Hard constraints on a Chinese nuclear breakout

ABSTRACT

Arguments about China's nuclear-modernization program and its implications for US nuclear policy rest on a fundamental misunderstanding. American experts have resurrected fears of a Chinese nuclear “breakout”—defined here as either development of a nuclear-warfighting capability or a significant and rapid increase in the scale of China's nuclear arsenal—to justify significant changes to US nuclear policies. In addition to the “soft” political constraints on China's nuclear arsenal, Beijing also faces a number of “hard” technical constraints, especially on a “sprint to parity” with the United States and Russia. A Chinese nuclear breakout would require significant changes to China's nuclear infrastructure that are likely to be highly detectable. The United States, while being vigilant for indications of a future Chinese nuclear breakout, should also adopt policies that decrease the likelihood of such changes.

KEYWORDS:

• China
• plutonium
• fissile material
• missiles
• deterrence
• nuclear testing

Notes

1 Donald Rumsfeld, Testimony before the Committee on Armed Services of the United States Senate, July 25, 2002, p. 7, <http://armed-services.senate.gov/statemnt/2002/july/rumsfeld.pdf>.

2 Senator John Kyl's remarks quoted in David Wright, “More Confusion about China,” Union of Concerned Scientists, December 19, 2010 <http://allthingsnuclear.org/dwright/more-confusion-about-china>; Gordon Chang, “START-ing without China,” Wall Street Journal, January 27, 2010, <www.wsj.com/articles/SB10001424052748703906204575027821767691054>.

3 See New Start Treaty Resolution of Advice and Consent, US Senate, 22 December 2010, section (c)(7), <www.state.gov/documents/organization/154123.pdf>.

4 I thank one of the anonymous reviewers for this point.

5 See, for example, Matthew Kroenig, “Think Again: American Nuclear Disarmament,” Foreign Policy, September 3, 2013, <http://foreignpolicy.com/2013/09/03/think-again-american-nuclear-disarmament/>; Elbridge Colby, “Nuclear Weapons in the Third Offset Strategy: Avoiding a Nuclear Blind Spot in the Pentagon's New Initiative,” Center for a New American Security, February 2015, pp. 9–11.

6 For the argument that China's underground tunnel network suggests a much larger arsenal, see Phillip A. Karber, “Strategic Implications of China's Underground Great Wall,” September 26, 2011, <https://fas.org/nuke/guide/china/Karber_UndergroundFacilities-Full_2011_reduced.pdf>. For reporting on the Karber analysis, see William Wan, “Georgetown Students Shed Light on China's Tunnel System for Nuclear Weapons,” Washington Post, November 29, 2011, <www.washingtonpost.com/world/national-security/georgetown-students-shed-light-on-chinas-tunnel-system-for-nuclear-weapons/2011/11/16/gIQA6AmKAO_story.html>. For a thorough rebuttal of the Karber argument and its related reporting, see Jeffrey Lewis, “Collected Thoughts on Phil Karber,” Arms Control Wonk, December 7, 2011, <www.armscontrolwonk.com/archive/204799/collected-thoughts-on-phil-karber/>. Kristensen and Norris estimate China has a stockpile of 260 nuclear warheads, while Zhang estimates a stockpile of 170, with 110 deployed. See Hans M. Kristensen and Robert S. Norris, “Chinese Nuclear Forces, 2016,” Bulletin of the Atomic Scientists, Vol. 72 (July 2016), <http://thebulletin.org/2016/july/chinese-nuclear-forces-20169627>; Hui Zhang, “China's Nuclear Weapons Modernization: Intentions, Drivers, and Trends,” presentation at the Institute for Nuclear Materials Management 53rd Annual Meeting, Orlando, July 15, 2012, <http://belfercenter.ksg.harvard.edu/files/ChinaNuclearModernization-hzhang.pdf>.

7 The essential history of the Chinese nuclear program is John W. Lewis and Xue Litai, China Builds the Bomb (Stanford, CA: Stanford University Press, 1988). For more on China's distinctive views of nuclear weapons, see Jeffrey Lewis, Paper Tigers: China's Nuclear Posture (New York: Routledge, 2014); Li Bin and Tong Zhao, eds., Understanding Chinese Nuclear Thinking (Washington, DC: Carnegie Endowment for International Peace, 2016).

8 For a recent overview of ongoing changes to China's nuclear forces, see Eric Heginbotham, Michael Chase, Jacob Heim, Bonny Lin, Mark R. Cozad, Lyle J. Morris, Christopher P. Twomey, Forrest E. Morgan, Michael Nixon, Cristina L. Garafola, and Samuel K. Berkowitz, China's Evolving Nuclear Deterrent: Major Drivers and Issues for the United States (Santa Monica, CA: RAND, 2017). For a discussion of possible constraints, see chapter 8. For an assessment of potential changes to China's nuclear arsenal, strategy, and policies, see chapters 9 and 10.

