... Introduction: Motivation and Context...

... The United States and the world face serious threats to nuclear stability and peace, now and in the coming decades, and U.S. policy makers will need to make strategic decisions to assist with long-term planning as well as react to unanticipated rapid changes in the ... arena. Examples include the following: the current war in Ukraine, nuclear developments in the Middle East and Northeast Asia over the years, potential nuclear proliferation by countries friendly to the United States, and the past decades’ security crises in ... Asia. The threat of nuclear war erupting from non-nuclear conflict is not just a Cold War or immediate post–Cold War relic, but a matter of current and even urgent concern....

... Despite the Nuclear Nonproliferation Treaty, nuclear arsenals are an openly documented reality in South and Northeast Asia; military conflicts among nuclear-armed Pakistan, India, and China have been ongoing since the 1990s;1 and Iran’s stockpile of ... ). Russian troops’ unprecedented occupation of a commercial nuclear power plant in Ukraine presents yet another form of nuclear danger as of 2022 and 2023 (Granholm 2023), as does the documented interest in nuclear-explosive technologies expressed by certain terrorist groups since at least 2000....

... Defense Strategy (Biden 2022; DoD 2022; see Box 1-1).3 Nuclear terrorism threats are also evolving as nuclear proliferation and an anticipated expanded use of advanced nuclear reactors among nation-states increases opportunities for loss of material control and insider threats or possibilities of a ...

... 1 There have been numerous crises and military clashes and one war (Kargil War in 1999)....

... 2 Great powers flexing their strength and regional conflicts of smaller states with great power allegiances are reminiscent of 1914 leading up to World War I. The difference between today and ...

... 3 Previous administrations have similarly highlighted the need to expand the scope of deterrence across diplomatic, military, and nonmilitary (technological) domains; the U.S. interagency spectrum of deterrence failures; and allies and partners—as noted in their National ... Strategies and Nuclear Posture Reviews (Clinton 1994; Bush 2002)....

... strategy documents, expanding and broadening deterrence while still encompassing nuclear deterrence as a key component of national defense.a This expanded definition of deterrence also intertwines conventional weapon and nuclear weapon use and strategies. As described in the Biden administration’s ...

... Integration across domains—military (land, air, maritime, cyber, and space) and nonmilitary (economic, technological, and information)....

... Integration across regions—understanding that our competitors combine expansive ambitions with growing capabilities to threaten U.S. interests in key regions and in the homeland....

... Integration across the U.S. government—to leverage the full array of U.S. advantages, from diplomacy, intelligence, and economic tools to security assistance, and to force posture decisions....

... Integration with allies and partners—through investments in interoperability and joint capability development, cooperative posture planning, and coordinated diplomatic and economic approaches....

... a Integrated deterrence is first formally defined in the 2022 unclassified National Defense Strategy and its associated Nuclear Posture Review, but similar concepts were highlighted in Nuclear Posture Reviews of previous administrations....

... Nuclear and Radiological Materials.”4 Approaches to assessing risks of both nuclear war and nuclear terrorism to guide policy decisions should be able to accommodate and adapt to these shifts....

... To navigate the coming decades, U.S. government leaders will need to rely on nuclear security and deterrence experts who will use a variety of methods to broadly assess the short- and long-term risks of conflict, anticipate adversary actions that ... lead to nuclear war or nuclear terrorism, and understand the potential impact of U.S. responses to those actions. A key objective of risk analysis applied to nuclear war and nuclear terrorism is to avoid ... catastrophic consequences of nuclear events, the challenge being to help decision makers identify, understand, and mitigate the impacts of a wide range of scenarios. Risk analysis, done well, provides important tools and results that can help address this ... systematically, thereby offering decision makers a wider array of options and choices and potentially reducing the chances of nuclear destruction. It could address an issue identified by Thomas Schelling in his foreword to Roberta ... ’s Pearl Harbor: Warning and Decision (Wohlstetter 1961):...

... BACKGROUND AND CHARGE FOR THE STUDY...

... The committee was established and managed by the National Academies of Sciences, Engineering, and Medicine in response to a congressional mandate (P.L. 116-92, 2019). Biographies for committee members are listed in Appendix A. Congress tasked the Department of Defense (DoD) to contract with ... National Academies to independently explore U.S. government methods for assessing nuclear war and nuclear terrorism risks and how those assessments are used to develop strategy and policy. The committee’s statement of task is reprinted in Box 1-2....

... Sheet, available at https://www.whitehouse.gov/briefing-room/statementsreleases/2023/03/02/fact-sheet-president-biden-signs-national-security-memorandum-to-counter-weapons-of-massdestruction-terrorism-and-advance-nuclear-and-radioactive-material-security....

... The National Academies of Sciences, Engineering, and Medicine will convene an ad hoc committee of experts to examine whether a risk assessment framework is applicable to determining the potential risks ... nuclear terrorism and nuclear war; and to examine assumptions in nuclear policy and doctrine and their implications on national security. During this examination, the committee will undertake the following:...

... Identify risks associated with nuclear terrorism and nuclear war;...

... Explore the prior literature relevant to assessing risks of nuclear terrorism and nuclear war;...

... Assess the role that quantitative and nonquantitative analytical methods can play in estimating such risks, including the limitations of such analysis;...

... Identify and examine the assumptions about nuclear risks that underlie the national security strategy of the United States; and...

