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Could Terrorists Produce Low-Yield Nuclear Weapons? Stanislav Rodionov * Russian Academy of Sciences Space Research Institute Quite recently, most specialists have implied that terrorists would try to produce nuclear weapons using existing scientific knowledge and technical po- tential. For example, the Committee on International Security and Arms Control (CISAC) of the U.S. National Academy of Sciences came to the following con- clusions concerning the unauthorized use of plutonium: i . Possible proliferators could produce nuclear explosive devices even from reactor-grade plutonium; a simple design (i.e., implosive systems) would pro- vide a yield from one to a few kilotons, while a more modern design could provide a higher yield. In assessing security threats, it is necessary to understand who is trying to acquire and misuse plutonium. Terrorists might care little about the differences between reactor-grade and weapons-grade plutonium. Small nations would be likely to care more, in the sense of preferring to make weapons from weapons- grade plutonium, if everything else were equal. I would like to focus on the fact that the situation might be much simpler. Indeed, although terrorism in general is an unpredictable and uncontrolled phe- nomenon, nuclear terrorism itself may have some specific features. First, it might be dangerous and risky to keep stolen nuclear explosives for a long time. In this case, one could not spend the extra time needed to develop (not to mention, test) a reliable nuclear bomb. It is highly probable that terrorists * Translated from the Russian by Kelly Robbins. 156

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NUCLEAR TERRORISM 157 would need just an explosive device and a very simple device at that to carry out a single action. Second, the explosion itself might be the most effective factor in achieving the terrorists' objectives, rather than the nuclear blast yield. Moreover, an enor- mous number of victims could have a negative effect on that part of the interna- tional community that adopts a positive or neutral attitude toward terrorists. Therefore, low-yield nuclear explosive devices might be rather attractive for terrorists, barring any serious technical barriers to their construction. We shall see later that under some conditions this problem may have a solution. Let us consider two approaches to lowering the yield of a nuclear explosion. The first is based on extremely high compression of a fissile material. It is well known that its critical mass is inversely proportional to the square of its density. For example, plutonium density in modern weapons designs is three to four times higher as a result of implosion.2 At higher compressions, there is no limit on the minimum amount of fissile material required to construct a nuclear explo- sive. One can imagine micronuclear explosives with yields in the ton range, requiring fissile materials on the order of hundreds or even tens of grams. But what can actually be achieved along this line of development is limited only by available implosion technologies. Thus, it does not seem that this straightfor- ward approach could be used by terrorists, because it requires a very high degree of technical expertise. The other approach is connected to the so-called fizzle effect, which really is a preinitiation of a nuclear chain reaction in a fissile material in a supercritical state (due to the occurrence of "accidental" neutrons). As a result, the yield of the explosion is reduced in comparison with its nominal value. It should be noted that all types of nuclear weapons have a nonzero fizzle probability. One can categorize all types of nuclear weapons as either fast (implosive systems) or slow (gun-type assemblies) depending on the "waiting time" between the start of criticality and the moment of optimal condition. The fizzle effect is more proba- ble in slow systems and for fissile materials with a high level of neutron self- emission (due mostly to the process of spontaneous fission). Therefore, nuclear terrorists could be very interested in a gun-type nuclear device with reactor- grade or weapons-grade plutonium. Estimates of the fizzle yield were made by Dr. Carson Mark, former Theo- retical Division Leader of Los Alamos National Laboratory.3 He considered "as a purely hypothetical example" a weapons-grade plutonium assembly of the im- plosion type used at Trinity (the first American nuclear test, July 16, 1945), with the nominal yield of 20 kilotons. The fizzle yield in this case might be 0.5 kiloton. A similar assembly in a gun-type system would produce a fizzle yield of some 10-20 tons. The fizzle phenomenon is of a statistical nature where the main parameter would be the moment of neutron occurrence during the waiting period. The fizzle could be managed to some extent, but management of this kind requires

