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Risk Analysis Methods for Nuclear War and Nuclear Terrorism (2023)

Chapter: 4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism

« Previous: 3 The History and Literature of Risk Assessment for Nuclear War and Nuclear Terrorism
Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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4

The Use of Risk Assessment for Nuclear War and Nuclear Terrorism

The threats of nuclear war and terrorism have evolved over the years as the geopolitical landscape and the leadership, capabilities, and objectives of U.S. adversaries have changed. The nuclear capabilities of the United States have also evolved. This chapter offers a snapshot of some of the current threats contributing to the risks of nuclear war or terrorism and a discussion of what is known about the range of possible consequences of nuclear war and nuclear terrorism.

THREATS OF NUCLEAR WAR

Though the world’s stockpile of nuclear weapons has been reduced by more than 80 percent since its Cold War peak, unclassified estimates suggest that some 13,000 nuclear weapons remain in the world. Clearly, numbers alone do not tell the whole story: for instance, the role of new weapons and technologies has become increasingly significant. Yet, as the destruction of Hiroshima and Nagasaki showed in 1945, one nuclear weapon can instantly destroy much of a city and kill tens or hundreds of thousands of people. Much has changed since then, in world politics, in technology, and in nuclear policies—including, not least, the collapse of the Soviet Union, the end of the global Cold War, the acknowledgment of a larger number of nuclear states, and the emergence of other technologies (e.g., hypersonic weapons, cyber capabilities, artificial intelligence, and disinformation).

While no nuclear weapons have been detonated in conflict since 1945, the risk of nuclear war and nuclear and radiological terrorism remains very real. The challenge presented to this committee is how one might assess those risks. In an

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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analytical process, a risk analyst would begin by identifying the risks associated with nuclear war and nuclear terrorism, describing classes of scenarios that involve either nuclear war or nuclear terrorism. The next steps would include exploring what is known about the probabilities of their occurrence, the uncertainties, other military and nonmilitary options, and the possible consequences of the use of nuclear weapons. This section explores those factors.

The scenarios of potential future nuclear conflicts considered here happen in the context of a roughly 10-year time horizon. Key questions include the following: How will these open or latent conflicts evolve? What scenarios could lead to different directions? How can risk analysis shed some light on various risk reduction options?

The committee does not attempt to assess the magnitude of the risks of nuclear war posed at this time by these different conflicts. The order in which the issues are discussed below is based on the total number of nuclear weapons that could potentially be targeted on the United States.

The United States and Russia

The United States and Russia possess approximately 90 percent of the known nuclear weapons in the world. Tensions between the two countries are high as this report is being written, having taken a severe downturn after Russia’s annexation of Crimea in 2014 and the U.S. and European imposition of sanctions in response. These tensions have been exacerbated by Russia’s 2022 invasion of Ukraine, during which Russia has threatened the use of nuclear weapons.

Former senator Sam Nunn and former Secretary of Energy Ernest Moniz argued in 2019 that this geopolitical hostility, the cutoff of various forms of communication and cooperation, and evolving technologies may undermine the stability of deterrent balances (Moniz and Nunn 2019). Because of ongoing hostility between the two sides, nearly all military-to-military contact below the highest levels, nearly all legislator-to-legislator contact, and nearly all contact between nuclear scientists has been ended. As of 2022, the world’s two largest nuclear complexes are modernizing and developing new weapon systems, increasing some of their arsenals’ capabilities, and proceeding in virtually total isolation from each other. One particularly worrisome scenario involves Russia’s attempt to reestablish control over the Baltic states, which could start with a fast-paced conventional invasion followed by a limited nuclear attack intended to consolidate military gains and prompt the North Atlantic Trade Organization to come to the negotiating table rather than countering Russia’s advances directly.

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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The United States and China

Tensions between the United States and China have also risen dramatically in recent years. China’s economic and military power has grown, and China’s foreign policy has become more assertive. As with Russia, China and the United States each have a long list of complaints about the other’s behavior.

China possesses hundreds of nuclear weapons, compared with the thousands in the U.S. and Russian nuclear arsenals, but a major modernization of its nuclear forces is under way. As of early 2021, the U.S. intelligence community expected China to at least double the size of its nuclear stockpile over the next decade (ODNI 2021). Nongovernment analysts have also noted that China appears to be building hundreds of new silos for what may be intercontinental ballistic missiles (Korda and Kristensen 2021).

China has declared a policy of no-first-use of nuclear weapons and contends that this greatly reduces the risks of nuclear conflict. But it is easy to imagine scenarios in which a regional conflict might escalate to higher levels of violence, such as a Chinese attempt to force Taiwan reunification (Talmadge 2017). Chinese conventionally armed missiles would pose a serious threat to U.S. naval forces in such a conflict, possibly leading to a U.S. decision to attempt to destroy many of these missiles. But as China, like other countries, has missiles and command systems with dual nuclear and conventional roles, such a U.S. attack might be seen as the beginning of an effort to destroy China’s nuclear forces, calling for a nuclear response. This mixing of nuclear and conventional forces (which exists in most nuclear states) has come to be known as entanglement and could increase the risks of escalation that neither side initially intended (Acton 2018). Some U.S. analysts argue that China is overconfident in its ability to control escalation, potentially increasing risks (Cunningham and Fravel 2019).

China has not participated in nuclear arms control, so there are no treaties or political commitments limiting its nuclear forces, except for (1) the broad Nuclear Nonproliferation Treaty obligation to negotiate in good faith toward both general and nuclear disarmament and (2) the Comprehensive Nuclear Test Ban Treaty, which—like the United States—it has signed but not ratified. Moreover, as of late 2021, there were no strategic stability talks under way between the United States and China, and there were few confidence-building measures, such as notification of missile launches or military-to-military cooperation.

The United States and North Korea

It is estimated that North Korea now has dozens of nuclear weapons and a variety of missiles to deliver them. It has tested missiles with the range and payload necessary to reach nearly all of the continental United States. While it has not

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

carried out full-scale tests of reentry vehicles at that range, it is probably safest to assume that North Korea could deliver nuclear weapons to the United States; and it could, with greater certainty, threaten South Korea, Japan, and China.

