In May 2008, officials of the International Atomic Energy Agency, commonly known as the United Nation nuclear watchdog, said to the press that they had been helping China with training exercises to ensure an adequate response in case of a dirty bomb attack during the Beijing Olympics. Do you know what a dirty bomb is, and how it differs from conventional explosives?

A dirty bomb contains radioactive materials and conventional explosive. When the explosive is set off, the blast carries radioactive materials into the surroundings. Since a dirty bomb can be made from nuclear wastes, it is a relatively accessible radiological device in terrorist attack.

The radioactive materials used in a dirty bomb will probably not create enough radiation exposure to cause immediate serious illness. The conventional explosive itself would be more harmful to people than the radioactive materials. However, a dirty bomb explosion would create fear and panic, contaminate property, and require potentially costly cleanup.

In the event of a dirty bomb explosion, radiation in the form of alpha particles, beta particles and gamma rays is emitted. The health effects from exposure to radiation can be minimized by (a) keeping away from the source of radiation; (b) staying away from the scene to keep shielding from external shining and inhaling radioactive material; and (c) removing clothes or/and taking a shower to reduce the radioactive contamination on the body.

There is a big difference. A dirty bomb is a conventional explosive device that when set off, contaminates a limited area near the blast. A nuclear bomb sets off huge explosion resulting from nuclear fission^[1] or nuclear fusion^[2]. In both fission and fusion, a large and highly concentrated amount of energy is released. It will release radioactive fallout and great heat, and cause widespread death, injury and massive damage.

The likelihood of a terrorist attack using a dirty bomb in Hong Kong is low. Nevertheless, the Hong Kong Special Administrative Region Government has a detailed contingency plan for possible terrorist attacks. The Hong Kong Observatory has equipment to monitor ambient radiation levels in Hong Kong and conduct radiological measurements on air, soil, water and food samples. It also provides the emergency departments with weather information to assess the spreading of radioactive materials. Details of the advice provided by the government on terrorist attacks can be obtained here.

Notes:
[1] Nuclear fission - Nuclear fission is a nuclear reaction in which a relatively large nucleus such as uranium splits into two smaller nuclei and releases energy in the process. Nuclear fission can be spontaneous or induced. Very large nuclei are unstable normally so some of them have a chance of splitting up spontaneously. Nuclear fission can also be initiated when the nucleus of an atom is hit by other particles. For example, uranium may split up when hit by a neutron.
[2] Nuclear fusion - Nuclear fusion is a nuclear reaction in which two nuclei join to form a larger nucleus and release energy. The energy released in nuclear fusion is usually larger than that in nuclear fission. It is hydrogen fusion (producing helium) in the Sun that results in light and heat coming out of it.