What does “nuclear terrorism” really mean?

There are few scarier pairs of words: “nuclear,” evoking the great 20th century fear of atomic annihilation, and “terrorism,” the bogeyman of the 21st. Put them together and you’ve got a frightening specter. Since European authorities revealed that the group behind the November 2015 Paris terrorist attacks was also spying on a senior nuclear official in Belgium, many news sources have reported that the threat of “nuclear terrorism” is upon us.

But what does that actually mean? The news stories don’t always say, and sometimes they fail to distinguish among events that would look completely different from one another, if they ever came to pass. In fact, “nuclear terrorism” can refer to several possible occurrences, all of which are best avoided. But if you’re the glass-half-full type, you may take some solace in knowing that the most dire scenario is also the least probable.

Here is what nuclear terrorism most likely won’t look like: A self-styled Islamic State caliph successfully launching a ballistic missile with a nuclear warhead at Washington, incinerating millions of people in a giant mushroom cloud. There are so many technical, financial, military, and logistical barriers that it would be extremely unlikely that even the most dogged, nuclear-obsessed extremist group could make that happen.

But just because nuclear terrorism won’t look like a Cold War nightmare come to life doesn’t mean we should rest easy. In a March 2016 report, the Harvard Kennedy School’s Belfer Center for Science and International Affairs laid out three potential types of “nuclear or radiological terrorism.” One possibility—the hardest to achieve, but by far the most devastating if it were to occur—is that terrorists will acquire or build and then detonate a nuclear bomb in a major city. A second possibility is that they will set off a “dirty bomb,” a weapon made of radioactive material attached to conventional explosives, sometimes referred to as a radiological dispersal device or RDD. Executing this scenario would be so easy that many experts are surprised it hasn’t happened already. A third possibility, which the Belfer Center estimates would fall somewhere between the other two in terms of severity and likelihood, is that terrorists will sabotage a nuclear facility, releasing radioactive material over a wide area.

Least likely: a nuclear weapon. The reason the first scenario is improbable is that it’s difficult to steal, buy, or make a nuclear weapon. While there are about 10,000 nuclear warheads in the world, most are heavily guarded and don’t lie around fully assembled. To steal one would require the cooperation of more than just one corrupt or coerced person.

Some policy analysts do worry that terrorists might be able to buy an atomic weapon from a nuclear power hostile to Western interests, perhaps North Korea or Pakistan. In 2013, though, political scientists Keir A. Lieber of Georgetown University and Daryl Press of Dartmouth College published one of the few papers to rigorously examine that likelihood and found the fear overblown. As they write, “a terrorist nuclear strike would not remain anonymous for long and would soon be traced back to the originating state.” Few national leaders are crazy or naïve enough to think they wouldn’t be found out, or that if they were, there wouldn’t be massive repercussions.

As for building an atomic weapon, it’s unlikely that terrorists could make anything as sophisticated as the warheads owned by governments, but making a crude nuclear bomb—an improvised nuclear device, or IND—is “potentially within the capabilities of a technically sophisticated terrorist group,” according to the Belfer Center report. However, in addition to equipment and know-how, the atom-bomb-seeking terrorist would need—the largest obstacle—some quantity of either plutonium or highly enriched uranium (HEU). Highly enriched uranium is present in fewer than 25 countries, according to a new report from the Nuclear Threat Initiative. Even Al Qaeda, which in the 1990s and early 2000s had deep pockets, a centralized command structure, and many scientists in its employ, was not able to acquire material suitable for a nuclear weapon despite its best efforts. There have been reports of attempts to sell nuclear material in countries in the Black Sea area, but none has been successful, as far as has been made publicly known.

Most likely: a dirty bomb. None of this is to suggest that the international community shouldn’t worry about the world’s nuclear arsenals; we would all be unequivocally safer if there were fewer atomic weapons and less nuclear-weapon-ready material around. But we’re far more likely to see the second scenario—a dirty bomb attack—than a nuclear explosion in the near future.

So what will that look like? Nothing like the aftermath of a nuclear weapon attack. As the US Nuclear Regulatory Commission explains, “A dirty bomb is in no way similar to a nuclear weapon.” The latter relies on fission or fusion to create an explosion millions of times more powerful than the former. A nuclear bomb could spread radiation over hundreds of square miles, whereas a dirty bomb could only do so over a few square miles. Dirty bombs have more in common with nuclear medicine than nuclear war.

A dirty bomb wouldn’t immediately kill any more people than an ordinary explosive. It is a weapon ideally suited to terrorism, though, part of the very purpose of which is to sow fear. In fact, in the perverse psychology of terrorism, a mere claim that a bomb had spread radioactive material would have some of the same effect as a bomb that actually did so.