9 For a detailed articulation of China's nuclear strategy and policies, see “China's National Defense in 2006,” Information Office of the State Council, December 29, 2006, <http://fas.org/nuke/guide/china/doctrine/wp2006.html. Some American experts and officials have expressed skepticism about the credibility of China's pledge in a crisis, especially if China believed its second-strike nuclear capability were placed at risk by an adversary's conventional assets. For skepticism about China's NFU pledge, see, for example, Brad Roberts, “China–U.S. Nuclear Relations: What Relationship Best Serves U.S. Interests?” Institute for Defense Analyses, August 2001, <www.au.af.mil/au/awc/awcgate/dtra/china_us_nuc.pdf>. For an argument that China might relax its NFU pledge in certain circumstances, see Thomas J. Christensen, “The Meaning of the Nuclear Evolution: China's Strategic Modernization and U.S.–China Security Relations,” Journal of Strategic Services, Vol. 35, No. 4 (2012), pp. 474–81. US experts who often raise concerns about a possible Chinese nuclear breakout also tend to be dismissive of the credibility of China's NFU policy.

10 See, for example, M. Taylor Fravel and Evan S. Medeiros, “China's Search for Assured Retaliation: The Evolution of Chinese Nuclear Strategy and Force Structure,” International Security, Vol. 35, No. 2 (2010), pp. 48–87; Fiona S. Cunningham and M. Taylor Fravel. “Assuring Assured Retaliation: China's Nuclear Posture and U.S.–China Strategic Stability.” International Security, Vol. 40, No. 2 (2015), pp. 7–50; Jeffrey Lewis, The Minimum Means of Reprisal (Cambridge, MA: MIT Press, 2007); Lewis, Paper Tigers.

11 Li Bin, “China and Nuclear Transparency,” in Nicholas Zarimpas, ed., Transparency in Nuclear Warheads and Materials: The Political and Technical Dimensions (New York: Oxford University Press, 2003), p. 52.

12 “China's National Defense in 2008,” Information Office of the State Council, January 2009, sections II and VII, <http://english1.english.gov.cn/official/2009-01/20/content_1210227.htm>.

13 For a discussion of Chinese views of nuclear weapons and conventional military operations, see Liu Chong, “The Relationship between Nuclear Weapons and Conventional Military Conflicts,” in Li Bin and Tong Zhao, eds., Understanding Chinese Nuclear Thinking (Washington, DC: Carnegie Endowment for International Peace, 2016), pp. 149–69. For discussion of how China's evolving concepts of deterrence may increasingly be blurring the lines between the conventional and nuclear domains, see Michael S. Chase and Arthur Chan, “China's Evolving Approach to ‘Integrated Strategic Deterrence,’” (Santa Monica, CA: RAND, 2016).

14 Mark Stokes, “China's Nuclear Warhead Storage and Handling System,” Project 2049, March 12, 2010, <https://project2049.net/documents/chinas_nuclear_warhead_storage_and_handling_system.pdf>.

15 Li Bin, “China's Potential to Contribute to Multilateral Nuclear Disarmament,” Arms Control Today, March 3, 2011, <www.armscontrol.org/act/2011_03/LiBin>.

16 For a general discussion of China's nuclear modernization, see Gregory Kulacki, “China's Nuclear Arsenal: Status and Evolution,” Union of Concerned Scientists, October 2011, <www.ucsusa.org/sites/default/files/legacy/assets/documents/nwgs/UCS-Chinese-nuclear-modernization.pdf>. For a discussion of the effects of some emerging conventional precision-strike capabilities on the US–China nuclear relationship, see Joshua H. Pollack, “Boost-Glide Weapons and US–China Strategic Stability,” Nonproliferation Review, Vol. 22, No. 2 (2015), pp. 155–64. For an examination of some Chinese concerns regarding US ballistic missile defense architecture, see Wu Riqiang, “China's Anxiety about US Missile Defence: A Solution,” Survival, Vol. 55, No. 5 (2013), pp. 29–52. For an argument that China's core nuclear strategy and policies would remain unchanged despite its ongoing modernization program and advancing US military capabilities, see Yao Yunzhu, “China Will Not Change Its Nuclear Policy,” China–U.S. Focus, April 22, 2013, <www.chinausfocus.com/peace-security/china-will-not-change-its-no-first-use-policy/>.

17 US Department of Defense, Annual Report to Congress: Military and Security Developments Involving the People's Republic of China 2017 (Washington, DC, May 15, 2017), p. 31.

