Summary

The assessment of risk is complex and often controversial. It is derived from the existence of a hazard, and it is characterized by the uncertainty of possible undesirable events and their outcomes. Few outcomes are as undesirable as nuclear war and nuclear terrorism. Over the decades, much has been written about particular situations, policies, and weapons that might affect the risks of nuclear war and nuclear terrorism. The nature of the concerns and the risk analysis methods used to evaluate them have evolved considerably over time.

In recognition of the risks that both nuclear war and nuclear terrorism pose, the National Defense Authorization Act for Fiscal Year 2020 directed the Department of Defense to contract with the National Academies of Sciences, Engineering, and Medicine to undertake a study to explore the nature of risk analysis methods and their use in assessing nuclear war and nuclear terrorism risks.

This report represents the first phase of the study, which discusses risks, explores the risk assessment literature, highlights the strengths and weaknesses of risk assessment approaches,¹ and discusses some publicly available assumptions that underpin U.S. security strategies, all in the context of nuclear war and nuclear terrorism. The second phase of the study will expand the focus to include an analysis of the role that the assumptions and methods in risk analysis may play in U.S. security strategy. Phase II of the study will produce a classified report and an unclassified summary. Table S-1 details the committee’s work.

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¹ The terms “risk assessment” and “risk analysis” are used interchangeably in this report.

TABLE S-1 The Phase I and Phase II Committee Tasks

^a Phase II may revisit Tasks 1–4, as needed, to address Task 5.

It is important to note that the study does not include performing a risk analysis in either phase of its work. This report will also not address current geopolitical events, such as Russia’s 2022 invasion of Ukraine, although these events illustrate the importance of understanding nuclear risks during international conflicts.

The U.S. government and international community have invested significant resources and time in trying to understand and reduce the risks of nuclear war and nuclear terrorism. The current commander of U.S. Strategic Command, as well as campaigners for nuclear disarmament, have asserted that the risks of nuclear war remain very real. Similar statements have been made about the risks of nuclear and radiological terrorism. Moreover, the risks are becoming more complex as new technologies and new adversaries arise.

To identify the threats and consequences associated with nuclear terrorism and nuclear war, an analyst would confront numerous challenges while conducting a risk analysis of nuclear war or nuclear terrorism. The committee identified seven classes of scenarios that might lead to nuclear war: preventive, preemptive, escalatory, catalytic, accidental, unauthorized, and misinformed. The committee also identified three classes of scenarios that might lead to nuclear terrorism: improvised nuclear device, radiological dispersal device or radiological exposure device, and sabotage of a nuclear facility. These classes of scenarios are not mutually exclusive as other interactions among categories could also occur, such as between accidental and misinformed scenarios. These dependencies have to be reflected in any assessment of the risks. The classes of scenarios identified by the committee are used here as examples and are not collectively exhaustive; however, an analyst will

have to include all classes of scenarios that they can envision so that the risk results are not underestimated. Estimates of the immediate physical consequences from the use of nuclear weapons have relied on mathematical models based on nuclear physics, past experience, nuclear test data, and other available information. Much is known about some of the physical effects of nuclear weapons (such as immediate estimates of injuries and deaths), though some effects (such as fires, damage in modern urban environments, electromagnetic pulse effects, and climatic effects such as nuclear winter) are not yet well known or difficult to quantify (Frankel et al. 2015). Methods for assessing societal, psychological, and longer-term effects of the use of nuclear weapons have relied heavily on surrogate data for human behavior in response to other catastrophic events.² Analyses that use these methods typically contain large uncertainties and strong interdependencies.

The committee examined the history of risk assessments and analyses related to nuclear war and nuclear terrorism, including an exploration of historical attempts to understand the risks of nuclear war and nuclear terrorism, as well as the significant sources of uncertainties involved in assessing the overall risks of nuclear war and nuclear terrorism. Key insights from the historical literature are reflected throughout this report, but a notable gap is the lack of knowledge about the less-well-understood physical effects of nuclear weapons, as well as the assessment and estimation of psychological, societal, and political consequences of nuclear weapons use.