The Kim family dictatorship that rules North Korea has long shown a savvy instinct for survival—illustrating how deterrence can be effective. They have a history of violent provocations against South Korea, such as the 2010 shelling of a South Korean island and the sinking of a South Korean ship in the same year. In the future, one could easily imagine another such provocation escalating to large-scale conflict, which could lead North Korea to believe its survival depended on disabling South Korean and U.S. attacks by using nuclear weapons. In such a conflict, North Korea might use a small number of conventionally armed missiles against U.S. and South Korean air bases to interfere with joint air operations. Once North Korea had begun firing its missiles, the United States and South Korea might launch an air campaign to destroy the remaining North Korean missiles before they could do more damage. That air campaign would create pressure on the North to use its nuclear weapons before they were destroyed; the air campaign might be seen by the North as preparation for an all-out invasion that had to be stopped by destroying key U.S. bases with nuclear weapons. That initial use might then lead to a larger-scale nuclear conflict.1

India and Pakistan

India and Pakistan are each thought to have more 100 nuclear weapons, as well as a range of systems to deliver them. The relative proximity of each country to the other may complicate effective early warning. Neither is likely to attack the United States, but nuclear war between the two of them is believed to be quite possible. The two have fought four wars since independence; they share a hotly disputed border; and they have hostile relations, little communication, and few agreements or confidence-building measures to help manage their conflict. A 2019 clash led to the interception of an Indian military aircraft over Pakistan and heated public calls for war on both sides. In addition, a technical malfunction during maintenance in March 2022 launched an Indian missile into Pakistan and reignited the debate on risk mitigation and nuclear weapon responsibility in the region (Shahzad et al. 2022).

India and Pakistan’s respective military and nuclear doctrines—if implemented as each side threatens—could lead directly to nuclear war and potentially draw in other nuclear powers, such as Russia or the United States, into an escalated global war. India, seeking to deter Pakistan from sponsoring terrorist attacks, could posture its conventional forces to pose a threat such that an attack could lead to a rapid

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1 For a plausible scenario of how a U.S.–North Korean nuclear war might begin, see Lewis (2018).

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

Indian incursion into Pakistani territory. Pakistan, seeking to deter such an Indian invasion, could threaten to use tactical nuclear weapons to stop Indian forces from entering Pakistan. India, seeking to deter Pakistan from using nuclear weapons, could reinforce warnings that if Pakistan uses any nuclear weapons, the war will not remain limited and India will launch full strategic nuclear strikes against Pakistani targets. This could result in a full Pakistani retaliation with whatever forces survived the Indian strike.

Recently, some Indian officials have suggested that India might not wait for such a series of events, but might try to attack Pakistani nuclear weapons and destroy them before they could be used.

THREATS OF NUCLEAR TERRORISM

As discussed in the historical review of the literature on assessment methods for nuclear terrorism in Chapter 3, the U.S. approach toward nuclear terrorism threats has evolved with the threats: from a limited focus on clandestine actions by the Soviet Union to terrorist groups with motivations to inflict great harm to a large number of people. A consistent theme across decades of analysis is that it is the difficulty in obtaining nuclear material that remains one the most effective means of preventing nuclear terrorism. While much work has been done over the past decades to successfully secure nuclear materials, threats of nuclear terrorism still remain.

To understand the extent of the current possibilities for nuclear terrorism, one must consider terrorists’ motives, capabilities, and opportunities. For motive, there is an enduring goal for a variety of terrorist groups to inflict great harm on the United States, its population, and allies. Capabilities of a terrorist group can change rapidly, either with acquisition of nuclear materials or devices, or with sudden surges in influence. Finally, opportunities to obtain nuclear materials persist as some materials are not well secured, and insider threats remain real (Bunn 2021).

Evidence collected from terrorist groups indicates that there is interest in developing and using improvised nuclear devices and radiological dispersal devices (Bunn et al. 2019). As noted in Chapter 3, for example, the Japanese terror cult Aum Shinikyo conducted nerve gas attacks in Matsumoto in 1994 and in Tokyo subways in 1995 and had previously focused efforts to get nuclear and biological weapons. Core al-Qaeda pursued nuclear weapons, carried out conventional explosive tests, and considered attacks on nuclear reactors, and their affiliates pursued radioactive material for a radiological dispersal device. Chechen terrorists planted a dangerous radiological source in a Moscow park as a warning, threatened to use radiological dispersal devices, and repeatedly threatened and planned attacks on reactors. In addition, Russian officials have reported catching terrorist teams scoping nuclear weapon storage sites and transports.

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

Current understanding of terrorism threats will continue to change as the United States and others continue to address the problem, and as adversaries react to those improvements. Risk analysis is one way to identify what components of the problem may be addressable.

Contributors to Nuclear Material Availability

It is challenging for terrorist groups, as they exist today, to produce their own plutonium or highly enriched uranium. To achieve nuclear weapons capability, they would likely have to secure a state-made nuclear weapon and figure out how to detonate it, or get plutonium or highly enriched uranium and figure out how to make a crude nuclear bomb of their own. Hence, ensuring that nuclear weapons and weapons-usable nuclear material are protected from theft, and blocking smuggling of any nuclear material that is outside of state control, remain critical to preventing nuclear terrorism. Similarly, ensuring effective security for major nuclear facilities and for radiological materials is important for reducing the dangers of those types of terrorism.

As discussed in Chapter 3, the problem of security for nuclear weapons or materials came to widespread attention in the 1970s, and what were loosely termed “loose nukes” became an even greater concern after the collapse of the Soviet Union in 1991.

How states and organizations managing these materials decide what types of security measures to implement and how much is enough is still not well understood. The results of such decisions vary widely, from states that do not require any armed guards at nuclear facilities to states that require on-site armed protection capable of fighting off a substantial force of attackers. These decisions are not necessarily correlated with resources and experience. It often takes a major incident to drive improvements in nuclear security arrangements, but responses to such incidents are modulated by the particular regulatory arrangements and organizational cultures of different countries.2 Many efforts—ranging from bilateral technical cooperation to recommendations and review and training programs of the International Atomic Energy Agency (IAEA) to the nuclear security summits of 2010–2016—have strengthened nuclear security around the world and eliminated weapons-usable nuclear material entirely from many countries. However, much remains to be done, and the underlying factors that lead states to adopt and maintain stringent nuclear security measures remain mysterious.

Two of the most important and difficult nuclear security challenges are coping with an insider threat and maintaining strong security cultures. All the cases of theft of highly enriched uranium or plutonium (where enough is known about the

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2 For an initial attempt at collecting data in this area, see Bunn and Harrell (2014).

