Grand Challenges for Engineering  -  Jul 23, 2014

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Prevent nuclear terror

The need for technologies to prevent and respond to a nuclear attack is growing.

Long before 2001, defenders of national security worried about the possible immediate death of 300,000 people and the loss of thousands of square miles of land to productive use through an act of terror. 

From the beginnings of the nuclear age, the materials suitable for making a weapon have been accumulating around the world. Even some actual bombs may not be adequately secure against theft or sale in certain countries. Nuclear reactors for research or power are scattered about the globe, capable of producing the raw material for nuclear devices. And the instructions for building explosive devices from such materials have been widely published, suggesting that access to the ingredients would make a bomb a realistic possibility. 

“It should not be assumed,” write physicists Richard Garwin and Georges Charpak, “that terrorists or other groups wishing to make nuclear weapons cannot read.”

Consequently, the main obstacle to a terrorist planning a nuclear nightmare would be acquiring fissile material — plutonium or highly enriched uranium capable of rapid nuclear fission. Nearly 2 million kilograms of each have already been produced and exist in the world today. It takes less than ten kilograms of plutonium, or a few tens of kilograms of highly enriched uranium, to build a bomb.

Fission, or the splitting of an atom's nucleus, was discovered originally in uranium. For a bomb, you need a highly enriched mass of uranium typically consisting of 90 percent uranium-235, a form found at levels of less than 1 percent in uranium ore. Fuel for nuclear power reactors is only enriched 3 percent to 5 percent with respect to this trace form of uranium, and so is no good for explosions. Highly enriched bomb-grade uranium is, however, produced for some reactors (such as those used to power nuclear submarines and for some research reactors) and might be diverted to terrorists.

Besides uranium, another serious concern is the synthetic radioactive element plutonium. Produced by the nuclear “burning” of uranium in reactors, plutonium is a radioactive hazard in itself and also an ideal fuel for nuclear explosives. Worldwide, more than 1,000 reactors operate nowadays, some producing electric power, others mostly used for research. Plutonium produced in either reactor type could be extracted for use in weapons.

Nuclear security therefore represents one of the most urgent policy issues of the 21st century. In addition to its political and institutional aspects, it poses acute technical issues as well. In short, engineering shares the formidable challenges of finding all the dangerous nuclear material in the world, keeping track of it, securing it, and detecting its diversion or transport for terrorist use.

What are the challenges to preventing nuclear terror attacks?

Challenges include: (1) how to secure the materials; (2) how to detect, especially at a distance; (3) how to render a potential device harmless; (4) emergency response, cleanup, and public communication after a nuclear explosion; and (5) determining who did it. All of these have engineering components; some are purely technical and others are systems challenges.

Some of the technical issues are informational — it is essential to have a sound system for keeping track of weapons and nuclear materials known to exist, in order to protect against their theft or purchase on the black market by terrorists.

Another possible danger is that sophisticated terrorists could buy the innards of a dismantled bomb, or fuel from a nuclear power plant, and build a homemade explosive device. It is conceivable that such a device would produce considerable damage, with explosive power perhaps a tenth of the bomb that destroyed Hiroshima.

With help from renegade professional de­signers, terrorists might even build a more powerful device, equaling or exceeding the force of the Hiroshima bomb. Detonated in a large city, such a bomb could kill 100,000 people or more.

Building a full-scale bomb would not be easy, so terrorists might attempt instead to cause other forms of nuclear chaos, possibly using conventional explosives to blast and scatter radioactive material around a city. Such “dirty bombs” might cause relatively few immediate deaths, but they could contaminate large areas of land, cause potential economic havoc to the operation of a city, and increase long-term cancer incidence. There are millions of potential sources of radioactive material, which is widely used in hospitals, research facilities, and industry -- so preventing access is extremely difficult. Responding to a “dirty bomb” attack would also involve engineering challenges ranging from monitoring to cleanup, of both people and places.

