [DSP] satellites), which are procured and operated by the U.S. Air Force. A decade ago when the 2002 Report was written it was anticipated that there would continue to be an effective satellite nuclear detonation detection capability with improvements timed to coincide with Air Force plans to modernize GPS and DSP. There is currently uncertainty about whether the requirement for nuclear detection is still a sufficiently high priority relative to other military requirements (See Chapter 2 and Appendix G for further discussion). 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

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8 The CTBT- Technical Issues for the U.S. interest of the United States and its allies, regardless of whether the CTBT enters into force. 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. On-Site Inspection Under the CTBT after entry into force, each State Party has the right to request an on- site inspection (OSI) to determine whether a nuclear explosion has been carried out in violation of the treaty and to gather any facts that might assist in identifying any possible violator. Inspection requests must be based on data collected by the IMS, NTM, or both. The OSI request must be approved by the CTBTO Executive Council. 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. SUSTAINING U.S. TECHNICAL CAPABILITIES UNDER THE CTBT Two technical programs that are essential to maintaining U.S. technical capabilities under the CTBT are the U.S. nuclear weapons program, including both DOE/NNSA and DOD components, and the U.S. monitoring and verification program. Chapters 1 and 2 of this report discuss these programs, and Chapter 3 discusses challenges to sustaining them. Sustaining the U.S. Nuclear Weapons Program Finding 3-2: A strong national commitment to recruiting and sustaining a high-quality workforce; recapitalizing aging infrastructure and force structure; and strengthening the science, engineering, and technology base is essential to sustaining a safe, secure, and reliable stockpile, as well as necessary explosion-monitoring capability for the United States. Recommendation 3-1: The Administration, in concert with Congress, should formulate and implement a comprehensive plan that provides a clear vision and strategy for maintaining the nation’s nuclear deterrence capabilities and competencies, as recommended in the 2010 Nuclear Posture Review and related studies. Recommendation 3-2: The DOE/NNSA should re-evaluate the current contract system for carrying out the tasks of the nuclear weapons program. At a minimum, any new approach should:  Reduce the number of requirements in directives and simultaneously transform those requirements to performance goals (prescribing what must be done, not how to do it).

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9 Summary  Shift the balance of incentives in contracts for the weapons laboratories to emphasize successful implementation of the technical mission. Sustaining the U.S. Monitoring and Verification Program Finding 3-6: Continued enhancement of the USAEDS is necessary to monitor the CTBT. Research and development of advanced monitoring capabilities are needed, including research and training at universities of the next generation of scientists and engineers. Recommendation 3-5: A sustained, predictable program of investment in nuclear- explosion monitoring R&D should be coordinated among the responsible U.S. agencies. This program should specifically include investments in university research and training programs focused on technical disciplines critical for treaty monitoring. Finding 3-10: The OSI capability of the CTBTO lags behind the readiness of the IMS; however, steps have been taken, such as the 2008 Integrated Field Exercise, which have improved OSI capabilities significantly. Recommendation 3-9: The United States should support the CTBTO OSI work by participating fully in all of its aspects, including training and field exercises. CTBT Safeguards Six CTBT safeguards were proposed in 1995. These safeguards make no mention of nuclear weapon production capabilities. There is also no mention of a means to assess whether the safeguards are adequately implemented. Finding 3-11: Without agile production capabilities, it is not possible to promptly correct deficiencies revealed by surveillance or to remanufacture components or weapons when required. Recommendation 3-10: The U.S. CTBT safeguards should include the maintenance of adequate production and non-nuclear-explosion testing facilities. Finding 3-12: There is currently no mechanism that would enable Congress to assess whether the U.S. CTBT safeguards were being fulfilled after entry into force. Recommendation 3-11: Under the CTBT, the Administration should prepare an annual evaluation of the ongoing effectiveness of safeguards and formally transmit it to Congress. POTENTIAL TECHNICAL ADVANCES FROM NUCLEAR-EXPLOSION TESTING A critical technical issue is whether the risk of adversaries developing new or improved nuclear weapons capabilities is greater with a CTBT or without the CTBT. Finding 4-1: The Nuclear Weapon States have been able to maintain their nuclear weapons programs under a nuclear-explosion-test moratorium and are likely to be able to make nuclear weapons modifications that fall within the design range of their test experience without resorting to nuclear-explosion testing.