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

incident to know how it happened) were perpetrated by known and trusted insiders at the facility, or with the help of insiders. The same is true of most of the known cases of attempted sabotage of nuclear power plants. But insider threats are difficult to cope with: insiders have authorized access to go through many of the layers of a facility’s security system; they may understand the facility’s security system and its weaknesses; and a variety of cognitive and organizational biases often lead organizations not to suspect these insiders until it is too late (Bunn and Sagan 2017). The growing problem of violent nationalist extremists in advanced democracies may increase the insider threat. More work is still needed to understand how best to address insider threats without undermining the cooperation and trust essential for organizations to do their work.

Security culture—the degree to which all personnel take security seriously and are constantly on the lookout for threats that need to be addressed or for weaknesses that need to be corrected—remains a critical and difficult problem. The best security technology will not provide effective security if the people in the security system are not paying adequate attention. In 2012, for example, an 82-year-old nun and two other protestors went through three layers of alarmed fencing at the Y-12 nuclear site in Oak Ridge, Tennessee, and reached the building where thousands of bombs’ worth of highly enriched uranium is stored. They pounded on the building with sledgehammers, sang peace songs, and poured blood and paint on the building before finally being accosted by a single guard. It turned out that the site had been directed to install a new alarm system and had tried to save money by combining it with the old system; the result had been some 10 times as many false alarms as before. There was, in short, a profound breakdown of security culture at one of the most secure sites in one of the countries with the most experience implementing nuclear security and some of the world’s most stringent nuclear security rules. Many nuclear organizations around the world are attempting to strengthen nuclear security culture, but keeping people constantly on alert for threats that almost never occur is a major challenge (see, e.g., WINS 2016).

Social science theory and past experience related to black markets have provided important insights about potential nuclear black markets.3 But while markets for items such as illicit drugs involve thousands of transactions taking place over years, with both criminals and law enforcement having the opportunity to observe each other and learn over time about what works and what does not, a seller with weapons-usable nuclear material continues to be a rare event, as is a real buyer.

How states decide how much effort to put behind steps to block nuclear smuggling—for example, implementing effective laws, creating dedicated police or intelligence teams trained and equipped to handle such cases, proactively organizing

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3 For a good summary of what is known about smuggling of stolen highly enriched uranium and plutonium, see Zaitseva and Steinhäusler (2014).

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

investigations and sting operations, or putting radiation detectors at key locations—remains another area of uncertainty, as is how to maintain strong security cultures and avoid insider threats in these areas.

Efforts to counter nuclear smuggling, too, vary widely among countries. Efforts such as the Global Initiative to Combat Nuclear Terrorism, the nuclear security summits, bilateral cooperation, and IAEA programs have helped eliminate some of the weakest points in the global system, but there is a great deal more to be done, and only modest understanding of the factors that are most important in determining state decisions.

Understanding Whether States Will Help with Nuclear Terrorism

There is a question of whether states would help terrorists get nuclear weapons, either for money or to carry out attacks for which a state hoped to avoid blame (Butler 2001). Some analysts consider the possibility that a country like North Korea (or Iran, were it to acquire nuclear weapons) might provide nuclear materials or weapons to terrorists to be a major part of the nuclear terrorism risks (Allison 2004). The fear that Saddam Hussein’s Iraq might have been helping terrorists with nuclear, chemical, or biological weapons was one of the key public arguments for the U.S.-led invasion of Iraq in 2003 (Blitzer 2003).

Others argue that conscious state decisions to help terrorists with nuclear weapons are extremely unlikely and probably only a small part of the overall nuclear terrorism risks. The argument is that any such help would carry an enormous risk of being discovered, and dictators attempting to control everything in their states are unlikely to give the greatest power they have ever acquired to terrorist groups who might use it in ways that could provoke retaliation that would remove them from power. In that sense, there is a very big difference between selling a nuclear weapons capability to another state (such as North Korea’s plutonium reactor export to Syria, which was ultimately destroyed by Israel) and providing such a capability to a terrorist group. While a wealthy terrorist group might be willing to offer substantial sums for such a capability, it is highly unlikely that terrorists would be able to provide enough money for the effect to be that the transaction would increase, rather than decrease, the chance of regime survival (Bunn 2006; Lieber and Press 2013).

CONSEQUENCES OF NUCLEAR AND RADIOLOGICAL WEAPONS USE

The consequences of nuclear war and terrorism can be described across several different dimensions, including the device or event type, the scale of the event, and the timelines and magnitude of the effects.

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

Timelines and Magnitude of Effect

The magnitude of the effects of the use of a nuclear or radiological weapon varies according to the time elapsed since the event occurred. The immediate effects of a nuclear explosion include an intense burst of gamma and neutron radiation; a fireball (anything inside the fireball is likely to be totally consumed); an intense, blinding flash and a pulse of thermal radiation (causing burns and igniting fires); a powerful blast wave, accompanied by intense winds; and an intense electromagnetic pulse (localized for blasts within the atmosphere, but more far-reaching for blasts in space).

Near- and long-term effects are wide reaching. In the near term (roughly, 1 hour to 1 week after the event), widespread evacuations and grid instability are possible along with initial radiation effects on humans. In the long term (weeks to several months or years after the event), effects include social and economic unrest, political and governance crises, health effects, infrastructure failures, negative environmental and climate effects, migration, and psychological distress (Glasstone and Dolan 1977; Jervis 1988; Katz and Osdoby 1982).

Radiological Dispersal Devices and Nuclear Sabotage

Nuclear or radiological terrorism could take several forms with different consequences. Recent studies have explored the consequences of various types of nuclear terrorism. Bunn and colleagues (2019) outline three types of nuclear terrorist events: improvised nuclear devices, radiological dispersal devices, and sabotage of nuclear facilities. They found that the latter two types have similar consequences that are smaller in magnitude than the consequences from improvised nuclear devices. Another study explored the effects of a single terrorist nuclear bomb (Bunn and Roth 2017). Effects of a radiological dispersal device and nuclear sabotage have also been studied (NAS 2015; Purvis 1999; Rosoff and von Winterfeldt 2007; Salter 2001; von Hippel and Schoeppner 2017; Whitehead et al. 2007).