Risk information can be a crucial input for decision makers when making a variety of decisions, including the identification of priorities, the development of new policies or procedures, and the allocation of resources or time. In both natural and engineered systems, especially when statistical data are available and reliable, risk analysis based on frequencies in samples of events can readily produce estimates of future risks. As noted in previous National Academies studies, however, the application of traditional risk methodologies for nuclear war and nuclear terrorism—with limited direct evidence; great uncertainties in contexts; and intelligent, adaptive adversaries (NASEM 2016; National Research Council 2008, 2011)—represents a significant challenge. Among many assumptions, assessments of risks in such contexts have to account for the intentions and interests of the actors, their capabilities, the information and intelligence available to them, and their adaptive responses—all of which may be difficult to assess.

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² Surrogate data are information about similar phenomena but often from a different context.

The committee considered risk to refer to four key questions³:

1. What can happen? Specifically, what can go wrong?
2. How likely is it that these events will happen?
3. If these events happen, what are the potential consequences?
4. What is the time horizon in which these events might happen?

Risk analysis can be a powerful tool for clarifying assumptions; structuring and systematizing thinking about complex, interrelated factors; describing uncertainties; and identifying what further evidence or information might be needed to inform the decisions to be made. However, using risk analysis methods to assess the overall risks of nuclear war and nuclear terrorism is difficult for several reasons.

In addition to the specific conclusions detailed in the body of this report (and listed in Chapter 8), the committee reached three overall conclusions.

1. Past examples of nuclear war and nuclear terrorism are rare. As such, there is little direct evidence that can be relied on to make empirical estimates about the probability of either.

Analysts attempt to describe the resulting uncertainties by applying different methods and using multiple sources of information to supplement this limited body of evidence. Similarly, the historical record includes limited examples of attempts at nuclear or radiological terrorism, and analyses of the risks of nuclear terrorism often draw on these. The uncertainties introduced by limited direct evidence are compounded by the important role that human intentions, perceptions, and motivations play. The policy relevance of an overall risk analysis is unclear, given the significant uncertainties involved and the different possible risk attitudes of the decision makers.

While much is known about the physical consequences of nuclear and radiological weapons, the indirect consequences are not as well understood. This includes the social, economic, political, infrastructure, climate, and psychological effects, which are affected by the immediate physical effects of these weapons.

The dynamic interactions among these factors are complex, and methods of analysis for them are less developed. The minimal direct evidence about these effects represents a challenge for assessing the consequences of nuclear weapons used by a state or a terrorist. Even the bombings of Hiroshima and Nagasaki offer only limited information about the likelihood and consequences of conflicts involving modern nuclear weapons.

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³ Questions 1–3 are adapted from Kaplan and Garrick (1981). Question 4 is adapted from Paté-Cornell (2011).

Information elicited from experts is often all that is available for assessing some of the risks associated with nuclear war and nuclear terrorism. Analysts and decision makers need to be aware of the sources of that information, of the biases and limitations that the experts could introduce in the analysis, and of the resulting effects of this information on the risk results. Best practices for expert elicitation can be adapted from other risk analysis disciplines, although some aspects of nuclear war and nuclear terrorism may pose challenges for the adequate application of these methods.

2. The scenarios that might lead to nuclear war and nuclear terrorism are numerous and involve many interdependent factors, and the assessment of their risks often depends on the capabilities, values, perceptions, and intentions of many experts and actors.

The risks of nuclear war and nuclear terrorism depend in part on the effectiveness of deterrence, which reflects the capabilities, beliefs, motivations, intentions, anticipation strategies, and information available to all parties involved. The unavailability and inaccuracy of information in the throes of a crisis can potentially increase the risks faced by both aggressors and defenders. The risks of nuclear war and nuclear terrorism scenarios vary in terms of the justification or initiating reason by states or actors involved, the type and number of weapons used, and the target(s), among many other highly interdependent factors. Because there are a large number of scenario possibilities, they are often grouped together and analyzed as classes of scenarios that share some key common factors.

