In This Issue
Nuclear Dangers
June 15, 2010 Volume 40 Issue 2

A Nuclear Explosion in a City or an Attack on a Nuclear Reactor

Tuesday, June 15, 2010

Author: Richard L. Garwin

A surface detonation of a 10-kiloton nuclear bomb would be far more deadly than either of the bombs dropped on Hiroshima and Nagasaki.

In this article, I discuss two types of nuclear terrorism: (1) the detonation of a nuclear weapon or improvised nuclear device (IND, also called an improvised nuclear explosive) in a city; and (2) an attack on a nuclear reactor, spent-fuel pond, or reprocessing facility with the intent of releasing the vast amount of radioactivity they contain.

Explosion of an Improvised Nuclear Device

Nuclear terrorists are likely to use an IND rather than a weapon from the inventory of a nuclear state. An IND is an explosive device created by a sub-national group that contains weapons-usable material, such as highly enriched uranium metal or plutonium, combined with a means of rapidly assembling fissionable material that exceeds a critical mass and causes a nuclear explosion (for a detailed description, see Garwin, 2010).

For the purposes of this article, I assume that terrorists or criminals have managed to assemble a gun-type device of highly enriched uranium and to detonate it with a yield of 10 kilotons at a location and time of peak population density in a major city. This scenario was the focus of President Obama’s Nuclear Security Summit in Washington on April 12–13, 2010 (White House, 2010).

A photo of Hiroshima in October 1945 (Figure 1), taken two months after the city was destroyed by a 13-kiloton nuclear explosion at an altitude of about 570 meters (m), can give an impression of the destruction. The blast knocked down buildings, and the radiant heat from the explosions ignited fires and burned or incinerated people. Because the fireball did not touch the ground, there was essentially no radioactive material (“fallout”) on the city. “Prompt radiation” (i.e., radiation emitted within a few seconds of the explosion) added relatively little to the death toll, but it was a new and frightening phenomenon.

Figure 1

A surface burst of a nominal 10-kiloton explosive in a densely populated modern city would be even more devastating. Because of the heavy local fallout of radioactive material associated with a ground burst, a ground-level detonation would greatly increase the number of deaths and injuries from radiation. In addition, there would be a fallout spot, or plume, delayed by, perhaps, 30 minutes, at a distance of 5 to 20 kilometers (km) from the ground burst. Another new phenomenon would be a crater, which, on dry soil or dry soft rock, would have a diameter of about 75 m and a depth of about 17 m (Glasstone and Dolan, 1977).

A blast of invisible nuclear radiation would be released within microseconds, followed within milliseconds by thermal radiation from the surface of the expanding fireball. Winds and destructive overpressure would follow, knocking down buildings in the destroyed area, breaking windows out to a radius of 5.3 km (at 0.5 psi = 0.03 bar overpressure from a surface burst of 10 kiloton yield), and converting people and objects into lethal missiles (Glasstone and Dolan, 1977).

About six seconds later, the nearest potential survivors would feel an enormous blast and wind. The intensely bright fireball would be long gone by then and some fires would be burning, but more would later be ignited by broken gas mains and the ignition of combustible materials from buildings.

The crater material would give off intense, but unfelt, radiation in the immediate area; a total dose of 4 Sieverts (4 Sv or 400 rem)1 would be lethal to at least 50 percent of the people exposed. The bomb debris, mixed with hundreds of thousands of tons of material from the crater, would rise in the prototypical mushroom cloud into the stratosphere from which coarse debris particles, along with much of the radioactive material, would fall out over a period of 30 minutes or so. With a nominal wind speed, there would be a fallout plume about 2 kilometers (km) wide to a downwind distance of about 20 km. The area affected by lethal fallout might be on the order of 20 km2. An example of such a plume with boundaries at the dose rate of 10 rem/hr is shown in Figure 2.

 Figure 2

Not much could be done to help people in the area of the 50-percent blast-casualty distance of 590 m. People within the 1.8 km radius, where there would be 50 percent mortality from thermal burns, would be lucky if they had been indoors and not in the direct line-of-sight of a window. But the realization that there had been a nuclear explosion would raise concerns about family members and others, and many people would be on the streets trying to gather their families or to leave the area. In tens of minutes a firestorm could develop, accompanied by strong in-rushing winds from the unaffected area, and evacuation by vehicle would be impossible except, perhaps, in areas where streets were not blocked by rubble.