Terrorists might use a variety of means to disperse radiological material (such as from one of the radiological sources used in medicine, industry, or agriculture) over an area. In most cases, there would be few if any radiation-induced deaths from such an event: however, the public fear of radiation means that such an event could have devastatingly disruptive social and economic consequences, potentially amounting to tens of billions of dollars in cleanup and disruption costs. Most of the impact would be the result of public fears of radiation and government reaction, and is therefore difficult to predict with any precision (see Box 4-1). Some analyses suggest that even modest radiation releases would have significant disruptive effects (Giesecke et al. 2012; Trost and Vargas 2020). A risk analysis that includes a characterization of the uncertainties would be helpful in response planning.

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

The effects of a nuclear sabotage event could cover a broad range, from purely economic (as in the insider sabotage that destroyed the turbine at the Doel-4 nuclear plant in Belgium in 2014) to a devastating radiation release comparable to those of the Chernobyl accident. Actions that lead to a fire in a spent fuel pool might be difficult to accomplish, but could be especially devastating. The National Academies of Sciences, Engineering, and Medicine (NASEM 2016) study of the Fukushima disaster focused on security, including security of spent fuel rods, and on the sociological and psychological effects of the accident. Two of its key messages relevant to sabotage and assessing its impacts are reproduced below (NASEM 2016):

The understanding of security risks at nuclear power plants and spent fuel storage facilities can be improved through risk assessment. Event trees and other representational formalisms can be used to systematically explore terrorist attack scenarios, responses, and potential consequences. Expert elicitation can be used to rank scenarios; develop likelihood estimates; and characterize adaptive adversary responses to various preventive, protective, or deterrence actions. The identification of scenarios may be incomplete, and the estimates developed through expert elicitation are subjective and can have large uncertainties. Nevertheless, risk assessment methods that focus on the risk triplet—scenarios, likelihoods, and consequences—can contribute useful security insights. (Finding 4.1)

The U.S. nuclear industry and the U.S. Nuclear Regulatory Commission should strengthen their capabilities for identifying, evaluating, and managing the risks from terrorist attacks. Particular attention is needed to broaden scenario identification, including asymmetric attacks; account for the adaptive nature of adversaries; account for the performance of plant security personnel in responding to the identified scenarios; estimate the potential onsite

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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and offsite consequences of attack scenarios, including radioactive releases and psychological impacts; and develop strategies for countering the identified threats [emphasis added]. (Recommendation 4.1A)

Analyses of the consequences of nuclear terrorism could also include the effects of a country’s actions taken in response to the initial attack. For example, damage of the 9/11 attacks inflicted by al-Qaeda on the United States was not only the loss of life and resources resulting from the direct airplane attacks, but also the subsequent actions by the United States, which led to fighting two prolonged wars with significant economic and human effects, as well as costly homeland security measures.

It is important to note that a concerted terrorist attack could involve several local events with attacks of a different nature, such as the release of radioactive material in one city and a biological attack in another city at the same time.

Damage Effects of Nuclear Weapons

The publicly available nuclear weapons effects analyses funded by the Department of Defense (DoD) between the late 1950s and late 1970s were largely focused on increasing accuracy of damage estimates for U.S. nuclear weapons against adversary targets and the potential effects of an attack on U.S. military forces (Binninger et al. 1974; Murphy et al. 1975). Mathematical models were developed to estimate direct damage and weapons effects that occur immediately or in the hours or days after an explosion. While the weapon effects codes have evolved significantly over time, the codes are still used today: see Binninger et al. (1974) for mathematical model descriptions; Moakler (2015) for descriptions of the evolution of the vulnerability number for thermonuclear kill system; and Helfand et al. (2002) for the consequences assessment tool set system developed by the Federal Emergency Management Agency and the Defense Threat Reduction Agency.

Some observers have noted that additional research is needed to better understand overlooked physical effects. For example, Buddemeier (2010, p. 31) argued:

Updating our cold war understanding of blast damage in a modern city is another important area of research. The bombings of Hiroshima and Nagasaki demonstrated that the area of glass breakage is nearly 16 times greater than the area of significant structural damage. Injury from broken glass has not previously been well modeled, however, because cold war planners generally considered it not of military significance.

Dresch and Baum (1973) performed a similar analysis with the stated purpose of expanding the damage effects methodologies into analyses of potential economic recovery. Similarly, Eden (2006) documented that the U.S. military primarily emphasized blast effects in assessing the damage from different types of attacks and concluded that DoD systematically understated the impact of fire. For a recent

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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analysis of both the physical and nonphysical consequences of nuclear war, see Scouras et al. (2021).

Weapons Effects Beyond Immediate Damage

Other early consequence analyses expanded the timeline of effects to include those that occur weeks to years after the initial explosions and also expanded the list of effects that were studied. For example, the 1947 U.S. Strategic Bomb Survey’s medical effects section included the nature of casualties (e.g., radiation and fires/burns), environmental sanitation (e.g., water supply; sewage and waste disposal such as public sewerage, night soil collection and disposal, and garbage and refuse collection and disposal; milk and food sanitation; insect and rodent control; disposal of the dead; and evacuation), food supply and nutrition, effects on the medical system, communicable diseases, and psychological effects (U.S. Strategic Bombing Surveys 1947). Another early book on the subject expanded this list by assessing the social impact of nuclear bombings, including large-scale nuclear war and the ability for a society to reform and function (Iklé 1958); the book borrows heavily on the U.S. Strategic Bombing Survey.

Significant consequence analyses were conducted in the 1970s and 1980s. The Office of Technology Assessment (OTA 1979) examined the effects of nuclear war on the populations and economies of the United States and the Soviet Union and reached several major conclusions:

  • The effects of a nuclear war that cannot be calculated are at least as important as those for which calculations are attempted.
  • The impact of even a small or limited nuclear attack, particularly on cities, would be enormous.
  • The situation in which the survivors of a nuclear attack find themselves will be unprecedented.
  • From an economic point of view, and possibly from a political and social viewpoint as well, conditions after an attack would get worse before they started to get better. This postwar damage could be as devastating as the damage from the actual nuclear explosions.

A compendium of lectures, The Medical Implications of Nuclear War (IOM 1986), includes articles on genetic, psychological, social, and economic effects. These early studies show that some researchers (and funders) recognized the importance of and were beginning to explore the social and psychological effects of nuclear war. It is noteworthy and disturbing that there has been so little attention to further deepening understanding of these vital impacts since 1986.