... Describe the consequences or impacts of the methods and assumptions that have been, are, or could be used in developing the nuclear security strategy of the United States....

... The committee will issue an unclassified Phase I report, which may include findings and recommendations regarding the use of analytical methods to assess the risks of nuclear terrorism and nuclear war....

... At the conclusion of the study, the committee will issue a final Phase II report that expands upon the use of analytical methods to assess the risks of nuclear terrorism and nuclear war and the role such approaches may play in U.S. security ... [Tasks 4 and 5]. This final report may include findings and recommendations supported by classified information....

... in two phases, without congressional oversight. Phase I produced an unclassified report (NASEM 2023a) focused on Tasks 1–3 of the statement of task and relied on unclassified, publicly available information. Specifically, within the Phase I report, Chapters 2 and 4 outline classes of scenarios and ... leading to nuclear war or nuclear terrorism; Chapter 3 explores the prior literature, and Chapters 5 and 6 discuss risk analysis, including qualitative and quantitative methods and their applications to the nuclear risks problem set. The Phase I report ... for Phase II by outlining methods that are used in risk assessment for a variety of applications. In Phase II, with access to classified briefings and documents, the committee focused on Tasks 4 and 5 by exploring the use of risk methods to estimate the risks of nuclear war and nuclear terrorism and ...

... a set of conclusions. Those conclusions were further supported by information gathered by the committee during Phase II (the classified phase), and the committee determined that the Phase I conclusions required no changes....

... to change—in response to the evolving threats of Russia’s invasion of Ukraine,6 as well as the emergence of two near-peer adversaries (Russia and China) and a nuclear-armed North Korea (USSTRATCOM 2021b)....

... 2023b) of a longer report determined to contain Controlled Unclassified Information. Because additional findings and conclusions are included in this expanded abbreviated report, the numbering differs from the previously released report (NASEM 2023b)....

..., so the Phase I committee did not have an opportunity to collect information on the invasion’s impact on risk methods for assessing nuclear war and nuclear terrorism within the U.S. government, as noted in the Phase I Preface....

... This report is intended to provide an overview, guidance, and advice to the U.S. government on the development of risk assessments for nuclear war and nuclear terrorism in changing environments. The report will ... equally relevant to the federal agencies and congressional committees who play a critical role in guiding and contributing to nuclear strategy and policy. In responding to the committee’s statement of task and in the context of this historical moment, the aim of this report is to identify how ... analysis tools are useful and can (1) improve the development of strategy for nuclear deterrence in the context of integrated deterrence and (2) support decision making for countering nuclear terrorism. Necessarily, the committee’s study is limited to the documents and briefings to which ...

... The committee was briefed on and investigated the methods used by U.S. government and contractor analysts to assess risks of nuclear war and nuclear terrorism as well as the context and breadth of these analyses. It did not perform a risk assessment of nuclear war or nuclear terrorism, nor ... those risks, but it did consider how risk assessments are conducted—by whom, on whose request, and with what assumptions—and how the assessments are used to guide strategy development....

... In Phase I, the role of risk analyses (Task 2) was explored by considering and identifying approaches for assessing both the overall risk as well as more focused risks of nuclear war and nuclear terrorism. These focused risks, ... the detection probability of a particular model of radiation detector at Ports of Entry. In Phase II, with access to classified information and discussions with U.S. analysts and decision makers, the committee was able to more deeply explore the roles that analytical methods play in ... overall and more focused components of nuclear risks, the benefits of the risk analysis process, and the interface between risk analysis output and nuclear strategy development (Tasks 4 and 5). The timing of global events directly influencing stability and nuclear threats in ways not seen in past ... allowed the committee a unique opportunity to explore in near real time how the U.S. government risk assessment efforts were used and modified to guide decisions....

... Finally, the committee interpreted its statement of task as focusing on the methods of risks and strategies involving nuclear conflicts and risks of nuclear terrorist attacks against the United States. Nevertheless, it recognizes that a nuclear war or terrorist act anywhere would have ...

... This report contains four chapters and three appendixes. Chapter 1, this chapter, contains one conclusion. Chapter 2, “Risk Analysis,” outlines what risk analysis is, what it can do, ... summarizes the results of the committee’s data collection efforts in Table 2-1 (nuclear war risk analysis methods used within the U.S. government) and Table 2-2 (nuclear terrorism risk analysis methods) and describes a subset of the risk methods being used; eight findings, three conclusions, and one ... are made. Chapter 3, “Development of Risk-Informed Strategies,” explores the interface between risk assessment and decision making in the context of changing U.S. deterrence strategy and countering weapons of mass destruction strategy guidance; one finding and ...

... 7 The committee had access to information up to the SECRET level, so it may be possible that information critical to its findings, conclusions, and recommendations exists at higher levels of classification. Additionally, several key SECRET-level documents were not made available to the committee, ... the classified versions of the National Security Strategy, National Defense Strategy, Nuclear Posture Review, and National Security Memorandum 19....

... Appendix A contains the biographies of Phase II committee members, consultants, and staff. Appendix B is the set of questions developed by the committee to guide the content of presentations for invited speakers and briefers and for ... requests to U.S. government agencies. Appendix C comprises the list of presenters and briefers during Phase II data collection....

... As the United States continues to implement the expanded scope of integrated deterrence, there are multiple events that could signal deterrence failure. For the purposes of this report and its focus on the ...