Beyond the blast-damage area, the power and communications infrastructure would be largely intact, but the instantaneous loss of load on the electrical system would be likely to cause a blackout of uncertain duration; in principle, it need not last for more than a few seconds. The electromagnetic pulse from a ground-burst explosion would cause little damage outside the blast area, so cell towers in the suburbs and beyond should be capable of carrying traffic.

Roads would be clogged and emergency equipment almost absent. In 1946, Philip Morrison, a scientist with the Manhattan Project, described the effects of such an explosion in a U.S. city (Morrison, 1946). A more modern description appeared in a 2006 RAND paper for the U.S. Department of Homeland Security (DHS), which posited a 10-kiloton nuclear explosion in a cargo container in Long Beach harbor. Because the harbor region has relatively low population density, only about 60,000 deaths were predicted. Nevertheless, about 6 million people would be evacuated, and losses would amount to $1 trillion. The paper included graphics showing the range-to-effect for blast effects, burns, and prompt radiation, as well as the contours of the lethal fallout area (Meade and Molander, 2006). The area exposed to near-lethal fallout levels of 300 rem would be about 30 km2, with the orientation of the fallout pattern dependent on the winds aloft.

A further useful report was prepared for the Homeland Security Council. This report included information about how much protection could be provided by buildings. As Figures 3a and 3b show, taking shelter in the basement of an ordinary, one-story wooden house would reduce exposure to radiation by a factor of 10. A high-rise office building, even with windows, would reduce exposure by a factor of 50, even as low as the third floor (Homeland Security Council, 2009).

Figure 3

Responding to a Nuclear Attack

During the early stages of the cold war, a Soviet attack was much on the minds of national leaders, and understanding the effects of nuclear weapons on U.S. cities was considered essential. In a nuclear exchange with the Soviet Union, all significant U.S. cities would have been attacked and destroyed. Thus there would have been no possibility of bringing in resources from undamaged areas to help the one or two affected areas.

Indeed, it is impossible for each municipality to organize and put aside resources to respond to such an explosion. Nevertheless, it is important that the public be educated on how to protect themselves, especially from exposure to lethal fallout. But a thorough understanding of the variations in destruction and the training and mobilization of resources to provide water and food to the surviving population in little-damaged regions of the city, must be a federal responsibility.

A nuclear explosion would be fundamentally different from a disaster like the earthquake in Haiti, or even the Katrina-induced flooding of New Orleans. Because the United States has a developed economy, roads and railroads will be able to provide necessary transport capacity, and within a few days, it should be possible to obtain adequate water from suburban areas to supply the interior of the city. The federal government would take the lead in planning and creating capabilities for mapping (probably by drone aircraft or helicopters operating at an altitude of about 150 m) the distribution of radioactive fallout within 100 km or so of ground zero.

On April 19, 2007, the Preventive Defense Project, co-chaired by Ashton B. Carter of Harvard University and William J. Perry of Stanford University, convened The Day After Workshop, the purpose of which was “to address the actions that can and should be taken in the 24 hours following a nuclear blast in a U.S. city.” The insightful published report of that workshop is consistent with the estimates presented here and with my own comments at the workshop (Carter et al., 2007a,b):

Although there are some unknowns and variations, the broad outlines of the grisly effects of a 10-kiloton groundburst are clear. The downtown area, about one mile in radius, would be obliterated. Just outside the area leveled by blast, people wounded by flying debris, fires, and intense radiation would stand little chance of survival. Emergency workers would not get to them because of the intense radiation, and in any event, their burns and acute radiation exposure would require sophisticated and intensive medical care to offer any chance of survival. Further downwind from the detonation point, a plume of radioactive debris would spread. Its shape and size would depend on wind and rain conditions, but within one day, people within five to 10 square miles who did not find shelter or flee within hours would receive lethal radiation doses. This area, for example, could include Brooklyn, New York; northwest Washington, D.C.; or the upper peninsula of San Francisco.