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Weapons Effects: Post–Cold War Studies and New Methods

Serious government and academic studies of nuclear weapons consequences all but ceased after the Cold War (Frankel et al. 2015). The terrorist attacks of 9/11 spurred new studies on nuclear terrorism effects, and the “humanitarian consequences of nuclear weapons” movement in the 2000s and 2010s focused renewed attention on the potential effects of nuclear war through a series of international conferences. This led to new examinations of some of these potential effects from scholars all over the world (e.g., Løvold et al. 2013). There was also renewed interest in using quantitative methods for analyzing risks of catastrophic events (Garrick 2008).

Long-Term Effects and Social Phenomena

Some studies show that the long-term and societal consequences of either nuclear war or nuclear or radiological terrorism can far outweigh the immediate and near-term effects in terms of economic and psychological impact. However, the present understanding of these indirect consequences and the ability to model and estimate them remains limited. Furthermore, challenging problems of assessing general societal or psychological consequences have not been identified as a high priority for researchers or funding agencies (Frankel et al. 2015).

Methods for assessing societal and psychological effects have relied on limited data and mostly on the analysis of human behavior in response to other kinds of events. New large-scale computing approaches are beginning to integrate human decision making with infrastructure following an event, and this work offers new insights.

Estimating and calculating the effects of small- and midscale events can be informed by past events, such as Chernobyl, Hiroshima and Nagasaki, or the 9/11 terrorist attacks. However, there are no past events to inform estimates of the consequences of a large-scale nuclear conflict. In short, the ability to comprehend and estimate the magnitude of the effects in the future and to determine their value using analysis is limited, and some argue impossible (Slovic and Lin 2020). Fictional interpretations and human imagination have proven useful media for comprehending, if not predicting, the magnitude of the impact of a large-scale nuclear conflict.4 This narrative and artistic approach is accessible to a wide audience and conveys emotional content not readily captured by analytical methods.

Estimates of the consequences of nuclear explosions have relied first and foremost on engineering-type methods. Weapons were tested, effects measured, and

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4 Ronald Reagan’s reaction that the film The Day After both depressed him and affirmed his belief in the importance of a strong deterrent to prevent nuclear war is just one example (Frankel et al. 2015).

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

calculations done of the impact of such effects on various kinds of structures, on people, and on other items of interest. Nuclear weapons effects analyses funded by DoD between the late 1950s and late 1970s (which are publicly available) were focused on the development of increasingly accurate damage estimates for U.S. nuclear weapons against adversary targets and potential effects on U.S. military forces (Binninger et al. 1974; Dolan 1972; Glasstone and Dolan 1977; Murphy et al. 1975; P.L. 81-920). The effects studied were mainly the direct effects that occur immediately or in the hours or days after an explosion (Frankel et al. 2015).

Mathematical models were developed and refined as better data or more powerful computers became available. However, the more substantive consequences of these weapon effects have not been deeply or extensively explored, if for no other reason than the lack of empirical evidence that can be brought to bear on this problem.

Methods for assessing societal and psychological impacts have relied on surrogate data, including analyzing human behavior in response to other kinds of events. These effects might be heavily dependent on the societies that are affected, their government, and their general attitude and response to catastrophes. New large-scale computing approaches are beginning to offer new insights by better integrating human decision making into analyses of possible events. However, all approaches are limited by the lack of historical data and the corresponding uncertainties (Quinlan 2009).

Prominent among these studies are environmental impact analyses using current climate models, which suggest that in cases in which tens to hundreds of cities are attacked with nuclear weapons, the smoke from burning cities would rise into the upper atmosphere and block a portion of sunlight, altering the global climate for a period of years. The impact on land and in oceans would likely affect agriculture and other food resources, potentially putting hundreds of millions of people at risk of starvation.

This scenario is reminiscent of a nuclear winter, first described in the 1980s; the conclusion of the more recent studies has been to reinforce the earlier findings that were based on far less advanced computer simulations. Indeed, the underlying process of self-lofting of soot proposed by Crutzen and Birks (2016) has now been documented through observations above wildfires: large fires release soot into the lower atmosphere (troposphere), where it absorbs sunlight and becomes warm, thereby warming the surrounding air that then rises, pulling the soot into the upper atmosphere (stratosphere) where it can remain for weeks or months.

Many details about the consequences of nuclear explosion–produced soot remain uncertain, but a key point is that, to this committee’s knowledge, this topic has not been extensively studied by the U.S. government, especially regarding the

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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impact on highly interconnected, technologically dependent modern society, as well as on climate.5

A study of the social, economic, and health impacts of a 10-kiloton improvised nuclear device was undertaken as a part of a U.S. National Planning Scenario. Individual and collective behaviors were modeled and their interactions with the built infrastructure were investigated from up to 48 hours to roughly 1 week after the event.6 Agent-based models, social network analysis, and machine learning technologies were developed to build digital twins of functioning city-scale systems. Individuals’ behaviors were represented as a partially observable Markov decision process, in which the evolution of a system from one state to another is assumed to depend only on the first, and the system state at any time is uncertain. The model allows for an analysis of the effects of future decisions. Collective behavior is represented by connecting these individual models using an underlying network. Each individual is part of multiple networks. Conceptually, a person has a social network and associated behavioral representation corresponding to an infrastructure. For example, individual behavior related to a communication network would represent whom a person might call, how often, ownership of a cell phone, and other characteristics of human-communication interactions.

These systems are data driven, and the models are informed by available numerical and procedural data. Analytical techniques are then developed to derive insights from these agent-based simulations. Ideas from statistical designs are used to create a set of computational experiments and the outcomes of the analysis scenarios are computed, not just as point estimates but as trajectories and collective behavioral outcomes, along with the corresponding uncertainties. For example, one may be interested in how a family is reconstituted, or the number of injured individuals who seek medical care at a given triage center.

Psychological and Social Effects

Yet another type of study involves the psychological effects of nuclear events: for example, Becker (2012) reviews and reexamines earlier social and psychological studies with new analogies and updated analyses. The social and psychological effects of large-scale nuclear weapons events continue to be difficult to assess (e.g., Slovic and Lin 2020). The methods used for psychological and social impact studies

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5 This committee did not further consider climate impact in its work because the National Defense Authorization Act for Fiscal Year 2021 called for the National Academies to conduct a separate, detailed study of the nonfallout atmospheric effects of scenarios for nuclear war, ranging from low-quantity regional exchanges to large-scale exchanges between major powers.