18 For official reference to China's emerging SSBN fleet, see US Department of Defense, Military and Security Developments, pp. 24, 60. For discussions of China's emergent ballistic-missile submarine force, see Tong Zhao, “China's Sea-Based Nuclear Deterrent,” Carnegie–Tsinghua Center for Global Policy, June 30, 2016, <http://carnegietsinghua.org/publications/?fa=63909>; David C. Logan, “China's Future SSBN Command and Control Structure,” Institute for National Strategic Studies, National Defense University, November 2016. For reference to a future nuclear-capable strategic bomber, see US Department of Defense, Military and Security Developments, p. 61.

19 Based on an average increase in China's overall stockpile of six nuclear warheads per year. This assumes a stockpile of two hundred warheads in 2006 and a stockpile of 260 warheads in 2016. Estimates from Natural Resources Defense Council, “Chinese Nuclear Forces, 2006,” Bulletin of the Atomic Scientists, 2006; Hans M. Kristensen and Robert S. Norris, “Chinese Nuclear Forces, 2016,” Bulletin of the Atomic Scientists, June 13, 2016.

20 See, for instance, Elbridge Colby, “Welcome to China and America's Nuclear Nightmare,” National Interest, December 19, 2014, <http://nationalinterest.org/feature/welcome-china-americas-nuclear-nightmare-11891>; Elbridge Colby, “Nuclear Weapons in the Third Offset Strategy: Avoiding a Nuclear Blind Spot in the Pentagon's New Initiative,” Center for a New American Security, February 2015, pp. 9–11. Colby argues that China's ongoing nuclear-modernization efforts could eventually allow China “tailored use” of nuclear weapons in a “narrower band … of escalation,” including possibly using nuclear weapons in an otherwise conventional conflict.

21 Walter Pincus, “NATO's Atomic Stockpile: Primarily Political Weapons,” Washington Post, December 11, 1978, <www.washingtonpost.com/archive/politics/1978/12/11/natos-atomic-stockpile-primarily-political-weapons/9fceeb3b-3b62-484b-8f2b-26ecf0f35616/>.

22 Paul Schulte, “Tactical Nuclear Weapons in NATO and Beyond: A Historical and Thematic Examination,” in Tom Nichols, Douglas Stuart, and Jeffrey D. McCausland, eds., Tactical Nuclear Weapons and NATO (Carlisle Barracks, PA: Army Strategic Studies Institute, 2012), p. 48.

23 William C. Potter, Nikolai Sokov, Harald Muller, and Annette Schaper, “Tactical Nuclear Weapons: Options for Control,” United Nations Institute for Disarmament Research, UNIDIR/2000/20 (2000), p. 57.

24 These include the estimated weight of the reentry vehicle. Hans M. Kristensen and Robert S. Norris, “Chinese Nuclear Forces, 2016,” Bulletin of the Atomic Scientists, June 13, 2016, p. 2; Jeffrey Lewis, “China's Belated Embrace of MIRVs,” in Michael Krepon, Travis Wheeler, and Shane Mason, eds., The Lure & Pitfalls of MIRVs: From the First to the Second Nuclear Age (Washington, DC: Stimson Center, 2016), p. 95.

25 Lewis, Paper Tigers, p. 43.

26 For an argument that China possesses the technical base to develop tactical nuclear weapons but has decided not to do so, see Charles D. Ferguson, Evan S. Medeiros, and Phillip C. Saunders, “Chinese Tactical Nuclear Weapons,” in Brian Alexander and Alistair Millar, eds., Tactical Nuclear Weapons: Emergent Threats in an Evolving Security Environment (Dulles, VA: Brassey's, 2003), pp. 110–26.

27 Lewis, Paper Tigers, pp. 44, 69.

28 However, these later warhead designs were plutonium based. As discussed below, testing may still be required for miniaturization and to ensure credibility of designs using HEU as fuel. For a good history and discussion of China's ERW program, see Jonathan Ray, Red China's “Capitalist Bomb”: Inside the Chinese Neutron Bomb Program (Washington, DC: National Defense University Press, 2015).

29 According to Ray, “The prospect of halting nuclear testing alarmed the weaponeers because they had not turned success in mastering principles into tested final designs. As of 1986, [head of theoretical research for the ERW program] Yu Min reported that despite 10 years of research on miniaturization, the weaponeers still needed to weaponize the designs.” Quote from Ray, Red China's “Capitalist Bomb”, p. 26. See also p. 30 for a discussion of problems with weaponization.

30 The limited plutonium stockpile is addressed more in the next section on a sprint to parity.

31 Hisham Zerriffi, “Tritium: The Environmental, Health, Budgetary, and Strategic Effects of the Department of Energy's Decision to Produce Tritium,” Institute for Energy and Environmental Research, January 1996, p. 1. Other boosted weapons are estimated to require an average of 4 grams of tritium.