People who were relatively close to the detonation point or who did not shelter themselves from the radiation, which would be most intense on the day of the blast and [would] subside with time, would receive large but varying doses of radiation. If the dose was intense (more than 400 rem), they would get sick and die; if strong but moderate (50–400 rem), they would get sick but probably recover; if moderate (less than 50 rem), they would not notice the effect[s] immediately but would have a greater chance of contracting cancer over their lifetime than if they had received no dose. [At 50 rem, an additional lifetime mortality from cancer of about 2.5 percent.] Because there is little that could be done for those in the area in and around the blast zone, responders would concentrate on minimizing the radiation dose to the population further downwind and preventing chaos among the rest of the population, which would be physically unaffected but traumatized and deprived of whatever utilities and services were located in the affected area.

In the months and years following the attack, policy-makers would face a trade-off in the large downwind plume area. If they allowed residents to return early, those residents would experience a higher average cancer rate later in their lives, resulting in many additional deaths when averaged over a large population. If not, or if those people were unwilling to accept a larger lifetime cancer rate, their homes would have to be abandoned. The city center itself would remain too radioactive to rebuild for a year or longer.

The Importance of Planning on the National Level

According to a recommendation by the authors of the report cited above, planning by the federal government is the key to reducing potential losses from an IND explosion (Carter et al., 2007a,b):

The federal government should stop pretending that state and local officials will be able to control the situation on the Day After. The pretense persists in Washington planning for the Day After that its role is to “support” governors and mayors, who will retain authority and responsibility in the affected area. While this is a reasonable application of our federal system to small and medium-sized emergencies, it is not appropriate for large disasters like a nuclear detonation.

Effective planning will require realistic modeling of the specific potential local impact of an explosion, as outlined above, but also of the effects on the larger society. For example, the loss of 300,000 people in an IND attack on Manhattan would be a large, but not unprecedented death toll from a natural disaster, such as a tsunami, an earthquake, or a pandemic. Sophisticated modeling can be used now to determine if the concentration of talent, data, or capability among those 300,000, just 0.1 percent of the total population of 300 million Americans, could imperil the functioning of the entire society.

If there is a significant potential of the collapse of the whole society, modifications and expenditures must be made immediately to ensure that the federal government can meet its first responsibility—to protect the people against unnecessary harm. This will require a change in our thinking and a realistic recognition of the magnitude of the problem.

Prevention of a Nuclear Detonation in a City

Prevention will be imperfect, so it makes sense to consider the consequences of failure and how to mitigate the harm. First of all, reducing the numbers of nuclear weapons in the world, particularly weapons that are not essential to a nation’s security, will reduce their availability for theft and terrorist use.

To prevent the detonation of an IND, it will also be helpful to reduce the amount of weapon-usable highly enriched uranium (HEU) and plutonium in the world. The Obama administration proposes to lock up all such HEU within four years, except for HEU in nuclear weapons or, perhaps, fuel for naval reactors on ships and submarines. A particular focus of this effort is to convert research reactors to use low-enriched uranium instead of HEU, and quickly. The four-year goal was unanimously adopted by the April 2010 Nuclear Security Summit.

Vast numbers of radiation detectors are being deployed in the United States and elsewhere to attempt to detect the transport of HEU or plutonium to a location where it might be fabricated into a bomb or smuggled to the chosen site for an IND. Unfortunately, HEU produces very few gamma rays, and gamma rays from plutonium can be easily shielded, so prospects are not bright for intercepting these materials with high probability.

Reducing the Damage from Exposure to Radiation

The best way to reduce damage from exposure to radiation is to reduce the exposure—and preventing a successful attack is by far the best approach. Despite everyone’s best efforts, however, an attack may succeed, and then exposure may still be reduced with sufficient knowledge and training. The distribution of radioactive material in an attack will not be uniform.

Given the location and magnitude of the release of radioactivity, the National Atmospheric Release Advisory Center (NARAC) at Lawrence Livermore National Laboratory is capable of predicting, within a few minutes, the distribution of radioactive material on the ground as determined by the wind profile of the moment. Therefore, it should be a high priority to provide NARAC with data on the location and yield of the nuclear explosion. These data would be available from sensors operated by the U.S. government for that purpose.

Attack on a Nuclear Reactor

Nuclear reactors are purposely sited in areas with low population density. Thus an attack on a nuclear reactor would cause no physical damage to an urban area. The nuclear-reactor scenario might involve a team of 20 or 30 dedicated terrorists on a suicide mission—probably a “beyond-design-basis threat” as defined by the U.S. Nuclear Regulatory Commission (USNRC). A few members of the team would have expert knowledge of the plan and of the reactor technology, at least to the point of having gathered information about locations and vulnerabilities.