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158 HIGH-IMPACT TERRORISM some extra technical complications that might be unacceptable for terrorists. Therefore, the "natural" fizzle seems to be more attractive for them. The above- mentioned natural-fizzle yield value of 10-20 tons was estimated for a rather high speed of bringing together two subcritical masses of plutonium (about 300 m/s). A yield about five times lower would be expected at a relative speed of 100 m/s. The corresponding yield value (a few tons) seems to be quite acceptable for terrorists. Let us assume that the mass of a single plutonium piece would be, say, 5 kg. In this case, its kinetic energy (at a speed of 100 m/s) would be equal to 25 kJ. Such energy may be provided either by high explosives (the explosive energy of TNT is about 4 MJ/kg) or by some source of stored mechanical energy (a com- pressed spring, for example). For systems with natural fizzle, the idea of testing makes no sense since every subsequent result can, in principle, differ from the preceding one. The yield value of few tons is comparable with the explosive energy release in some instances where terrorists used chemical high explosives (as in the Okla- homa City case, for instance). So, a natural question arises, What could be the advantages of a low-yield nuclear device compared to a few-ton blast produced by chemical high explosives? In fact, one could identify certain advantages. First of all, it would be a direct demonstration of the fact that terrorists do posses a nuclear explosive. From the psychological point of view, this action might produce the most important effect on public opinion. Second, the nuclear explosion is characterized by higher effective tempera- tures. This results in a more powerful shockwave and thermal effects. One can estimate the "kill range" of a 2-ton nuclear blast using corresponding scaling laws and reference data about the consequences of the nuclear explosions in Hiroshima and Nagasaki. Such an estimated value will hardly exceed 100 meters. Third, the nuclear explosion inherently produces radioactive contamination by fission products (not to mention radioactivity induced by fast neutrons). The yield of 2 tons would correspond to total fissioning of only 0.1 gram of plutoni- um. As a result, about 0.3 Ci of cesium-137 and 0.1 Ci of strontium-90 (the most abundant long-lived fission products) would be generated. The initial activity of short-lived fission products (which decay mostly within a few months after the explosion) would be greater by nearly two orders of magnitude (about 20-30 Ci). Plutonium itself is a toxic material as well, especially in the form of plutonium oxide aerosol, which is produced by a high-temperature blast. This aerosol could disperse over larger distances and be dangerous to the population.4 However, these low-yield nuclear devices cannot be "invisible." It has been shown that neutron emission from an ordinary plutonium warhead can be detect- ed at distances of 50-70 meters.5 The corresponding detection range would be two to three times greater for devices using reactor-grade plutonium. It is impor- tant to note that the detection range and kill range of a low-yield device are

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NUCLEAR TERRORISM 159 comparable. This makes it possible to protect some very important targets from terrorist nuclear attacks. In conclusion, potential nuclear terrorists would encounter no serious tech- nical problems in constructing a simple low-yield (in the order of few tons of TNT equivalent) and low-weight (in the order of a hundred kilograms) gun-type nuclear explosive device using weapons-grade or reactor-grade plutonium. A device of this kind would have destructive and thermal kill ranges of about 100 meters. Moreover, it would produce radioactive fallout with a total intensity of a few tens of curies as well as a cloud containing a few kilograms of plutonium oxide aerosol. The "threshold" amount of plutonium for such a device might exceed to some extent the mass of plutonium for an ordinary nuclear warhead. This hypothetical example emphasizes the vital importance of very strict control over nonproliferation of any amounts of plutonium (both weapons-grade and reactor-grade material of any isotope composition). It also emphasizes the potential importance of very sensitive neutron detectors. NOTES 1. Committee on International Security and Arms Control, National Academy of Sciences. 1995. Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options. Wash- ington, D.C.: National Academy Press, p. 44. 2. Cochran, T.B., C.E. Paine. 1995. Nuclear Weapons Databook: The Role of Hydronuclear Tests and Other Low-Yield Nuclear Explosions and Their Status Under a Comprehensive Test Ban. New York: Natural Resources Defense Council, p. 6. 3. Mark, J.C. 1993. Explosive properties of reactor-grade plutonium. Science and Global Security 4(1):111-124. 4. Fetter, S., F. von Hippel. 1990. The hazard from plutonium dispersal by nuclear-warhead accident. Science and Global Security 2(1):21-41. 5. Occasional Report. 1990. The Black Sea Experiment. Science and Global Security 1(3- 4):323-333.