6 The outcome of these initial studies will play a role in developing future models that range from weeks to years.

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

largely extrapolate behaviors from other radiation emergencies (e.g., nuclear accidents) and from other relevant situations (e.g., releases of invisible toxic agents, etc.). In addition, the work takes into account the literature on the mental health effects of Hiroshima and Nagasaki. The studies of these effects are essential since there were many lessons learned from the immediate and long-term effects of the nuclear bombing and the recovery paths of both cities. Nuclear events are closely tied to stress, anxiety, depression, and other mental and emotional health outcomes, all of which can lead to other health complications (Lambiase et al. 2014).

Areas of social and psychological research have expanded to help explore such questions as the ability and willingness of responders to be involved in radiological and nuclear incidents, or how to provide effective messaging and communication to the general public and responders following a nuclear event. This work uses a variety of methods, from focus groups, interviews, and surveys to analysis of how text messages are received and processed.7 However, the long-term and psychological, societal, and political consequences of nuclear weapons use are not well understood.

THE CHALLENGES OF ASSESSING THE OVERALL RISKS OF NUCLEAR WAR AND NUCLEAR TERRORISM

Factors of Complexity in Risk Analysis

The consequences of nuclear weapons use are qualitatively different from those of any other form of conflict. Therefore, the risks of triggering nuclear weapons use and the conditions under which it might arise are also profoundly different from those of nonnuclear (i.e., conventional) war or terrorism.

This section aims not to address the causes and associated risks of war or terrorism in general, but to focus on those that might lead to nuclear detonations or dispersal. All evaluations of potential causes and risks are necessarily speculative. It is possible and perhaps even likely that a future nuclear conflict may erupt for reasons completely overlooked by current analyses. Nevertheless, the implications of nuclear weapons use are so enormous that they deserve thorough attention, ranging from technical and military analysis to legal, social, political, environmental, and ethical considerations.

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7 For collections of information on nuclear and radiological response, see the TRACIE Healthcare Emergency Preparedness Information Gateway, hosted by the Department of Health and Human Services at https://asprtracie.hhs.gov.

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Coevolution of Risk Assessment and Actions

Assessing the risks of a nuclear war or a nuclear terrorism event is fundamentally different from assessing the risks of natural events, such as an earthquake or tornado, because decisions, policies, and actions of individual and collective human agents affect the risks more directly. The assessment for natural events is challenging because of uncertainties in understanding the physical processes, but complex computational models (complete with the assessment of uncertainties) have been in use for more than 50 years (Cornell 1968; McGuire 2008). There are some analytical lessons to be learned about the effect of large disasters from the assessment of seismic risks. In the case of earthquakes, society can affect the outcome to the degree that it understands the seismic loads and the uncertainties involved by enforcing building codes, which can increase the capacity of the different structures to absorb these loads, to some extent. This is true, in particular, for the seismic reinforcement of nuclear power plants, which is supported in each case by a site-specific risk analysis. The effects of social and political actions in preparation for and response to natural catastrophes can be observed across different countries. However, the social and political actions involved in nuclear war and nuclear terrorism and the scale, as well as the coevolution of realities and risk perceptions in such complex systems, makes assessing the risks especially challenging.

Moreover, there are significant disagreements among experts regarding the effects of relatively fundamental nuclear force decisions on nuclear security. For example, does procuring more nuclear weapons or developing ballistic missile defenses increase or decrease risks? These policy choices are interdependent, co-evolving, and difficult to assess.

Cross-Domain Interactions and the Impact of Modern Technologies

The unique characteristics of nuclear weapons may have complex interactions with emerging or disruptive technologies, which can affect the initiation of war, the perceived value or necessity of preemption, incentives for nuclear weapons use, the restoration of deterrence, and conflict termination (Roberts 2021).

Types of threats can be intertwined, such as some nations having threatened nuclear retaliation in order to deter attacks with biological or chemical, as well as nuclear, weapons. Cyber and space infrastructures are closely and increasingly connected, and an attack on one domain can compromise the other. For example, a nuclear crisis could involve a cyber attack, such as an attack on space capabilities associated with nuclear surveillance and warning; an attack on command, control, and communications; an intrusion into cyber systems for purposes of intelligence gathering; or intrusions into conventional capabilities that may also threaten nuclear command and communications systems. In these and other instances, a cyber

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

attack could be misunderstood or misattributed, especially if it takes place during a period of otherwise increased tension between nuclear adversaries.

One way of trying to deter threats to the U.S. nuclear infrastructure is to protect critical military or civilian infrastructure. A nuclear, conventional, or cyber attack on such infrastructure could be considered serious enough to trigger nuclear retaliation, because the published postures of both the United States and Russia could be taken to imply this possibility. Yet what constitutes critical infrastructure and what conditions would justify a nuclear response remain unspecified.

To some observers, ambiguity in posture is a benefit for deterrence, because avoiding specificity about the actions that would trigger nuclear response is viewed as complicating prospective adversaries’ planning of an attack. To others, such ambiguity is an invitation for misunderstanding and misinterpretation, especially at times of tension, let alone crisis between nuclear powers, and all the more so in the presence of accidents or human error.

New technologies have vastly improved sensing, monitoring, communications, and warning—all of which can reduce risks of crisis or conflict. However, these technologies may offer new paths to crisis and, potentially, to conflict and therefore the possibility of nuclear weapons use. The rapidly increasing diversity of new technologies in space, cyber, and other domains provides new pathways to instability and crisis, through the increased power and reach of these technologies, their interconnectedness and widespread commercial usage, and their increased accessibility to nations around the world. One of the key uncertainties when assessing the nuclear risks over the next 10 years is anticipating emerging and developing technologies and their impacts.

Likely Pathways to Nuclear War

Many dangerous pathways could lead to the use of nuclear weapons. Exploring these scenarios is part of developing risk analysis methods, and the list of examples described here is neither complete nor definitive. Analysts have to address how evolving technologies and changing geopolitics are changing those dangers, and what steps could be taken to reduce the risks that such technologies pose. Each nation develops its own assessment of those risks, as well as its own assessment of what its adversaries’ potential actions might be.

Some pathways may involve escalation from a crisis to a conventional military conflict, and from that conflict to nuclear weapons use. Events of recent years seem to make clear that future serious crises and conflicts will probably be complex, multidomain events, and they are likely to feature cyber attacks and disinformation campaigns starting in “the gray zone,” as well as more traditional military operations (Gerasimov 2016). In addition, multiple parties may be involved, including

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

allies of the contending nuclear states or other states or terrorist groups seeking to take advantage of the situation.