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10 The CTBT- Technical Issues for the U.S. Hydronuclear Testing The term hydronuclear refers to a test in which criticality is achieved but the nuclear yield is less than the energy released by the high explosive. In this report the committee distinguishes hydronuclear tests as a subset of nuclear-explosion tests, most of which have nuclear yield far greater than the energy released by the high explosive but all of which are banned under the CTBT. Finding 4-2: Hydronuclear tests would be of limited value in maintaining the United States nuclear weapon stockpile in comparison with the advanced tools of the Stockpile Stewardship Program. Finding 4-3: Based on Russia’s extensive history of hydronuclear testing, such tests could be of some benefit to Russia in maintaining or modernizing its nuclear stockpile. However, it is unlikely that hydronuclear tests would enable Russia to develop new strategic capabilities outside of its nuclear-explosion test experience. Given China’s apparent lack of experience with hydronuclear testing, it is not clear how China might utilize such testing in its strategic modernization. Evasive Nuclear-Explosion Testing Finding 4-4: An evader determined to avoid detection would test at levels the evader believes would have a low probability of detection. Finding 4-5: Mine masking is a less credible evasion scenario than it was at the time of the 2002 Report because of improvements in monitoring capabilities. Finding 4-6: With the inclusion of regional monitoring, improved understanding of backgrounds, and proper calibration of stations, an evasive tester in Asia, Europe, North Africa, or North America would need to restrict device yield to levels below 1 kiloton (even if the explosion were fully decoupled) to ensure no more than a 10 percent probability of detection for IMS and open monitoring networks. Finding 4-7: For IMS and open monitoring networks, methods of evasion based on decoupling and mine masking are credible only for device yields below a few kilotons worldwide and at most a few hundred tons at well-monitored locations. Finding 4-8: The States most capable of carrying out evasive nuclear-explosion testing successfully are Russia and China. Countries with less nuclear-explosion testing experience would face serious costs, practical difficulties in implementation, and uncertainties in how effectively a test could be concealed. In any case, such testing is unlikely to require the United States to return to nuclear-explosion testing. Finding 4-9: Better technical understanding of the decoupling process in various types of geologies would likely improve the capability to detect evasive nuclear-explosion testing.

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11 Summary Recommendation 4-1: If the possibility of evasive nuclear-explosion testing through cavity decoupling continues to be a concern, the United States should:  Apply modern computational and experimental methods to understand the decoupling process in various geologies;  Identify areas such as geologic salt domes advantageous for decoupling and consider the need for additional monitoring; and  Identify indicators that a country is using—or may be planning to use— decoupling as an evasion strategy. Technical Advances Finding 4-10: Threats could arise by clandestine nuclear weapons activity. For instance, a country with no testing experience and a modest industrial base could confidently build and deploy a single-stage, unboosted nuclear weapon without any testing, if it had access to sufficient quantities of fissile material. These advances could be made whether or not the CTBT were in force. However, it is highly likely that the United States could counter these threats without returning to nuclear-explosion testing and thus could respond equally well whether or not the CTBT were in force. Finding 4-11: The value of low-yield evasive underground testing to a particular country depends on that country’s nuclear-explosion test experience and/or design sophistication.  Nuclear Weapon States could use low-yield evasive testing to partially validate design codes and modernize their arsenals.  Countries with lesser test experience could build confidence with weapons physics experiments or develop and certify inefficient, unboosted fission weapons that might pose a regional threat. Because such tests may be undetectable, these advances could be made whether or not the CTBT were in force. Finding 4-12: Russia and China are unlikely to be able to deploy new types of strategic nuclear weapons that fall outside of the design range of their nuclear-explosion test experience without several multi-kiloton tests to build confidence in their performance. Such multi-kiloton tests would likely be detectable (even with evasion measures) by appropriately resourced U.S. national technical means and a completed IMS network. Finding 4-13: Other States intent on acquiring and deploying modern, two-stage thermonuclear weapons would not be able to have confidence in their performance without multi-kiloton testing. Such tests would likely be detectable (even with evasion measures) by appropriately resourced U.S. national technical means and a completed IMS network. CONCLUSIONS First, although there are legitimate concerns about maintaining the capabilities needed to sustain U.S national security into the future, the results of this committee’s deliberations have shown that these concerns are not the result of intrinsic technical limitations and are not limited by a possible future under the CTBT. Indeed, this committee has found that the SSP has been more successful than was anticipated in 1999. Similarly, the status of U.S national monitoring and the International Monitoring System has improved to levels better than predicted in 1999.