32 Ray, Red China's “Capitalist Bomb”, p. 27.

33 This would also be a limiting factor for any potential sprint to parity. Hans M. Kristensen, “No, China Does Not Have 3,000 Nuclear Weapons,” Federation of American Scientists, December 3, 2011, <https://fas.org/blogs/security/2011/12/chinanukes/>. Reliable estimates of China's tritium production or stockpile are scarce. However, rough calculations suggest its tritium stockpile is limited. China is believed to have produced tritium at its 125 MWt High Flux Experimental and Test Reactor (HFETR) outside Chengdu. Assuming an annual tritium production rate of 1–5 kg per thermal gigawatt, the HFETR, which operated with HEU from 1979 to 2007, could have produced between 125 and 625 grams of tritium per year. Using median values, China would have produced enough tritium for five hundred neutron bombs. However, this assumes the reactor operates continuously and is configured solely for tritium production. In addition, China's existing warheads would have consumed this stockpile in both initial production and periodic replenishment. Tritium decays at a rate of 5.5 percent per year and must be replenished; half of a warhead's tritium is gone in twelve years. In short, China likely has little tritium available to support an ERW arsenal. For estimates on tritium production and decay rates, see Martin B. Kalinowski and Lars C. Colschen, “International Control of Tritium to Prevent Horizontal Proliferation and to Foster Nuclear Disarmament,” Science & Global Security, Vol. 5 (1995), pp. 151, 134. For information on China's HFETR, see “HFETR,” Research Reactor Database, <https://nucleus.iaea.org/RRDB/RR/HeaderInfo.aspx?RId=54>.

34 For some discussion in the context of the country's neutron-bomb program, see Ray, Red China's “Capitalist Bomb”, especially p. 30. For an argument that China has been similarly conservative in the design and testing of its deployed warheads, see Lewis, Paper Tigers, chapter 2.

35 Caroline Reilly, “Assessing the Prospect of China's Potential ‘Sprint to Parity,’” in Nuclear Scholars Initiative: A Collection of Papers from the 2011 Nuclear Scholars Initiative (Washington, DC: Center for Strategic and International Studies, 2011), pp. 166–7.

36 For more on the CTBT and its associated monitoring system, see Andreas Persbo, “Compliance Science: The CTBT's Global Verification System,” Nonproliferation Review, Vol. 23, Nos. 3–4 (2016), pp. 317–28.

37 The use of an overpressure target of 5 psi (35 kpa) is illustrative, though, according to one well-known source, “A convenient rule of thumb for estimating the short-term fatalities from all causes due to a nuclear attack is to count everyone inside the 5 psi (35 kpa) blast overpressure contour around the hypocenter as a fatality.” See Carey Sublette, “Nuclear Weapons Frequently Asked Questions,” Nuclear Weapon Archive, updated May 15, 1997, <http://nuclearweaponarchive.org/Nwfaq/Nfaq5.html>. The amount of overpressure required for disablement varies significantly by kind of target. Aircraft are especially vulnerable and may be rendered inoperable at an overpressure as low as 1 psi (7 kpa). Wood-framed buildings will collapse at an overpressure of 5 psi (35 kpa), while nearly all buildings will collapse at an overpressure of 10 psi (70 kpa). At 15 psi (105 kpa), the probability of eardrum rupture is roughly 50 percent. Typical automotive vehicles may be rendered inoperable at an overpressure of 5 psi (35 kpa), though armored vehicles will likely still be operable. Surface ships generally sustain light damage at overpressures of 5 psi (35 kpa); it is estimated it would take an overpressure of 30 psi (210 kpa) to cause severe damage to older classes of aircraft carriers. See Samuel Glasstone and Phillip J. Dolan, eds., “The Effects of Nuclear Weapons,” 3rd ed, (US Department of Defense, 1977), pp. 154–230; “Overpressure Levels of Concern,” Office of Response and Restoration, National Oceanic and Atmospheric Administration, revised August 1, 2016, <http://response.restoration.noaa.gov/oil-and-chemical-spills/chemical-spills/resources/overpressure-levels-concern.html>.

38 Lethal radius estimate derived from the equation LR = 2.62 × Y⅓/H⅓, where LR is the lethal radius of the nuclear weapon, Y is the yield of the warhead in megatons, and H is the hardness of the target in psi (7 kpa).

39 Derived from the equation SSPK = 1 – 0.5(LR/CEP)^2, where SSPK is the single-shot kill probability measured as a percentage, LR is the lethal radius in meters, and CEP is the circular error probable of the missile in meters.

