By Carey Sublette
Last changed 18 May 2002
Adapted from Section 4.2 of Nuclear Weapon Frequently Asked Questions.
The prospect of terrorist acquisition of weapons has haunted the world since at least the late sixties, when international terrorism gained prominence. A variety of opinions have been expressed on the plausibility of these threats. Claims have been made that a terrorist weapon could:
These claims are all conditionally true: they may be valid, but only under a restrictive set of assumptions. And they also conflict strongly. Some are completely incompatible; others cannot be categorically eliminated as impossible in combination, but in any event it seems that no more two of them could be simultaneously possible under any scenario.
What technologies are plausible for terrorist use? And what types of weapons are reasonable threats?
The most fundamental constraint on a terrorist group is the type of fissile material that is available, and in what quantity. The key problem is to obtain any fissile material at all, a terrorist is not likely to have any choice in the matter as too what kind it is. The breakup of the Soviet Union has brought about a worrisome trade in fissile materials. A significant amount of weapons-usable material has turned up, although intelligence agencies running sting operations are the only actual "market" so far identified. Al-Qaeda operatives have actually tried to purchase fissile material during the 1990s, but failed to make any successful connections. The quantities of materials that have surfaced so far have not been nearly enough for a weapon (even if it were all pooled), but the quantities have been large enough to cause considerable concern. One of the most frightening problems has been Russian naval fuel. This contains highly enriched uranium (even higher than standard weapons grade!), and has been poorly secured at some locations.
In the long run, the availability of plutonium through commercial reprocessing for use in mixed oxide fuel (MOX) for commercial power reactors represents the major risk. Over one hundred tons of plutonium have already been commercially separated (an amount that will soon exceed the world's total weapons-grade plutonium production). This material will be in the hands of many nations, who will likely not all be equally vigilant in protecting their fuel stocks.
Clearly the most serious scenario is if weapons-grade HEU can be obtained by a terrorist group. Due to the very low neutron emission rate, very low technology can produce a substantial probability of full insertion and high yield detonation.
A weapon constructed from 40 kg of 93.5% HEU, with a 10 cm tungsten carbide reflector would produce a full yield of >10 kt. The required assembly time for a 50% chance of complete assembly is some 48 milliseconds, equal to a velocity of only 9 m/sec. This can be achieved by simply dropping the bullet 4.4 meters! Crude gun-type arrangements, along the lines of the IRA's makeshift mortars could easily achieve velocities of 100 m/sec or more.
A gun-type weapon is not a major concern if plutonium is used. Such a device might actually produce explosive yields in the range of a few tons, but would not be significantly more destructive than conventional truck bombs. On the other hand explosive compression, required for higher yields, is much more difficult to arrange. At the very least it requires a substantial quantity of good quality high explosive - at least a few hundred kilograms (unless the design and construction is rather sophisticated).
Now and in the future, reactor grade plutonium appears to be the material most likely to be available to a terrorist group. Given the spontaneous fission rate, and the limited technology for rapid assembly, predetonation is a foregone conclusion. In this scenario the yield of the system is not determined by the actual compression capability of the implosion system. Instead it is the rate of insertion that controls efficiency and yield. Any bomb design must emphasize making the insertion rate at the moment of criticality as fast as possible. In any case, rho (the density at the moment of disassembly relative to critical density) is going to be fairly small. Still, if insertion rates approaching those of the Fat Man design can be achieved then yields in the hundreds of tons, or even in excess of a kiloton, are possible. For an authoritative account of this by a major U.S. nuclear weapons scientist J. Carson Mark, see The Explosive Properties of Reactor-Grade Plutonium.
Despite hints to the contrary (for example Ted Taylor's comments in The Curve of Binding Energy among others), it is not plausible that true spherical implosion systems can be developed by a terrorist group. The difficulties in designing and making a working lens system appears to be simply insurmountable. Unfortunately, a spherical implosion system does not seem to be required for reasonably fast insertion at low levels of compression.
Consider an implosion of a system that may be in one dimension (linear implosion), two dimensions (cylindrical implosion), or three dimensions (spherical implosion). If delta represents the change in system dimension (i.e. size - radius or length) along the axis or axes of compression in n dimensions (n equals 1, 2, or 3), then the compression C achieved by the implosion is:
C = (r_0/(r_0 - delta))^nAt very low degrees of compression, this is roughly equivalent to:
C = n*(delta/r_0) + 1
That is, the excess density C - 1 is roughly proportional to the dimensional reduction ratio and the number of axes of compression. Thus for a given compression velocity, the actual rate of density increase for 3-D compression is three times faster than 1-D compression, but only 50% faster than 2-D compression. These differences are significant, but not dramatic.
Developing linear and cylindrical implosion systems fast enough to produce a highly destructive terrorist bomb appears to be feasible. The flying plate line-charge approach is sufficiently simple, and testable, that a low resource group could develop a workable system. Even plane or cylindrical explosive lenses are not out of the question, although they are probably more difficult.
Illicitly obtained plutonium would most probably be in the form of plutonium oxide, possibly as mixed oxide fuel. If the material were purified oxide powder, then it could be used directly in a bomb design. Fuel material, fabricated or not, would require chemical separation. A group sophisticated enough to attempt chemical processing would probably go on to reduce the plutonium to metal which is much more desirable for bomb construction.
Since the density of plutonium oxide is much since lower than plutonium metal, considerably more plutonium in this form would be needed. How much would depend on how highly compacted the plutonium oxide was at the moment of criticality. Although the crystal density of PuO2 is 11.4, the bulk density of unconsolidated oxide powder is only 3-4 (possibly even lower). To raise it as high as 5-6 would require compacting under substantial pressure.
The pressures generated by shock waves are much less efficient at compacting porous materials, compared to static pressures. This is due to the inherent strong entropic heating associated with large volume changes during shock compression. However the pressures in a strong high explosive shock (or generated by an explosive driven high velocity plate collision) are so high that densities approaching the theoretical crystal density are probably achievable. If it is assumed that a bomb builder could compress the powder to a density of 5 with moderate pressure, and that a density of 10 is achieved during implosion, then something like 50 kg of plutonium in the form of oxide would be required for a bomb without a reflector. Assuming a fairly good, readily available reflector (a few inches of iron or graphite), this could be reduced to 25-30 kg. Taking into account the explosive required, such a bomb (with a reflector) would be large - weighing on the order of a tonne.
Using plutonium metal would greatly reduce fissile material requirements, and lead to a much smaller bomb. A design might use the cylindrical collapse of a hollow ring of plutonium metal (as the delta or alpha phase), or cylindrical compression of a solid delta-phase aluminum-plutonium alloy disk. No more than about 10 kg of plutonium would be required in such a design, if a reasonably good reflector were used. Such a weapon might weigh as little as 200 kg.
Given that the system will disassemble well before compression is complete, an accurate symmetrical implosion is not really a necessity. Simply imploding the fissile material at a high rate even if imperfectly (that is, without a true plane or cylindrical shock wave), could produce the necessary rapid compression. For this to work, the fissile material would have to be fairly close to critical at the beginning of the implosion since an imperfect implosion would create unacceptable distortions if the compression factor were very large. As noted earlier in the discussion on nuclear testing, manufacturing a device that is close to critical is extremely hazardous and itself requires substantial sophistication.