The team would probably approach the area in sev-eral vehicles, would be armed with explosives, and would have undergone military-type engineering training, so it would be prepared to make its way through obstacles and to use rocket-propelled grenades and other shoulder-fired munitions to overcome the guards. The objective might be not only to cause a meltdown of the reactor by intentionally attacking the multiple safety systems that are designed to function independently in case of an accident (as they did in the 1979 Three Mile Island [TMI] incident), but also to blow a hole in the containment structure, which at TMI kept the large amount of radioactive material that escaped from the reactor pressure vessel from entering the environment as the reactor core melted.

There would be adequate warning that an attack on the reactor was in progress, not only through emergency communications from the reactor security force but, one hopes, also from a constant “all is well” communication among the USNRC, U.S. Department of Homeland Security (DHS), local security forces, and the reactor protective force.

A meltdown provoked by disabling the emergency core cooling system would mean the heat produced by radioactive decay within the reactor pressure vessel would no longer be removed (as it was at TMI) by water circulating through the pressure vessel. (At TMI, a gas bubble at the top of the pressure vessel prevented cooling of the upper part of the core, which melted.)

In the event of a complete meltdown, the radioactive contents of the core, on average two tons of fission products resulting from two years of full-power operation, would enter the containment building and would escape to the atmosphere through the hole that might have been blown in the containment structure. The USNRC-DHS could have organized and trained emergency response teams with the equipment necessary to close such a hole (e.g., an external crane to insert a gasketed steel umbrella-like device through the hole, just as one plugs a hole in a water heater or mounts a heavy-duty support on a hollow-core door).

If the reactor emergency systems have been overcome, the residual “decay heat” of the reactor core must be dissipated to the environment. Immediately after shutdown, the decay heat would be 7 percent of the normal 3,000 megawatts-thermal (MWt) output of the reactor—about 220 MWt. After one day, radioactive decay would reduce the heat generation to about 0.6 percent of the operating thermal output—about 20 MWt, corresponding to the evaporation (at 2.3 megajoule per kg of water) of about 9 kg/s or 0.5 tons per minute or 700 tons per day. However, this amount of water could not simply be allowed to boil freely to the atmosphere, because admixed debris from the reactor core would be intensely radioactive. One possible solution would be to create an emergency heat exchanger by allowing river water or pond water to remove heat by boiling.

Attack on a Spent-Fuel Storage Pool

An attack on an at-reactor storage pool for spent-fuel elements would be much like an attack on the reactor, in that it would be carried out by a suicide team armed with explosives. Again, it seems feasible to have stockpiled protective equipment at the site to prevent the rapid escape of water from the storage pool through a fissure or hole in the pool wall created by the attackers.

Because attackers might also cause a breach in the pool wall that would allow the release of thousands of tons of water that serve to shield and cool the spent-fuel elements, gasketed steel sheets might be put in place by a mobile crane provided for that contingency. Indeed, if one took seriously the prospect of an attack on a storage pool, analysis might show that such plates might be stored inside the pool along the wall, ready to be moved into place when necessary.

The Importance of Preparedness

Although it would be good to develop (even in secret) ways of responding to attacks on reactors and storage pools, it would be much better to make it publicly known that the consequences of an attack would be minimized or nullified, thus reducing the likelihood of such an attack.

Unlike a nuclear explosion in a city, the hazards created by an attack on a reactor or spent-fuel storage pool would be strictly radiological, and all physical systems would function normally, although fleeing staff could be a problem. There would be much less short-term radioactivity in the reactor core than in a nuclear explosion (i.e., primary fission products with half lives of less than a few hours), although a typical core of average operating age of two years contains long-lived fission products (e.g., 30-year strontium-90 and cesium-137) from 2,000 kg of uranium fissioned, equivalent (at 17 kilotons of energy release per kg of fission) to 34 megatons of explosive yield (i.e., 3,400 times the long-lived radioactivity of a 10-kiloton explosion).

