Nuclear weapons are the most destructive devices ever created. Could terrorists detonate one in a city?
Since the first atomic bomb was dropped on Hiroshima on 6 August 1945 the threat of nuclear weapons has been a key element in international affairs. Through the Cold War both superpowers were held in check by the fear of each other’s nuclear arsenal, with tens of thousands of warheads deployed on both sides. As the USSR fell apart in the early 1990s the danger seemed to recede, but it didn’t take long for it to return in a new form – the threat of a nuclear weapon being developed by a rogue state or terrorist group.
Terrorist nukes have been a theme in many movies and novels, but is there a real risk? Could a criminal group actually manage to build an atomic bomb? The answer is complicated – in some ways it’s extremely easy, in others almost impossible. A lot of it depends on the contacts the group can develop and the level of capability they’re willing to settle for.
The entry level to the radiological weapon game isn’t actually a nuclear device at all – it’s what’s usually known as a “dirty bomb.” This is a large conventional bomb that’s surrounded with radioactive material, which can be any one of a large number of isotopes and doesn’t need to be high grade. The explosion itself is simply a blast, but it scatters the surrounding material over a large area and causes widespread contamination. A bomb like this could cause casualties from radiation exposure but the main effects would be psychological and economic – if one was detonated in a major city the cost of the decontamination effort could run into billions of Euros. The material necessary to build one is fairly easy to obtain; radiation sources stolen from hospitals or research labs could provide suitable isotopes, such as caesium-137 or cobalt-60. Thousands of radioactive objects are believed to be missing in Russia including high-powered strontium-90 thermal generators used to power remote lighthouses.
There are technical challenges in building a dirty bomb though. The main one is that if the radioactive material is dangerous enough to create heavy contamination it’s also dangerous to those planting the bomb. If it’s shielded well enough to contain the danger – usually several inches of lead – the device may be too heavy to transport and the container can actually interfere with the explosion. Of course if the terrorists are willing to die that helps, but the radiation may be dangerous enough to incapacitate them before they can plant it. In 2001 three woodcutters in Georgia discovered a strontium-90 generator and, incredibly, decided to use it as a heater in their hut. All three developed acute radiation poisoning within hours. There are also issues with finding material that will disperse evenly. While dirty bombs are often seen as a simple weapon of mass destruction the difficulties are actually formidable, which is probably why none have been used yet.
Moving up a level, the next step is an actual nuclear device. The simplest variety is a single-stage fusion bomb, similar to the ones used in 1945. The first atomic bombs were huge, up to six metres long in some cases, but it would now be possible to build a simple, relatively crude device that would fit inside a one metre cube. The technology involved in building such a device is not beyond the reach of most engineers and most of the required materials and components are easily available. Standard machine tools and a small workshop would be adequate for the project, and a weapon with a yield of up to 50 kilotons could probably be built. This would create a fireball nearly 600 metres in diameter and cause almost 100% casualties inside a circle 3km wide.
A boosted fission device uses a small amount of fusion fuel – usually tritium – to increase the efficiency of a fission reaction. In military designs this method is used to reduce the amount of fissile material needed, but in a terrorist weapon it could significantly boost the yield by allowing more of the material to react before the bomb’s own power blew it apart – in a crude device it’s common for only about 1-2% of the fuel to be consumed in the reaction. The problem is that a boosted fission device is more complex – although still achievable for a good engineer – and tritium is extremely expensive. It’s not hard to get – many watches and other devices with luminous dials contain it – but enough for a device could easily cost millions. On the other hand a 200kt weapon might be possible, and this would be powerful enough to vaporise everything inside a fireball a kilometre across. Few people within 2km of the explosion would survive, with most of those who didn’t die from the fireball or blast killed by radiation within a week.
The ultimate terrorist goal would be a multi-stage fusion device – a hydrogen bomb. These use a fission bomb as a trigger to create immensely high pressure, which in turn ignites a massive fusion “burn” in a supply of tritium. A weapon like this would be far more complex to produce and would require a talented nuclear physicist to design. The cost would also be enormous. There are people out there who can do it, though; the bar is high but not impassable. The reward is the huge power of a thermonuclear explosion – even a terrorist device could easily reach a yield of 5 megatons. The fireball from a blast this size would be over 3.5km wide and there would be little chance of survival less than 5km away; the thermal pulse would cause third degree burns at out to 25km. The immediate radiation hazard would actually be reduced though; anyone close enough to collect a dangerous dose of radiation would already have been killed by blast or heat. There would be a significant fallout hazard on the other hand, caused by irradiated debris thrown up by the explosion.
Even a small fission bomb is clearly capable of devastating a city centre, so what has stopped terrorists using them? Mostly it’s the difficulty of getting hold of fissile material. Only two known isotopes will work in a bomb core – uranium-235 (U235) and plutonium-239 (Pu239). Supplies of both of these are very tightly controlled and only a few nations have the ability to refine them. International agencies monitor the development of the capability – like the controversy over Iran’s uranium enrichment plants – and severe pressure can be applied to any country that’s thought to be acting irresponsibly.
Dangers remain though. There are still suspicions that fissile material was lost during the breakup of the Soviet Union, and a well funded terrorist group might be able to buy a stockpile. The possibility of a rogue state like North Korea, or an unstable nuclear power like Pakistan, supplying material – or even a complete weapon – to terrorists can’t be discounted either. Sooner or later a terrorist group will manage to get their hands on what they need to build a nuclear device. The best defence against that is intelligence gathering.