U.S. NRC Backgrounder: Dirty Bombs
"A dirty bomb is one type of a radiological dispersal device (RDD) that combines a conventional explosive, such as dynamite, with radioactive material. The terms dirty bomb and RDD are often used interchangeably in the media. Most RDDs would not release enough radiation to kill people or cause severe illness – the conventional explosive itself would be more harmful to individuals than the radioactive material. However, depending on the scenario, an RDD explosion could create fear and panic, contaminate property, and require potentially costly cleanup. Making prompt, accurate information available to the public could prevent the panic sought by terrorists. A dirty bomb is in no way similar to a nuclear weapon or nuclear bomb. A nuclear bomb creates an explosion that is millions of times more powerful than that of a dirty bomb. The cloud of radiation from a nuclear bomb could spread tens to hundreds of square miles, whereas a dirty bombs radiation could be dispersed within a few blocks or miles of the explosion. A dirty bomb is not a Weapon of Mass Destruction but a Weapon of Mass Disruption, where contamination and anxiety are the terrorists major objectives."
link
or
.
Dirty Bombs and Primitive Nuclear Weapons
"This publication examines two types of possible nuclear terrorist attacks: the making and detonating of a dirty bomb, and the fabrication and use of a primitive nuclear weapon. The author provides information about dirty bombs, describes a primitive nuclear explosive that could be constructed by a terrorist group, and discusses some international safeguard measures to counter nuclear terrorism. The paper underlines the need to develop effective and peaceful ways of preventing primitive nuclear attacks."
link
or look at:
Nuclear Terrorism: U.S. Perspective
"Al Qaeda-linked terrorists have explicitly announced their intent to acquire and use nuclear weapons against the United States. As early as 1998, Osama bin Laden issued a statement, 'The Nuclear Bomb of Islam,' in which he declared it the 'duty of Muslims to prepare as much force as possible to terrorize the enemies of God.' In August 2001 bin Laden asked two former Pakistani nuclear officials for help finding other Pakistani nuclear scientists to build a nuclear weapon. After the U.S. overthrew the Taliban, U.S. forces found documentation, including crude bomb designs, at an al Qaeda safe house in Kabul. Since 1992, al Qaeda representatives have been trying to buy highly enriched uranium, and have actively recruited technicians with at least some nuclear training, such as Adnan El Shukrjumah, for whose capture the FBI is offering a $5 USD million reward. There are three basic ways terrorists could stage a nuclear attack on the U.S: by stealing or buying a nuclear weapon; by buying or stealing weapons grade U-235 or plutonium and assembling a device; by obtaining quantities of any of a variety of radioactive isotopes and assembling them into a non-nuclear 'dirty bomb.' A successful attack on a civilian nuclear power reactor could replicate the effect of a dirty bomb."
link
a few from the same site HSDL