Prevention of an Attack on a Reactor or Storage Pool

Attackers would not need nuclear materials to attack a reactor or spent-fuel pool. The essential factor in protecting those facilities is to recognize the harm that could be done either by an external team or by collusion among internal personnel. In a sense, protection against an external team is more likely to be effective, if the attack is detected early and the use of lethal force, in the form of land mines, automatic guns, and so on, has been properly planned and authorized. But the first requirement is to acknowledge the damage that might be done and to build flexible systems to counter an attack.

In the event of an attack on a reactor or storage pool, the communication infrastructure would be untouched and would retain the capability of broadcasting contamination maps to cell phones and on the web to guide individual decisions as to where to stay and where to seek safety from what might be a narrow plume of radioactivity. Similarly, with information on the nature of the release of radioactive material from a failed reactor or spent-fuel storage pool, NARAC could estimate the rate of cumulative dose to an individual.


The greatest damage and lethality from a nuclear explosion in a city would result from radioactive fallout that might expose people in a 20 km2 area to radiation levels that would cause 50 percent mortality. People outside (not shielded) during the time of maximum fallout would be subject to extremely high exposure levels because of the “beta” (electron-decay) radioactivity in contact with clothing or skin. Shielding from buildings can greatly reduce those levels (by a factor of 2 or 3 from light-frame buildings and a factor of 50 for even the lower stories of high-rise buildings). Without those protections, there would be 160,000 deaths in New York City (density 8,000 per km2) and 80,000 deaths in Los Angeles (density 4,000 per km2) from fallout.

Preventing an attack by securing nuclear weapons and nuclear materials is highly desirable. Preventing it by reducing the number of people with evil intent would also be beneficial. However, if such an attack does take place, an adaptable, rapid communication system could inform people after the first hour or so where the fallout hazard was greatest, so they could move one or two kilometers, on foot, away from that area and reduce their exposure to radiation. A communication system through cell phones or smart phones could save many lives.

Unfortunately, little has been done to create and test such a system, and the received wisdom is that no federal help will be available for the first 24 hours, when it would be most useful. The “push technology” that has been implemented in some tsunami-prone areas would be a starting point for such a system, although more detailed information would be necessary to characterize fallout patterns and to automatically provide specific advice about which way to go (e.g., a “Fallout App” for the smart phone).

Although not everyone in such an attack can be saved, it is the federal government’s responsibility to do the analysis, planning, simulation, and communication that might be needed for an attack on any one of 20 or more target cities. It would fall to local governments to prepare regulations that would facilitate the temporary sheltering of people, within tens of minutes, in office space to which they do not normally have access.


Carter, A.B., M.M. May, and W.J. Perry. 2007a. The Day After: Action in the 24 Hours Following a Nuclear Blast in an American City. Available online at

Carter, A.B., M.M. May, and W.J. Perry. 2007b. The Day After: Action Following a Nuclear Blast in an American City. Washington Quarterly 30(4): 19–32.  Available online at

Garwin, R.L. 2010. Nuclear Terrorism: A Global Threat. Presentation at the Harvard-Tsinghua Workshop on Nuclear Policies, Beijing, China, March 16, 2010. Available online at

Glasstone, S., and P.J. Dolan. 1977. The Effects of Nuclear Weapons. 3rd Edition, pp. 255–256. Washington, D.C.: Government Printing Office.

Homeland Security Council. 2009. Planning Guidance for Response to a Nuclear Detonation. Available online at

Meade, C. and R.C. Molander. 2006. Considering the Effects of a Catastrophic Terrorist Attack. Washington, D.C.: RAND. Available online at

Morrison, P. 1946. If the Bomb Gets Out of Hand. In One World or None: A Report to the Public on the Full Meaning of the Atomic Bomb. Published 1946 and republished 2007. New York: New Press. Available online at

White House. 2010. Key Facts About the Nuclear Security Summit. Available online at


 1 Sievert: A measure of dose (technically, dose equivalent) deposited in body tissue, averaged over the body. Such a dose would be caused by an exposure imparted by ionizing x-ray or gamma radiation undergoing an energy loss of 1 joule per kilogram of body tissue (l gray). One sievert is equivalent to 100 rem. Rem: A unit of absorbed dose that accounts for the relative biological effectiveness of ionizing radiation in tissue (also called equivalent dose).


About the Author:Richard L. Garwin is IBM Fellow Emeritus at the Thomas J. Watson Research Center and an NAE member.