This is an interesting read. General Observations Two options for nuclear devices to be built by terrorists are considered here: that of using the earliest design principles in a so-called crude design and that of using more advanced principles in a so-called sophisticated design. A crude design is one in which either of the methods successfully dem onstrated in 1945---the gun type and the implosion type---is applied. In the gun type, a subcritical piece of fissile material (the projectile) is fired rapidly into another subcritical piece (the target) such that the final assembly is supercritical without a change in the density of the material. In the implosion type, a near-critical piece of fissile material is compressed by a converging shock wave resulting from the detonation of a surrounding layer of high explosive and becomes supercritical because of its increase in density. A small, sophisticated design is one with a diameter of about 1 or 2 feet and a weight of one hundred to a few hundred pounds, so that it is readily transportable (for example, in the trunk of a standard car). Its size and weight may be compared with that of a crude design, which would be on the order of a ton or more and require a larger vehicle. It would also be possible, in about the same size and weight as a crude model but using a more sophis ticated design, to build a device requiring a smaller amount of fissile material to achieve similar effects. For a finished implosion device using a crude design, terrorists would need something like a critical mass of uranium (U) or plutonium (Pu) or, possibly, UO2 (uranium oxide) or Pu02 (plutonium oxide). For a gun type device, substantially more than a critical mass of uranium is needed, and plutonium cannot be used. It may be assumed that the terrorists would have acquired (or plan to acquire) such an amount either in the form of oxide powder (such as might be found in a fuel fabrication plant), in the form of finished fuel elements for a reactor---whether power, research, or breeder--- or as spent fuel. For a small, sophisticated design, the terrorists may need a similar amount of fissile material since practically all the presumed reductions in size and weight have to be taken from the assembly mechanism, and, with a less powerful assembly, not only will it be important to have the active material in its most effective form, but its amount will have to be sufficient to achieve supercriticality. Alternatively, a smaller amount could be used in a sophis ticated design with a more powerful and heavier assembly mechanism. Conceivably oxide powder might be used as is, although terrorists might choose to go through the chemical operation of reducing it to metal. Such a process would take a number of days and would require specialized equip ment and techniques, but these could certainly be within the reach of a dedicated technical team. Fuel elements of any type will have to be subjected to chemical pro cessing to separate the fissile material they may contain from the inert clad ding material or other diluents. This process would also require specialized equipment, a supply of appropriate reagents, well-developed techniques spe cific to the materials handled, and at least a few days to conduct the operation. Spent fuel from power reactors would contain some plutonium but at such low concentrations that it would have to be separated from the other materials in the fuel. It would also contain enough radioactive fission fragments that the chemical separation process would have to be carried out by remote operation, a very complicated undertaking requiring months to set up and check out, as well as many days for the processing itself. The fresh fuel for almost all power reactors would be of no use, since the uranium enrichment is too low to provide an explosive chain reaction. The terrorists would need something like a critical mass of the material they propose to use. For a particular fissile material, the amount that con stitutes a critical mass can vary widely depending on its density, the char acteristics (thickness and material) of the reflector employed, and the nature and fractional quantity of any inert diluents present (such as the oxygen in uranium oxide, the uranium 238 in partially enriched uranium 235, or chem ical impurities). Crude Designs Crude designs are discussed primarily in the context of the problems facing a terrorist group. Schematic drawings of fission explosive devices of the earliest types showing in a qualitative way the principles used in achieving the first fission explosions are widely available. However, the detailed design drawings and specifications that are essential before it is possible to plan the fabrication of actual parts are not available. The preparation of these drawings requires a large number of man-hours and the direct participation of indi viduals thoroughly informed in several quite distinct areas: the physical, chemical, and metallurgical properties of the various materials to be used, as well as the characteristics affecting their fabrication; neutronic properties; radiation effects, both nuclear and biological; technology concerning high explosives and/or chemical propellants; some hydrodynamics; electrical cir cuitry; and others. It is exceedingly unlikely that any single individual, even after years of assiduous preparation, could equip himself to proceed confidently in each part of this diverse range of necessary knowledge and skills, so that it may be assumed that a team would have to be involved. The number of specialists required would depend on the background and experience of those enlisted, but their number could scarcely be fewer than three or four and might well have to be more. The members of the team would have to be chosen not only on the basis of their technical knowledge, experience, and skills but also on their willingness to apply their talents to such a project, although their susceptibility to coercion or considerations of personal gain could be factors. In any event, the necessary attributes would be quite distinct from the paramilitary capability most often supposed to typify terrorists. Assuming the existence of a subnational group equipped for the activist role of acquiring the necessary fissile material and the technical role of making effective use of it, the question arises as to the time they might need to get ready. The period would depend on a number of factors, such as the form and nature of the material acquired and the form in which the terrorists proposed to use it; the most important factor would be the extent of the preparation and practice that the group had carried out before the actual acquisition of the material. To minimize the time interval between acquisition and readiness, the whole team would be