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12 The CTBT- Technical Issues for the U.S. As a result, the committee concludes that the United States is now better able to maintain a safe and effective nuclear stockpile and to monitor clandestine nuclear-explosion testing than at any time in the past. This result has been achieved because the technical infrastructure that was developed during the Cold War has been sustained by the technical workforce trained at that time and by the successors that they have in turn nurtured. In investigating many of the concerns that were presented to this committee, the committee found that the majority reflected real or perceived inadequacy of support for this human (and the related physical) infrastructure. The committee noted in the report a number of specific examples of technical capabilities that are under pressure due to competing priorities. The committee did not attempt to judge the balance of priorities reflected, or the funding levels that might be needed to address any specific instance. Instead, the committee emphasizes that the decay of capabilities under resource stress has consequences equivalent to those of a direct policy decision. The most serious requirement for sustaining the U.S. stockpile and monitoring capabilities is a clear statement of policy regarding the capabilities that must be maintained, combined with management and support focused on achieving well-defined technical goals underpinning those capabilities. The need for such action arises whether or not the United States ratifies the CTBT. Second, the technical assessment of risks to national security that might arise as a result of the United States ratifying the CTBT must be addressed on an objective basis. Because of the results of the SSP, those risks are limited. The SSP has captured knowledge of the United States history of nuclear-explosion tests and systematized it into a discipline that could be used to recreate any of the previously tested competencies. As a result, the CTBT would not prevent the United States from responding effectively if military and political decisions required development of previously tested weapon types not now present in the stockpile. A technical need for a return to testing would be most plausible if the United States were to determine that adversarial nuclear activities required the United States to develop weapons that could not be confidently certified based on its nuclear-explosion testing experience. In such a situation, the United States could invoke the supreme national interest clause and withdraw from the CTBT.6 Third, surprise by clandestine nuclear weapons activity cannot be prevented with absolute certainty with or without the CTBT, but a fully functioning CTBTO after entry into force—with completion and sustainment of the IMS and a strengthened OSI capability—can help reduce that risk. The appropriate technical questions for this point are (1) What types of threats could arise without the United States having ample forewarning via its nuclear-test- monitoring capabilities? and (2) Would a return to testing ameliorate those threats? There are threats that could arise without detection, with or without the CTBT. For instance, a single-stage, unboosted nuclear weapon can be confidently built and deployed without nuclear-explosion testing by a nation with access to sufficient fissile material. In addition, a tested nuclear weapon design could be transferred to a proliferating nation or group (either deliberately by a sophisticated nuclear state or through espionage), which then might be able to manufacture the weapon without a test. Sophisticated nuclear states could also develop and deploy low-yield tactical nuclear weapons, based on their nuclear-explosion test experience, that could threaten U.S. allies, and the United States must consider such a possibility in its defense planning. In these cases, the committee judges that the United States would not need to return to testing to counter the resulting threat because it already has or could produce weapons of equal or greater capability, based on its own nuclear-explosion test experience. Thus, while such threats are of great concern, the United States would be able to respond to them as effectively under the CTBT as it could without the CTBT. 6 The CTBT requires a six-month delay after invoking the supreme national interest clause before a State could conduct a nuclear-explosion test. However, the time needed to prepare a test would be greater than six months.

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13 Summary The final type of threat—that of a new type of strategic weapon—would require the adversary to test at levels detectable by adequately resourced U.S. national technical means and a completed IMS network. This conclusion is based on the best present understanding of nuclear weapons development. As long as the United States sustains its technical competency and actively engages its nuclear scientists and other expert analysts in monitoring, assessing and projecting possible adversarial activities, it will retain effective protection against technical surprises. This conclusion holds whether or not the United States accepts the formal constraints of the CTBT.

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