40 According to some reports, China may have tested nuclear variants of its short-range ballistic missiles, though it is not believed to currently deploy any. According to one account by Kristensen and Norris, a declassified 1990 Central Intelligence Agency (CIA) assessment stated that China had conducted a nuclear test “related to the development of a warhead for a Chinese short-range ballistic missile,” which is believed to be a reference to the DF-15. They go on to cite a declassified 1993 CIA report that assessed “China will begin to field nuclear-armed CSS-X-6's [DF-15] next year.” However, there are few details on actual deployment of nuclear-armed short-range systems. Western estimates of Chinese nuclear forces generally do not include either the DF-15 or the DF-11. For reference to the 1990 and 1993 CIA assessments, see Hans M. Kristensen and Robert S. Norris, “Chinese Nuclear Forces, 2015,” Bulletin of the Atomic Scientists, Vol. 71, No. 4 (2015), p. 80. For a notional Chinese nuclear order of battle, see Lewis, Paper Tigers, p. 104; Hans M. Kristensen and Robert S. Norris, “Chinese Nuclear Forces, 2015,” Bulletin of the Atomic Scientists, Vol. 71, No. 4 (2015), p. 78; Fiona S. Cunningham and M. Taylor Fravel, “Assuring Assured Retaliation: China's Nuclear Posture and U.S.–China Strategic Stability,” International Security, Vol. 40, No. 2 (2015), p. 43. Accurate information on CEPs is notoriously difficult to acquire. Public estimates for CEPs of US missiles can vary 30 percent or more. For estimates of the design and performance characteristics of China's missile forces, see “Design Characteristics of China's Ballistic and Cruise Missile Inventory,” Nuclear Threat Initiative, 2014, <www.nti.org/media/pdfs/design_characteristics_of_chinas_ballistic_cruise_missiles_3.pdf>. For writing on the difficulty of estimating CEP, see Bruce Blair and Chen Yali, “The Fallacy of Nuclear Primacy,” China Security, Autumn 2006, p. 62.

41 Akira Marusaki, “Developments in China's Conventional Precision Strike Capabilities,” Project 2049, November 23, 2015, <http://project2049.net/publications.html>.

42 Personal communication with Phillip C. Saunders, August 23, 2016. The author thanks Dr. Saunders for this point.

43 For some discussion of the potential drivers and risks of conventional–nuclear entanglement in China's missile forces, see David C. Logan, “Reading Schelling in Beijing: The Escalation Risks of Nuclear–Conventional Entanglement in China's Missile Forces,” working paper.

44 Heginbotham et al., China's Evolving Nuclear Deterrent, pp. 135–7.

45 I thank one of the anonymous reviewers for this point. See US Department of Defense, Military and Security Developments Involving the People's Republic of China (Washington, DC, 2016), p. 62, 72.

46 Gregory Kulacki, “China's Military Calls for Putting Its Nuclear Forces on Alert,” Union of Concerned Scientists, January 2016, <www.ucsusa.org/sites/default/files/attach/2016/02/China-Hair-Trigger-full-report.pdf>.

47 For some discussion on the different interpretations of “strategic warning” in the Chinese context, see Tong Zhao, “Strategic Warning and China's Nuclear Posture,” The Diplomat, May 28, 2015, <http://thediplomat.com/2015/05/strategic-warning-and-chinas-nuclear-posture/>. For some discussion of Chinese early warning and possible changes to force alert status and operational practices, see James M. Acton, “China's Offensive Missile Forces,” testimony before the US–China Economic and Security Review Commission, April 1, 2015, p. 4, <www.uscc.gov/sites/default/files/Acton%20USCC%20Testimony%201%20Apr%202015.pdf>.

48 David C. Logan, “PLA Reforms and China's Nuclear Forces,” Joint Forces Quarterly, No. 83, No. 4 (2016), pp. 57–62.

49 For some discussion of different methodologies for assessing the size and configuration of China's nuclear forces, see Mark A. Stokes, “China's Nuclear Warhead Inventory: Alternative Approaches for Research and Analysis,” statement before the US–China Economic and Security Review Commission, March 26, 2012, <www.uscc.gov/sites/default/files/3.26.12stokes.pdf>.

50 See, for example, Senator Jon Kyl's remarks quoted in David Wright, “More Confusion About China,” Union of Concerned Scientists, December 19, 2010 <allthingsnuclear.org/dwright/more-confusion-about-china>; Gordon Chang, “START-ing without China,” Wall Street Journal, January 27, 2010, <www.wsj.com/articles/SB10001424052748703906204575027821767691054>; Matthew Kroenig, “Think Again: American Nuclear Disarmament,” Foreign Policy, September 3, 2013, <http://foreignpolicy.com/2013/09/03/think-again-american-nuclear-disarmament/>.

