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Much as the anthrax letters focused the nation’s attention on the threat of bioterrorism, the arrest in May of Jose Padilla threw a spotlight on another threat: “radiological dispersal devices,” or more colloquially, “dirty bombs.”

Such devices use explosives or other means to disperse solid or liquid radioactive materials. And there are numerous potential sources of radioactive materials. Rods of cobalt-60, for example, are used for irradiating spices and other foods to kill insects and germs, for medical radiation therapy to treat cancer and for industrial radiography to x-ray thick and dense materials. Strontium-90 provides heat for powering isolated instruments or radio relays. But for the most part, a dirty bomb poses little more immediate health threat than a conventional bomb.

Consider, for example, a hypothetical attack on Munich with one kilogram of plutonium dispersed by high explosives. Assuming a very pessimistic low wind speed so that the radioactive cloud remains over the city for 12 hours, the net result is that-after 40 years or so-120 people would die of cancer caused by the plutonium. The economic ramifications of a detonated dirty bomb, on the other hand, could be tremendous, as a very large area of contamination would have to be evacuated and cleaned up or left uninhabited for years.

Nuclear explosives, however, represent a much larger threat. A terrorist nuclear explosive would devastate a city, whether detonated in the hold of a ship in harbor, in a cargo container, in a cellar, or in an apartment. U.S. and Russian strategic nuclear weapons are built to yield explosions in the range of 150 kilotons, or the equivalent of 150,000 tons of TNT. But even if a terrorist set off a device that caused just a one-kiloton explosion, the effect on a city like Manhattan would be devastating. Eleven city blocks would be obliterated. People in a 53-block area would be killed outright by the heat of the explosion. Those in an 88-block area would immediately receive a lethal dose of radiation. During working hours in a densely populated part of Manhattan with some 2,400 people per block, some 210,000 people would die. For a 10-kiloton explosion, perhaps five times as many would die.

Hospitals would be overwhelmed by the number of people injured by flying glass, suffering from radiation exposure and the like. Transit and communications would be severely crippled. Organized medicine would be unable to cope. Even after the initial crisis had passed, public-safety personnel would face the daunting task of determining where high levels of radioactivity had rendered areas uninhabitable, and where contamination was slight enough that people could return to their homes.

How could such a terrorist explosion come about? Military nuclear weapons could be stolen or diverted, but they are usually provided with substantial protection against unauthorized detonation, and considerable skill would be required to bypass this protection. An improvised nuclear device would not have this problem but would require the acquisition of one essential ingredient-fissile material, either plutonium or highly enriched uranium.

Fissile material is not an article of commerce and itself would have to be stolen or diverted. The first plutonium bomb incorporated six kilograms of weapons-grade plutonium, of which more than 250 tons has now been made-enough for 40,000 such crude weapons. In addition, every large nuclear power reactor produces annually on the order of 200 kilograms of plutonium, which is not weapons grade and need not be to make an improvised nuclear device. Indeed, there are 100 tons or more of plutonium accumulated in Japan, France and the United Kingdom alone from the reprocessing of civilian power reactor fuel.

The low-enriched uranium used in U.S. nuclear reactors, on the other hand, can in no way be used directly to make a nuclear explosive. But highly enriched uranium as used in nuclear weaponry is also employed in some research reactors and in fuel for naval reactors, such as those that propel our aircraft carriers and submarines. Likewise, Russian nuclear-propelled ships use highly enriched uranium. And in Russia particularly, stocks of highly enriched uranium and plutonium (even weapons-grade plutonium) intended as nuclear fuel do not have nearly the security provided to nuclear weaponry.

The best single protection against the terrorist use of nuclear weapons is to block the acquisition of plutonium or enriched uranium. After some months of denigrating U.S. programs that have existed since 1994 to help Russia protect weapon-usable materials, the Bush administration in December 2001 recognized the seriousness of this problem and that something can be done to solve it, and it has increased the budget for such “cooperative threat-reduction activities.” In a separate deal, the United States is buying 500 tons of highly enriched uranium (diluted in Russia to low-enriched uranium to fuel U.S. reactors) over 20 years, at a cost of about $12 billion. Once diluted, this material is useless for the manufacture of nuclear weapons. But the delivery of the nuclear fuel will not be complete until 2014, and Russia had diluted only about 150 tons of highly enriched uranium by summer 2002. Here is a threat that will persist for much longer than necessary. This is a serious concern. Every 100 tons of bomb uranium can be used to build more than 1,000 nuclear weapons of the type that destroyed Hiroshima.

It would be a simple matter for the United States and/or the international community to advance Russia the much smaller amount of money required to blend down the remaining 350 tons (and perhaps another 700 tons not included in this deal) enough to render it unusable for nuclear weaponry. This could be done in about two years, and the money would be repaid by Russia with or without interest when this material was further blended and transferred to the United States.

Eliminating such large stores of weaponable materials is one important step. Detecting the illegal transport of such materials when they fall into the wrong hands is another. Can weapon-usable materials be detected in transit? Yes and no. Radiation detectors sensitive to low-energy gamma rays from plutonium are routinely deployed at the portals of plants processing plutonium. Plutonium detection can be foiled by the use of enough lead shielding, but that eliminates the possibility of accumulating a weapon mass of plutonium by routinely smuggling tiny amounts through a portal, since the shield would be too massive to conceal on the body. Uranium, however, is somewhat more difficult to detect than plutonium.

In the late 1940s and early 1950s, the threat of a Soviet nuclear weapon smuggled into the United States was taken seriously. As recounted in a recent Washington Post op-ed piece, J. Robert Oppenheimer, who led the Los Alamos effort to produce the nuclear weapons used in 1945, was asked in 1946 at a congressional hearing “whether three or four men couldn’t smuggle units of an [atomic] bomb into New York and blow up the whole city.” His reply: “Of course it could be done, and people could destroy New York.” Asked how such a weapon smuggled in a crate could be detected, Oppenheimer replied, “With a screwdriver.” Some years later the U.S. Atomic Energy Commission published a still classified study, the “Screwdriver Report.”

Currently, the United States has dedicated nuclear-emergency search teams with the ability to deploy about 600 people with devices to detect and disable nuclear weapons in the case of a credible bomb threat. But a terrorist with a mission to actually kill people would certainly not alert the authorities to the existence of a nuclear explosive; the device would need to be detected either in transit, following intelligence tips or by generalized search. This is a tall order for the nuclear-emergency search team, even granting substantial improvement in its capabilities. We must-and with proper research we can-develop improved sensing technology capable of detecting even shielded nuclear materials in cargo containers, trucks, luggage and so forth. Deployed widely, such technology would be the embodiment of Oppenheimer’s screwdriver.

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