required to prepare for a consid erable number of weeks (or, more probably, months) prior to acquisition. With respect to uranium, most of the necessary preparation and practice could be worked through using natural uranium as a stand-in. The time intervals might range from a modest number of hours, on the supposition that enriched uranium oxide powder could be used as is, to a number of days in the event that uranium oxide powder or highly enriched (unirradiated) uranium reactor fuel elements were to be converted to ura nium metal. The time could be much longer if the specifications of the device had to be revised after the material was in hand. For plutonium, the time intervals would be longer because of the greatly increased hazards involved (and the absolute need of foreseeing, preparing for, and observing all the necessary precautions). in addition, although uranium could be used as a stand-in for plutonium in practice efforts, there would be no opportunity to try out some of the processes required for handling plutonium until a suf ficient supply was available. To achieve a minimum turnaround time, the terrorists would, before acquisition, have to decide whether to use the material as is or to convert it to metal. They would have to make the decision in part in order to proceed with the design considerations, in part because the amounts needed would be different in the two cases, and in part to obtain and set up any required equipment. For the first option---using oxides without conversion to metal---the terrorists would need accurate information in advance concerning the phys ical state, isotopic composition, and chemical constituents of the material to be used. Although they would save time by avoiding the need for chemical processing, one disadvantage (among others) is the requirement for more fissile material than would be needed were metal to be used. This larger amount of fissile (and associated) material would require a larger weight in the assembly mechanism to bring the material into an explosive configuration. As to the second option---converting the materials to metal---a smaller amount of fissile material could be used. However, more time would be needed and quite specialized equipment and techniques---whether merely to reduce an oxide to the metal or to separate the fissile material from the cladding layers in which it is pressed or sintered in the nuclear fuel elements of a research reactor, for example. The necessary chemical operations, as well as the methods of casting and machining the nuclear materials, can be (and have been) described in a straightforward manner, but their conduct is most unlikely to proceed smoothly unless in the hands of someone with experience in the particular techniques involved, and even then substantial problems could arise. More Sophisticated Devices Most of the schematic drawings that are available relate to the earliest, most straightforward designs and indicate in principle how to achieve a fission explosion, without, however, providing the details of construction. Since 1945, notable reductions in size and weight, as well as increases in yield, have been realized. Schematic drawings of an entirely qualitative sort are also available that indicate the nature of some of the principles involved in these improvements. Merely on the basis of the fact that sophisticated devices are known to be feasible, it cannot be asserted that by stealing only a small amount of fissile material a terrorist would be able to produce a device with a reliable multikiloton yield in such a small size and weight as to be easy to transport and conceal. Such an assertion ignores at least a significant fraction of the problems that weapons laboratories have had to face and resolve over the past forty years. It is relevant to recall that today's impressively tidy weapons came about only at the end of a long series of tests that provided the basis for proceeding further. For some of these steps, full-scale nuclear tests were essential. In retrospect, not every incremental step taken would now seem necessary. Indeed, knowing only that much smaller and lighter weapons are feasible, it is possible at least to imagine going straight from the state of understanding in 1945 to a project to build a greatly improved device. The mere fact of knowing it is possible, even without knowing exactly how, would focus terrorists' attention and efforts. The fundamental question, however, would still remain: that of whether the object designed and built would or would not actually behave as pre dicted. Even with their tremendous experience, the weapons laboratories find on occasion that their efforts are flawed. Admittedly, weapons designers are now striving to impose refinements on an already highly refined product, but they have had to digest surprises and disappointments at many points along the way. For persons new to this business, as it may be supposed a terrorist group is, there is a great deal to learn before they could entertain any confidence that some small, sophisticated device they might build would perform as desired. To build the device would require a long course of study and a long course of hydrodynamic experimentation. To achieve the size and weight of a modern weapon while maintaining performance and confidence in perfor mance would require one or more full-scale nuclear tests, although consid erable progress in that direction could be made on the basis of nonnuclear experiments. In connection with an effort to reduce overall size and weight as far as possible, it would be necessary to use fissile material in its most effectiveform, plutonium metal. Moreover, while reducing the weight of the assembly mechanism, which implies reducing the amount of energy available to bring the fissile material into a supercritical configuration, it would not be possible at the same time to reduce the amount of fissile material employed very much. In this case, the amount of fissile material required in the finished pieces would be significantly larger than the formula quantity. Alternatively, in an implosion device without a reduction in weight and size, it would be possible to reduce the amount of nuclear materials required by using more effective implosion designs than that associated with the crude design. In either case---a small or a large sophisticated device---the design and building would require a base or installation at which experiments could be carried out over many months, results could be assessed, and, as necessary, the effects of corrections or improvements could be observed in follow-on experiments. Similar considerations would apply with respect to the chem ical, fabrication, and other aspects of the program. For the complete article, follow the link. Can Terrorists Build Nuclear Weapons?