Summary: Nuclear and Radiation Terrorism - Real Option or Imagined Threat?
Annotations of a Russian Nuclear Physicist
Alexander B. Koldobskij
The assessment of technological terrorist threats aiming at killing a maximum number of people and inflicting spectacular damage is reason enough for dealing with this issue, which should, however, be done by scientists rather than politicians, as the latter often lack the knowledge needed. While it is clear, that nuclear and radiation terrorism may not be ruled out, simply because there are nuclear weapons and terrorists, there should nevertheless be a realistic assessment of the threats.
Only if an adequate amount of fissionable material is available, a nuclear bomb can be produced. This material can only be extracted from some isotopes and high atomic elements of the periodic table. Power reactor fuel or elements ranging below uranium in the periodic table are not usable for building such devices, and the use of transplutonium elements is unfeasible, due to the difficulty of extraction, even for the weapons industries of technologically advanced countries.
Only uranium-235 and plutonium-239 of weapons usable quality (i.e. at least 90% and 94%, respectively) are realistically suitable for building explosive devices. In order for a nuclear bomb to function, the socalled critical mass is decisive, which may be obtained either by increasing the mass of the fissionable material without changing the density or by increasing the density without changing the mass. The "canon type” uses an uncritical mass impacting another charge, causing an overcritical condition. With implosion charges this condition is achieved by exploding a chemical explosive charge arranged around a core of uncritical mass.
Plutonium-239 is unusable for building a canon type bomb, as the chain reaction would set in too early and produce hardly more than a bang. If reactor plutonium is used, the critical mass is more than double the amount, because of the presence of unwanted isotopes, ruling out an implosion charge, due to the high temperatures of the fissionable material. In principle reactor plutonium would lend itself to produce explosive devices, but the technical and financial investment would be of such an order that even the nuclear powers would not attempt to do so. To miniaturize nuclear explosive charges to the size of a few grams of plutonium-239 is limited by the fact that the necessary density cannot be reached with an explosive. With uranium-235 the main obstacles are the needed amount as well as the complexity of the enrichment.
Likewise, the diversion or illicit acquisition of a nuclear warhead is no viable alternative for terrorists, given the obstacle of safeguard systems, and even if terrorists should succeed in taking a warhead apart, it may be ruled out that they succeed in assembling another usable one. Because of these difficulties it may be assumed, that states like North Korea or Iraq actually do not have a nuclear bomb or, for that matter, will be in the possession of one in the near future. This leaves the socalled dirty bomb as a last option for nuclear terrorists. But as it uses conventional explosives and low radioactive materials, it can cause only limited contamination, thus not being very effective.
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