51 Acton, “China's Offensive Missile Forces,” p. 4.

52 For a list of Chinese nuclear tests, see Lewis, Paper Tigers, p. 69.

53 Thomas C. Reed, “The Chinese Nuclear Tests, 1964–1996,” Physics Today, September 2008, pp. 47–53.

54 “Global Fissile Material Report 2015: Nuclear Weapon and Fissile Material Stockpile and Production,” International Panel on Fissile Materials, 2015, pp. 23, 25.

55 “History of Nuclear Testing,” Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization, <www.ctbto.org/nuclear-testing/history-of-nuclear-testing/world-overview/>.

56 Lewis, Paper Tigers, p. 46.

57 This article leaves aside the issue of theft of warhead designs, which could conceivably ease the testing requirements. However, given the general difficulties of developing reliable nuclear weapons and China's generally conservative approach to nuclear-warhead designs, it would still likely want to engage in testing. For the initial report, see Select Committee, US House of Representatives, H. Repot. “U.S. National Security and Military/Commercial Concerns with the People's Republic of China,” H. Rept. 105–851, May 25, 1999. For a critique and rebuttal of the report's findings, see Alastair Iain Johnston, W.K.H. Panofsky, Marco Di Capua, and Lewis R. Franklin, “The Cox Committee Report: An Assessment,” Center for International Security and Cooperation, Stanford University, December 1999. For an argument that, even with theft of US warhead designs, China would face significant challenges in developing comparable weapons, see Richard L. Garwin, “Why China Won’t Build U.S. Warheads,” Arms Control Association, April 1, 1999, <www.armscontrol.org/act/1999_04-05/rgam99>.

58 This estimate of the amount of plutonium used per weapon is based on Lewis, Paper Tigers, p. 45. Lewis uses an estimate of 4–6 kg, though such a range appears more illustrative than definitive. For the technical basis for this estimate, see Lars-Erik De Geer, “The Radioactive Signature of the Hydrogen Bomb,” Science & Global Security, Vol. 2 (1991), pp. 351–63. Based on estimates that the November 17, 1976, Chinese nuclear test created 260 kilocuries of strontium-90, De Geer concludes there was “7.7 ± 2.6 kilograms of trigger plutonium left after the shot” (pp. 360–1). This estimate, however, includes a fairly substantial “one sigma estimated error” of 2.6 kg. There is, therefore, significant uncertainty. Indeed, there is a 5 percent chance that the debris from the test included less than 2.5 or more than 12.9 kg of plutonium. Using such numbers, and assuming a plutonium stockpile of 1.8 metric tons, the ceiling on Chinese uranium-fueled weapons could, theoretically, range from 140 to 720 weapons. However, the lower-bound two-sigma estimate of 2.5 kg per warhead is highly unlikely given both that US weapons are believed to use an average of 4 kg per weapon and the conservative character of Chinese nuclear policy.

59 Here I use the 4–7 kg per warhead estimate provided by Lewis (see footnote 62). For a range of estimates of the maximum potential size of China's nuclear arsenal, see the technical appendix.

60 This assumes no resumption of military plutonium production. Estimate of 260 warheads from Hans M. Kristensen and Robert S. Norris, “Chinese Nuclear Forces, 2016,” Bulletin of the Atomic Scientists, June 13, 2016, pp. 205–11. Hui Zhang provides a lower estimate of 170 total warheads and only 110 deployed warheads. See Hui Zhang, “China's Nuclear Weapons Modernization: Intentions, Drivers, and Trends,” presentation at the Institute for Nuclear Materials Management 53rd Annual Meeting, Orlando, FL, July 15, 2012, <http://belfercenter.ksg.harvard.edu/files/ChinaNuclearModernization-hzhang.pdf>.

61 See discussion in Lewis, Paper Tigers, chapter 2. The CHIC-4 has an estimated mass of up to 1,200 kg, which would make it too heavy for many of China's existing ballistic-missile variants.

62 Lewis, Paper Tigers, p. 52.

63 “Global Fissile Material Report 2015,” p. 15.

