Here. A lot of details.

Without additional data, it is impossible to say with an acceptable level of certainty whether any number of these weapons was stolen during and following the breakup of the Soviet Union, as Alexander Lebed and a few other Russians claimed. Available evidence suggests that these stories were most probably not true, and that they were generated by incomplete information or ulterior motives. The probability that such weapons could be used by terrorists (assuming some were stolen) appears even lower. The reasons for that conclusion are the following:

- Probably the most convincing evidence is the fact that no terrorist group has used such a device or even credibly threatened its use. There have been many instances, especially during the first and the second, ongoing wars in Chechnya. Yet, the worse case so far was limited to the burying of a container with radioactive isotopes in a Moscow park. Since the majority of feasible scenarios involve Chechens, and since the period of greatest risk was in the early 1990s, the inactivity of Chechens in this matter is significant.

- The scenario offered by Denisov, namely, that a number of portable nuclear devices were left outside Russia when nuclear weapons were withdrawn to its territory, appears unlikely. In most cases, the removal took place more than a year before the breakup of the Soviet Union, when the Soviet military, in particular troops charged with custody of nuclear weapons, were in relative order. In the only case in which an attempt to "conceal" a limited number of nuclear weapons could be suspected--in Ukraine--the removal was subject to enhanced accounting procedures.

- The risk of loss of nuclear weapons was the greatest in the early 1990s within the territory of Russia itself. At that time, large numbers of nuclear weapons were transported across the country, often by trains ill-suited for that purpose; accounting could be relatively lax, and personnel was undergoing the period of deepest uncertainty, depression, and precipitous drop in the standard of living. With time, the probability of the loss of portable nuclear devices decreased with gradual increase of stability and, even more important in terms of immediate effect, American assistance, which helped to improve accounting, transportation, and security of nuclear weapons.

- Yet another scenario offered by Denisov--assembly of portable nuclear devices from parts "forgotten" outside Russia--is the least believable. There was no nuclear weapons production outside Russia; consequently, the probability that parts of nuclear weapons could be found outside its territory is very unlikely. In addition, there was very little specialized expertise and know-how outside Russia, whereas portable nuclear devices were apparently very complicated in design and required highly skilled professionals to oversee their production and assembly.

Even assuming that some portable nuclear devices were lost, it would be very difficult to use them, and it is almost certain that the features that make portable nuclear devices so dangerous (small size and full-scale nuclear explosion effects) will not be taken advantage of.

- Information about unusually short maintenance periods for these weapons is probably true, although the extent and the pace of deterioration of nuclear weapons' features cannot be determined from open sources. Since, as it was noted above, the period of greatest risk was in the early 1990s, the stolen devices, if any, have already missed as many as 20 routine component replacement procedures and are probably nearing the end of their service life. Consequently, it is nearly certain that they will be unable to produce the design yield and maybe will not be able to produce any yield at all.

- Most, if not all, portable nuclear warheads are equipped with some protection devices (e.g., PALs), making their unauthorized use difficult, though not completely impossible.

In effect, portable nuclear devices, if stolen, will hardly be usable, at least not in the fashion that they were originally designed for. They could be, of course, dismantled to extract weapons-grade plutonium, which could then be used in a cruder nuclear device or for a "dirty bomb"; but in this case, the problem of suitcase nukes is virtually indistinguishable from the broader problem of safety and security of all Russian nuclear weapons and weapons-grade fissile materials.