A dirty bomb is likely to be a primitive device in which TNT or fuel oil and fertilizer are combined with highly radioactive materials. The detonated bomb vaporizes or aerosolizes the isotopes, propelling them into the air.
A fission bomb is a more sophisticated mechanism that relies on creating a runaway nuclear chain reaction in uranium 235 or plutonium 239. One type features a shell of high explosives. When the bomb goes off, the explosives produce an imploding shock wave that drives the plutonium pieces together into a sphere containing a pellet of beryllium/polonium at the center, creating critical mass. The resulting fission reaction causes the bomb to explode with tremendous force, sending high energy electromagnetic waves and fallout into the air.
Compared to a fission weapon blast, the most plausible dirty bomb attack would release one millionth as much radiation; and many fatalities could be avoided if people are evacuated properly. Afterwards, questions of what areas to abandon and what soil, water and air quality to attain have barely been explored by local and national authorities and scientists.
US must fix conflicting standards and jurisdictions that will make the chaos caused by a nuclear attack go on for years. FAS's Jaime Yassif reviewed the technology for cleaning up after a radiological attack and local and federal policies. Research and public debate is needed to correct these shortfalls before tragedy strikes.
Henry Kelly's March 6 2002 testimony Henry Kelly testified before the Senate Foreign Relations Committee on March 6, 2002 on the threat of radiological attack by terrorist groups. This excerpt is taken from the text of his written testimony, based on analysis by Michael Levi, Robert Nelson, and Jaime Yassif.
Michael A. Levi and Henry C. Kelly Weapons of Mass Disruption? Scientific American, November 2002, pp. 77-79.