The following are characteristics of technologies that could increase the risk that nuclear war would occur:

  • Technologies that make deterrent forces vulnerable or that create an incentive to strike first in a particular domain: Threats of both cyber and counterspace technologies can add to the risks of crisis escalation.
  • Technologies that blur the lines between peace and conflict, or between different levels of conflict: Cyber activities, in particular, are blurring conflict lines as the United States and its adversaries are in each other’s information systems every day, even in peacetime. It is unclear what kinds of attacks would represent an escalation. In some cases, cyber assaults can (perhaps inadvertently) damage computer systems, which might be seen as an escalation requiring a serious response.
  • Technologies that entangle conventional and nuclear forces and command and control: With dual-use weapons and command systems, attacks seen by one side as essential to preventing a conventional conflict can be seen by the other side as threatening its nuclear capabilities. Even worse, such attacks could be seen as part of a large-scale attack that might require a nuclear response.
  • Technologies that compress the time for decision or worsen the decision environment: In some circumstances, decisions in a potentially nuclear crisis will have to be made quickly. Future systems enabled by artificial intelligence may help the decision maker respond quickly but could also introduce biases and flawed information (e.g., “hallucinations”) into decision making.
  • Technologies that proliferate nuclear weapons: If there are more nations, groups, or individuals that can decide to use nuclear weapons, the routes to nuclear conflict increase rapidly in number, diversity, and complexity. The global effort to stem the spread of nuclear weapons—including keeping them out of terrorists’ hands— continues to be justified despite having been remarkably successful to date.

Finally, when a nation’s decision makers think about what forces to build and what operations to undertake, they are often thinking mainly about strengthening the deterrence threat that their nation poses to its adversaries and the capability that it offers to reassure its allies. But one must remember the security dilemma: what looks like a defensive step to strengthen one nation’s deterrent might look like an offensive threat to its adversaries, potentially provoking them to respond. Part of China’s motivation for its current nuclear buildup, for example, appears to

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

be intended to counter the threat to its forces from U.S. counterforce capabilities and missile defenses.

Traditional arms control is problematic for some of these evolving technologies. For example, one cannot count, measure, or control capabilities in the cyber domain. As a consequence, most risk reduction is likely to come from unilateral steps—each nation making sure that its own forces are secure and resilient against cyber and counterspace systems.

However, there may be room for rules of the road or confidence-building measures that reduce the risks posed by emerging technologies. Bilateral and multilateral talks on cyber security are ongoing, as part of a broader umbrella of strategic stability conversations. Unilateral steps can also play a crucial role in enhancing strategic stability, starting with building mutual confidence, especially during periods of animosity between rival nations.8 Still, future rounds of nuclear arms control may require at least some clearer understanding on how to control nonnuclear technologies that affect nuclear dangers—including hypersonic weapon systems, missile defenses, long-range precision conventional weapons, counterspace systems, and cyber capabilities.

Wargaming and simulations continue to have a role in improving each nation’s understanding of how these emerging technologies—applied in different geostrategic contexts—might contribute to risks of conflict and of escalation. While the complex factors involved in decision making are difficult to represent fully, these and other approaches may provide benefits from evolving technologies while moderating to some degree the risks they may cause.

Challenges in Assessing the Risks of Nuclear Terrorism

A wide range of challenges confront assessments of the probability of nuclear terrorism. There are strong disagreements even on the direction of the trend: some analysts argue that major improvements in security for nuclear weapons and materials and the killing or capture of the top of leadership of al-Qaeda and the Islamic State have greatly reduced the probability of nuclear terrorism, while others argue that factors decreasing the chances are more than balanced by factors increasing them.

The potential consequences from nuclear or radiological terrorism would cover a broad spectrum, from minor to catastrophic. While the direct consequences of physical effects of nuclear explosives are reasonably well understood, the long-term

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8 An example in the space domain is the April 18, 2022, U.S. announcement that it unilaterally “commits not to conduct destructive, direct-ascent anti-satellite (ASAT) missile testing” (White House 2022).

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
×

economic, environmental, political, and social consequences are perhaps more devastating and are poorly understood.

The challenges in assessing the risks of nuclear terrorism described below focus primarily on a terrorist detonation of a nuclear bomb, but similar challenges would face efforts to assess the risks of other forms of nuclear or radiological terrorism.

Uncertain Terrorist Capabilities and Motivations

As discussed above, past terrorist groups have pursued nuclear weapons and radiological dispersal devices and plotted to sabotage nuclear facilities. Nevertheless, most of those reported events are now decades in the past. Assessing current and future intentions is challenging because terrorist activities take place in secret and groups can change rapidly.

Uncertain Availability of Nuclear Weapons or Materials

In the 1990s, there were multiple seizures of kilogram quantities of stolen plutonium or highly enriched uranium, nearly all of which appear to have come from the former Soviet Union. It has been a decade since the last well-documented seizure of stolen highly enriched uranium, amounting to a few tens of grams (in Moldova in 2011). Does that mean that terrorists could no longer get weapons-usable nuclear materials? Or, as some suspect, is enough material for a bomb still outside state control, at unknown locations?

With ongoing incidents around the world in which terrorists manage to overcome security at very well-guarded nonnuclear sites, what are the odds that the same might happen at a nuclear site in the future? There have been significant advances in approaches to modeling and testing the effectiveness of nuclear security and accounting systems in recent decades, but the latest approaches are still applied unevenly around the world. And thieves at nonnuclear facilities often succeed in defeating security systems using unforeseen tactics. Modeling and assessment are better able to assess the effectiveness of security at one site relative to another than they are able to assess the absolute probability of a successful theft or sabotage at any particular location. Even judgments on relative effectiveness are uncertain, however, for at least three reasons: (1) the vulnerabilities that adversaries end up exploiting may be ones those doing security assessments have not thought of; (2) one site may have better protection against one type of threat and another better protection against a different type, and which approaches are most likely is not well understood; and (3) often, day-to-day security performance is different from performance when an inspection or assessment is being carried out.