That said, getting hold of the sort radioactive material needed to make a dirty bomb isn’t difficult; it has occasionally even been stolen by accident. Literally thousands of sites, in more than 100 countries, contain the kind of sources required, which have many uses in agriculture, industry, and medicine. Radioactive isotopes are commonly used, for example, to irradiate blood before transfusions and treat cancer tumors.

One reason governments worry about dirty bombs, even though the stakes are relatively low and none has ever been detonated, is that the materials needed to make them are so obviously in circulation, and apparently in demand. The International Atomic Energy Agency tracks radioactive material that governments discover to have been lost, stolen, or otherwise “outside of regulatory control.” The most recent fact sheet from the agency’s Incident and Trafficking Database reports 2,734 incidents between 1993 and 2014. (Only 49 involved HEU or plutonium.) The fact sheet also shows there has been a steady increase in annual incidents of theft and loss since the late 1990s. And because the IAEA fact sheet is based on voluntary reporting by governments reluctant to embarrass themselves or disclose sensitive information, it can be presumed to represent just the tip of the iceberg. The James Martin Center for Nonproliferation Studies, which also tracks incidents, counted 325 instances of radioactive material being outside of regulatory control in 2013 and 2014.

So what would happen if this stuff gets spewed around a city? The answer depends on many factors. To the untrained eye, the immediate aftermath of a dirty bomb explosion wouldn’t look much different than the aftermath of an attack perpetrated with regular explosives, like the Boston Marathon bombing in 2013, the Paris attacks in November 2015, or more recent terrorist bombings in Istanbul, Jakarta, Brussels, and Lahore. Law enforcement authorities would sweep for radioactive material right away, but depending on the isotopes used, the amount of smoke and debris in the air, and proximity to the blast, a member of the public might, for lack of visual evidence, have no idea that radioactive material was involved until an announcement was made.

Once the public knew the bomb was radioactive, it would be hard to stop fear and chaos from escalating. Authorities would have to decide whether to let people flee, which could reduce their radiation exposure and begin an evacuation, but might also spread radiation through the city and let perpetrators escape.

So many variables would be involved in a possible dirty bomb attack that it’s hard to definitively predict an outcome. The IAEA divides radioactive materials into five categories, from Category 1, which is so harmful that exposure for only a few minutes to an unshielded source may be fatal, to Category 5, which poses a relatively low hazard. But Category 5 materials—such as the americium-241 found in lightning detectors or the strontium-90 used in brachytherapy cancer treatment—are more readily available, and if enough are brought together in one place, they can add up to a harmful dose. An early task for first responders would be to figure out what kind of radioactive material was used.

Then there’s fear of the big C. Radioactive isotopes are associated with an increase in various cancers, but by how much and over what time period isn’t perfectly known. A great deal depends on the concentration to which a person was exposed. Much of what scientists have learned about radiation-caused cancer comes from studying the aftereffects of the 1986 Chernobyl nuclear disaster.

Various cities and government agencies, including the US Department of Homeland Security and the Centers for Disease Control and Prevention, have produced studies and briefings on how to respond to a dirty bomb attack, and many of these focus on the costs of evacuation and decontamination. “A radioactive dirty bomb would not cause catastrophic levels of death and injury,” the Nuclear Threat Initiative report says, “but depending on its chemistry, form, and location, it could leave billions of dollars of damage due to the costs of evacuation, relocation, and cleanup … Buildings could have to be demolished and the debris removed. Access to a contaminated area could be denied for years as a site is cleaned up well enough to meet even minimum environmental guidelines for protecting the public.” Businesses would close, shipping would halt, wages would be lost. This kind of upheaval has earned dirty bombs the moniker “weapons of mass disruption.”

It could happen: sabotage. It may be that the Brussels terrorists who spied on the nuclear official were aiming for option three, wreaking havoc by damaging a nuclear power plant. It’s hard to say how close they might have come. Belgium experienced a major incident of sabotage at its Doel-4 nuclear power reactor in 2014, when someone opened a valve and allowed lubricant to escape so that the turbine overheated and destroyed itself. No radioactive material was released, but the cost of the damage was estimated at between $100 and $200 million. As authorities investigated, they also happened to discover that a former contractor at the plant had left to fight for terrorists in Syria. (The jihadi contractor wasn’t responsible for the valve incident.) Needless to say, Belgium has since tightened security at its nuclear power plants, but at least as of March 2016, security elsewhere remained lax. “Some countries have no armed guards at all at nuclear facilities, relying on offsite response forces some distance away; others have no background checks before allowing employees access to reactor vital areas or nuclear security systems,” the Belfer Center report says.

As with a dirty bomb attack, results of an attack on a nuclear facility could vary wildly depending on many factors. The immediate death toll wouldn’t necessarily go beyond whatever was caused by the explosive itself. But the fear factor, long-term health effects, and economic consequences could be significant.