64 The estimate of 25 kg per warhead refers to the International Atomic Energy Authority's (IAEA's) definition of a significant quantity of HEU, or the amount of the material to produce a single nuclear device. Experts have estimated that US uranium-fueled weapons contain an average of 20 kg of weapon-grade uranium and 10–15 kg of weapon-grade uranium per thermonuclear primary and 15–25 kg per thermonuclear secondary. I thank one of the anonymous reviewers for this point. China could also opt for simpler gun-type weapon designs, though those are estimated to require as much as 50 kg of HEU per warhead, further lowering the ceiling of a potential Chinese nuclear buildup. For an estimate of average HEU per US warhead, see Steve Fetter, Valery A. Frolov, Oleg F. Prilutsky, and Roald Z. Sagdeev, “Fissile Materials and Weapon Design,” Science & Global Security, Vol. 1 (1990), p. 255. For an estimate of fissile material in gun-type devices, see “Preventing Nuclear Terrorism,” Union of Concerned Scientists, April 2004, p. 2, <www.ucsusa.org/sites/default/files/legacy/assets/documents/nwgs/nuclear_terrorism-fissile_materials.pdf>. For an estimate of fissile material in thermonuclear secondaries, see Harold A. Feiveson, “Unmaking the Bomb: A Fissile Material Approach to Nuclear Disarmament and Nonproliferation,” Physics and Society, Vol. 44, No. 3 (2015), p. 5.

65 This may be a generous ceiling, as the estimates of uranium per Chinese warhead apply to the primaries of China's nuclear weapons and do not take into account fissile material used in the secondaries of the country's thermonuclear weapons. Assuming consistency in weapon design, these figures of fissile material per warhead may even underestimate fuel requirements and mean that China might confront an even lower ceiling on the maximum feasible size of its nuclear arsenal using existing fissile material stockpiles.

66 Hans M. Kristensen and Robert S. Norris, “Status of Nuclear Forces,” Federation of American Scientists, <http://fas.org/issues/nuclear-weapons/status-world-nuclear-forces/>.

67 The author does not necessarily endorse US and Russian abrogation of arms control agreements in such a scenario, but rather notes their possibility.

68 Hui Zhang, “China's Fissile Material Production and Stocks,” in International Panel on Fissile Materials, Global Fissile Material Report 2010: Balancing the Books (Princeton, NJ: Princeton University, 2011), p. 106.

69 As early as 2000, experts demonstrated the ability of commercial satellite imagery to detect vapor plumes from reactor cooling towers and warm water discharges into nearby bodies of water. See Hui Zhang and Frank N. von Hippel, “Using Commercial Imaging Satellites to Detect the Operation of Plutonium-Production Reactors and Gaseous-Diffusion Plants,” Science & Global Security, Vol. 8 (2000), pp. 219–71.

70 Author's calculations based on information from Hui Zhang, “China's Fissile Material Production and Stocks,” in International Panel on Fissile Materials, Global Fissile Material Report 2010: Balancing the Books, (Princeton, NJ: Princeton University, 2011), pp. 97–102.

71 For a thorough treatment of China's recent expansion of uranium-enrichment capacity for commercial purposes, including the expansion and indigenization of centrifuge technology, see Hui Zhang, “China's Uranium Enrichment Capacity: Rapid Expansion to Meet Commercial Needs,” Project on Managing the Atom, Discussion Paper No. 2015-03, August 2015, <http://belfercenter.ksg.harvard.edu/files/chinasuraniumenrichmentcapacity.pdf>.

72 Figure does not include SLBMs. US Department of Defense, Military and Security Developments, p. 60.

73 This estimate focuses only on growth in the ICBM force. The two-hundred-year estimate that appears earlier in this article refers to overall average growth across the entire force. This estimate is based on past deployment rates and is illustrative of the relatively slow recent growth in the country's nuclear missile force. China could expand its missile production capability or use production facilities for conventional missiles.

74 “Why Is China Modernizing its Nuclear Arsenal?” event transcript, Carnegie Endowment for International Peace, March 24, 2015, pp. 16–19, <http://carnegieendowment.org/files/12-chinanucleararsenal240315wintro-formatted.pdf>. Retaining older systems may provide systems that are more amenable to multiple independently targetable reentry vehicle (MIRV) weapons, which might be viewed as having greater penetration ability against US ballistic-missile defense systems. US officials believe some of China's DF-5B silo-based ICBMs are now deployed with MIRV capability. See US Department of Defense, Military and Security Developments, p. 31.

75 For a recent discussion about how changes in China's nuclear policies might have a cascading effect on regional nuclear security dynamics, see Greg Thielmann and David Logan, “The Complex and Increasingly Dangerous Nuclear Weapons Geometry of Asia,” Arms Control Association, July 27, 2016, <www.armscontrol.org/files/Threat_Assessment_Brief_Nuclear_Weapons_Geometry_of_Asia.pdf>.

76 For some discussion, see David C. Logan, “China's Future SSBN Command and Control Structure,” Institute for National Strategic Studies, National Defense University, November 2016; Tong Zhao, “China's Sea-Based Nuclear Deterrent,” Carnegie–Tsinghua Center for Global Policy, June 30, 2016, <http://carnegietsinghua.org/publications/?fa=63909>.