Methods for assessing how likely it is that, after a theft, adversaries would be able to successfully get nuclear material or components to a terrorist group wanting

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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to make a bomb are much less mature. How would sellers with nuclear material and potential buyers find each other? How would they have confidence that the other party was not a government agent participating in a sting operation? What are the chances that nuclear material would be detected and recovered, and how do those chances vary from one part of the world to another? There are few data for answering these questions or for judging whether the stolen nuclear material that has been intercepted represents close to 100 percent or only a small fraction of the total material stolen over time. Various analogies (such as drug smuggling) have been used to try to make judgments on this point, but the situations are often quite different.

Uncertain Difficulty of Making, Delivering, and Detonating a Nuclear Bomb

Similarly, there is broad disagreement over how likely it is that, once a terrorist group had obtained nuclear material, they would be able to turn it into at least a crude nuclear bomb; how likely it is that they would be able to deliver that weapon to a target site in the face of efforts to stop them; how likely it is that they would choose to actually detonate such a weapon rather than keeping it for deterrence and blackmail; and how likely it is that the bomb would actually go off. Data for assessing these probabilities are simply not available.

Uncertain Economic, Political, Environmental, and Social Consequences

The consequences of acts of nuclear and radiological terrorism are difficult to fully grasp. In the case of an actual nuclear detonation, a good deal is known about the direct effects that would result. But, as discussed elsewhere in this report, the reverberating economic, political, and social consequences, not only in the country attacked but elsewhere around the world, are difficult to understand and predict. As just one of many possible consequences, after a nuclear detonation, once people understand that the nuclear material for the bomb would fit in a briefcase, traditional notions of civil liberties and protection from unreasonable search and seizure may be swept aside by demands for enhanced security.

There have been important efforts to model the potential impacts of an attack with a radiological dispersal device or a release from an accident or sabotage at a major nuclear facility. In the case of releases from nuclear facilities, there is real-world experience from the Chernobyl and Fukushima disasters to draw on. But understanding of the full scope of the impact on society of such events—particularly if they were malevolent rather than accidental, and people feared further attacks would occur—is still in its early stages. The conceptual framework known as the social amplification of risk may provide useful guidance for improved modeling (Kasperson et al. 1988; Pidgeon et al. 2003).

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Interacting Uncertainties

As with nuclear war, all the uncertainties about the risks of nuclear terrorism may interact with each other. If, for example, adversaries believe that nuclear weapons and weapons-usable nuclear material would be very hard to get, they are likely to put less effort into recruiting the people who might be able to help figure out how to detonate a stolen nuclear weapon or put together a crude nuclear bomb. If defenders are underestimating the potential consequences of nuclear terrorism, they are likely to require fewer measures to protect against it, which may increase its probability.

Lack of Direct Evidence

As described in Chapter 2, the evidence needed to support risk analyses can be drawn from many sources: statistical evidence, past near misses and false alerts, surrogate data, models, war games, red teaming, and expert judgment. In the case of nuclear war and nuclear terrorism, however, direct evidence is limited. Some past near misses and false alerts may involve factors that are still relevant, thus providing some statistical information, but expert judgment and assumptions are likely to be needed to bridge the inevitable gaps in the evidence available. For assessments of the overall risks of nuclear war and nuclear terrorism, uncertainties in the available evidence can be large, and experience in other fields may offer an important part of that evidence.

Uncertainties about the assumptions and inputs to a risk model can be represented in the risk analysis. The lack of direct evidence may make it difficult or impossible to accurately consider important dependencies across relevant scenarios, but the existence of these dependencies needs to be highlighted when presenting the results to decision makers.

CONCLUSIONS

CONCLUSION 4-1: There is a need to improve the understanding of less-well-understood physical effects of nuclear weapons (such as fires; damage in modern urban environments; electromagnetic pulse effects; and climatic effects, such as nuclear winter), as well as the assessment and estimation of psychological, societal, and political consequences of nuclear weapons use.

CONCLUSION 4-2: The U.S. government and the international community have invested significant resources and time in trying to understand and reduce the risks of nuclear war and nuclear terrorism. The risks remain real and are becoming more complex as new technologies and new adversaries arise.

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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CONCLUSION 4-3: There is a fundamental lack of direct evidence about nuclear war and nuclear terrorism. Analysts attempt to mitigate the resulting uncertainties by applying different methods and using multiple sources of information to supplement the limited body of evidence.

Given this lack of evidence, an analysis of overall risks of nuclear war or nuclear terrorism is likely to involve great uncertainties about the likelihood and the range of consequences of relevant scenarios. As discussed above, some physical consequences of nuclear weapons use are well understood, but significant uncertainties remain, especially regarding the psychological, societal, environmental, and political consequences of nuclear weapons use.

All risk analyses involve uncertainty, and communicating those uncertainties is essential. The specific quantitative result of an analysis of the overall risks of nuclear war or nuclear terrorism may have little or no policy relevance. If decision makers are reluctant or unable to consider the significant uncertainties and effects involved in such an analysis, the utility of such an overall estimate is unclear. Still, structuring the risk problem, exploring the possible scenarios, and identifying what gaps in evidence are addressable may be of great value to decision makers to prioritize risk mitigation actions.

CONCLUSION 4-4: 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.

Although overall analyses of the risks of nuclear war or nuclear terrorism may have limited policy relevance, analyses of specific risk problems related to nuclear war and nuclear terrorism are routinely requested and used by the U.S. government. As noted in Chapter 2, the framing and formulation of a risk problem is a critical first step in risk analysis. Well-formulated risk problems addressing specific questions can allow analysts to overcome the challenges posed by the lack of direct evidence. Chapter 6 addresses these specific risk questions and the methods analysts may use to address them.

CONCLUSION 4-5: The value of risk analysis is not solely in assessing the overall risks of nuclear war or nuclear terrorism. Risk analysis can also provide valuable input on many specific problems related to nuclear war and nuclear terrorism, including an understanding of the uncertainties involved.

Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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Suggested Citation:"4 The Use of Risk Assessment for Nuclear War and Nuclear Terrorism." National Academies of Sciences, Engineering, and Medicine. 2023. Risk Analysis Methods for Nuclear War and Nuclear Terrorism. Washington, DC: The National Academies Press. doi: 10.17226/26609.
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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.

At the request of the Department of Defense, Risk Analysis Methods for Nuclear War and Nuclear Terrorism discusses risks, explores the risk assessment literature, highlights the strengths and weaknesses of risk assessment approaches, and discusses some publicly available assumptions that underpin U.S. security strategies, all in the context of nuclear war and nuclear terrorism.

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