Concern for nuclear security complicates the use of nuclear energy for peaceful purposes, such as generating electricity. Ensuring that a nation using nuclear power for energy does not extract plutonium for bomb building is not easy. Diversion of plutonium is much more difficult when a country opts for a “once through” fuel cycle that keeps the plutonium with the highly radioactive spent fuel, rather than a “closed” fuel cycle where spent fuel is reprocessed and plutonium separated out. Simple record keeping could be faked or circumvented. Regulations requiring human inspection and video monitoring are surely not foolproof.

What are the possible engineering solutions?

A possible engineering solution would be the development of a passive device, situated near a reactor, which could transmit real-time data on the reactor’s contents, betraying any removal of plutonium. (This sort of device would be especially useful if it could also detect signs that the reactor was being operated in a way to maximize plutonium production rather than power.) Such devices are already being designed and tested.

Of course, if dangerous nuclear materials are diverted from a power reactor, or probably more likely from some other source, preventing their transportation to a possible point of use remains a serious problem. Protecting U.S. borders from nuclear transgression poses a formidable challenge, because so many imports are legitimately shipped into the country within large shipping containers. Individual inspection of each container would be costly and very disruptive — each can hold 30 tons, and roughly 10 million arrive in the United States every year.

Various ways of detecting nuclear materials hidden in such containers have been proposed or tested, but most are ineffective.

One new approach has shown promise, though. Nicknamed the “nuclear car wash,” it consists of a sophisticated scanning system that containers would pass by while on a conveyor belt, much as a vehicle glides through an automated car wash. As the containers pass the device, they receive pulses of neutrons, a common subatomic particle often used to induce nuclear reactions. In this case, the neutrons would induce fission in any weapons-grade nuclear materials within the container. The fission, in turn, produces radioactive substances that would emit gamma rays, a form of radiation that could be reliably detected from outside the container. Still, it leaves residual radioactivity and could be shielded with a large amount of water. High-energy X-rays leave no radioactivity, and may even be able to detect shields against their beams, but they damage sensitive material like photographic film. 

A simulation program might be engineered that would help shippers in packing, ensuring that it leaves acceptable detail for current security scanning of the contents. Incentives might include charging for more complex scans when items are shielded by their packing.

There are already real-time mutual surveillance systems in operation between Russia and the U.S.’s Sandia National Laboratory to ensure that there is no unauthorized access to storage containers of weapons-usable materials. A challenge for engineers would be to expand such schemes at a reasonable cost.

No doubt other nuclear challenges will surface and additional engineering methods will be needed to protect against the variety of possible nuclear assaults. But the ingenuity of systems and nuclear engineers, and the deep understanding of nature’s nuclear secrets provided by basic physics research, offer encouragement that those challenges can be met in the 21st century.

 

References 

Bernstein, A. et al.  2002.  Nuclear reactor safeguards and monitoring with antineutrino detectors,” Journal of Applied Physics 91: 4672-4676. DOI: 10.1063/1.1452775

Bowden, N.S., et al.  2007.  Experimental results from an antineutrino detector for cooperative monitoring of nuclear reactors.  Nuclear Instruments and Methods in Physics Research A, 572: 985-998.

Garwin, R. and G. Charpak.  2001.  Megawatts and Megatons.  New York: Knopf.

Hecker, S.S.  2006.  Toward a Comprehensive Safeguards System: Keeping Fissile Materials Out of Terrorists’ Hands.  The Annals of the American Academy of Political and Social Science, 607: 121-132.

National Research Council.  2002.  Making the Nation Safer: The Role of Science and Technology in Countering Terrorism.  Washington, D.C.: National Academies Press.  pp. 39-64.      

Nuclear Threat Initiative.  2006.  Seeing the Danger is the First Step: 2006 Annual Report.

Slaughter, D.R., et al.  2007.  The nuclear car wash: A system to detect nuclear weapons in commercial cargo shipments,” Nuclear Instruments and Methods in Physics Research A, 579: 349-352. DOI:10.1016/j.nima.2007.04.058

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