Assessing the overall risks of nuclear war and nuclear terrorism involves great uncertainties about the likelihood and consequences of different scenarios. The assessment and communications of these uncertainties are critical for policy decisions essential to managing these risks. However, the value of risk analysis is not solely in assessing the overall risks. Risk analysis can provide valuable input on many smaller-scale problems related to nuclear war and nuclear terrorism. Many analyses are intended to determine the relative or comparative risks of classes of scenarios (e.g., the risk of sabotage of a nuclear facility compared with the risk of a radiological exposure device; or the determination of the risk reductions associated with different investments or design changes) or to address specific questions confronting decision makers such as: What is the reliability of a particular country’s nuclear stockpile? What is the probability that a particular model of detector at an automobile border crossing will detect a specific level of radiation? Which nuclear facilities should be inspected and how often? For risk management problems that involve significant uncertainties and a need to make resource-constrained decisions, assessing the risk variations associated with different options can help inform decision making.

Analysts inevitably make assumptions in risk analysis, including assumptions about the definition and the framing of the risk problem; which models can be used effectively; the reliability of data; and the capabilities, intent, and potential actions of adversaries. Strategic assumptions can help define the boundaries of a risk problem. Some strategic assumptions address the nature or magnitude of risks, the effect of risk drivers, whether policies or actions increase or decrease the risks, the nature and the variety of threats that confront the United States, and the most likely scenarios. Strategic assumptions also include risks of nuclear wars outside the borders of the United States.

3. Different risk assessment methods are more or less suited to different situations and goals.

The committee identified the following methods relevant to analyzing these risks and considered the applicability and limitations of those methods:

• First-strike stability analyses compare the advantages to both sides of striking first in a crisis in which nuclear war appears imminent.
• Probabilistic risk assessment can explore interactions between adaptive adversaries, though extracting qualitative values from quantitative outputs may obscure some of the nuanced results.
• Order-of-magnitude estimates set extreme bounds on the probability of a nuclear incident, which can then be incrementally narrowed.
• Game theory can be used to model potential moves and their outcomes between intelligent adversaries based on information about their preferences and capabilities.
• Adversarial risk analysis can be used to evaluate possible choices of an intelligent adversary or small number of adversaries.
• Agent-based models can estimate behaviors of individuals given defined rules and uncertainties.
• Multi-attribute models assess the different elements (attributes) of the outcomes of different scenarios, according to defined and weighted criteria among the different attributes of the decision makers’ preferences.
• Network models use network analysis to explore multiple alternatives at nodes representing key events and scenarios in the path from start to end.
• Nuclear and conventional force exchange models can help assess deterrence by quantifying the outcomes of potential nuclear or conventional attacks.

Just as the structure, parameters, and assumptions in a risk analysis may color the results of a risk analysis, the ways that risk information is assessed, framed, or

presented has a powerful effect on how that information is understood and used in decisions. Risk analysis results are most valuable when the methods and assumptions by which they were generated is clear, the process is replicable, trust in the analytical process is established, and the results address the real questions or decisions that the decision makers are facing.

Risk information may be a valuable input to decision making, but it does not and cannot dictate decisions, which also depend on preferences and risk attitudes. Other considerations beyond risk need to be taken into account, such as legal, political, or budgetary consequences and constraints. Emerging technologies, such as new weapons systems and advances with artificial intelligence, are quickly changing the risk and deterrence landscape. The U.S. nuclear posture has evolved over time, taking into account new threats, developing deterrence strategies involving different U.S. adversaries, technological advancements, nuclear arms treaties, and changing geopolitical environments. U.S. assessments of the risks of nuclear terrorism have likewise evolved over time, taking into account new threats and emerging technologies.

As the context in which decisions about nuclear war and nuclear terrorism are made continues to evolve, risk assessment will continue to be a valuable tool for analysts and decision makers.

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