77 See, for example, Christensen, “The Meaning of the Nuclear Evolution,” pp. 460–66. For some general theoretical and empirical discussion, see Todd S. Sechser and Matthew Fuhrmann, “Crisis Bargaining and Nuclear Blackmail,” International Organization, Vol. 67, No. 1 (2013), pp. 173–95; Matthew Kroenig, “Nuclear Superiority and the Balance of Resolve: Explaining Nuclear Crisis Outcomes,” International Organization, Vol. 67, No. 1 (2013), pp. 141–71.

78 Logan, “Reading Schelling in Beijing.”

79 For some good discussion of implications for US policy, see Caroline Reilly, “Assessing the Prospect of China's Potential ‘Sprint to Parity,’” in Nuclear Scholars Initiative: A Collection of Papers from the 2011 Nuclear Scholars Initiative (Washington, DC: Center for Strategic and International Studies, 2011), pp. 156–73.

80 Mark Stokes, “China's Nuclear Force Infrastructure: A Notional Breakout Scenario,” draft paper, East Asian Alternative Weapons Futures Conference, February 21, 2014, <www.npolicy.org/article.php?aid=1258&rid=2>.

81 Stokes, “China's Nuclear Warhead Storage and Handling System.”

82 Fumihiko Yoshida, “Confronting Plutonium Nationalism in Northeast Asia,” Bulletin of the Atomic Scientists, June 30, 2016, <http://thebulletin.org/confronting-plutonium-nationalism-northeast-asia9617>; Frank von Hippel, “Reprocessing Policy and South Korea's New Government,” Bulletin of the Atomic Scientists, May 15, 2017, <http://thebulletin.org/reprocessing-policy-and-south-korea%E2%80%99s-new-government10768>; “Two British Ships Arrive in Japan to Carry Plutonium to U.S.,” The Telegraph, March 21, 2016, <www.telegraph.co.uk/news/worldnews/asia/japan/12199884/Two-British-ships-arrive-in-Japan-to-carry-plutonium-to-US.html>.

83 An analysis of the feasibility of constructing usable weapons with civilian plutonium is beyond the scope of this article. For some discussion of the general risks of ostensibly civilian plutonium programs, see Charles Ferguson, “Risks of Civilian Plutonium Programs,” Nuclear Threat Initiative, July 1, 2004, <www.nti.org/analysis/articles/risks-civilian-plutonium-programs/>. For an authoritative official assessment that reactor-grade plutonium could be used to construct a weapon, see Committee on International Security and Arms Control, National Academy of Sciences, Management and Disposition of Excess Weapons Plutonium (Washington, DC: National Academy Press, 1994), p. 33.

84 Brad Sherman, Jeff Fortenberry, and Adam Schiff, “Letter to President Regarding the Production of Fissile Material in East Asia,” June 10, 2016, <www.npolicy.org/article.php?aid=1317&rid=2>; Henry Sokolski, “Japan and South Korea May Soon Go Nuclear,” Wall Street Journal, May 8, 2016, <www.wsj.com/articles/japan-and-south-korea-may-soon-go-nuclear-1462738914>. House bill HR 3766, introduced by Representatives Ileana Ros-Lehtinen and Brad Sherman in the 113th Congress, would have required Congressional approval of nuclear cooperation agreements that did not prohibit reprocessing by the recipient country. However, that bill never made it out of committee. For bill text, see <www.congress.gov/bill/113th-congress/house-bill/3766>.

85 Nie Rongzhen, as quoted in Lewis, The Minimum Means of Reprisal, p. 1.

86 For a discussion of strategic restraint in the US–China security context, see David C. Gompert and Phillip C. Saunders, The Paradox of Power: Sino-American Strategic Restraint in the Age of Vulnerability (Washington, DC: National Defense University Press, 2011).

87 Hui Zhang, “China's Nuclear Weapons Modernization: Intentions, Drivers, and Trends,” presentation at the Institute for Nuclear Materials Management 53rd Annual Meeting, Orlando, FL, July 15, 2012, <http://belfercenter.ksg.harvard.edu/files/ChinaNuclearModernization-hzhang.pdf>.

88 Median estimate of current arsenal is simply the average of low and high estimates.

89 Hans M. Kristensen and Robert S. Norris, “Chinese Nuclear Forces, 2016,” Bulletin of the Atomic Scientists, Vol. 72 (July 2016), <http://thebulletin.org/2016/july/chinese-nuclear-forces-20169627>.

90 De Geer, “The Radioactive Signature of the Hydrogen Bomb,” pp. 351–63.

91 Ibid.

92 Ibid.

93 “Global Fissile Material Report 2015,” pp. 23, 25.

94 Ibid.

95 Ibid.