The area of damage of a dirty bomb is inherently limited to the spread of the bomb.The radioactive damage is not as much as you would think.
If you go through various events during the development of the atom bomb people were handling the seperated cores and there are only a couple of incidents when the cores were united manually by mistake and separated causing a partial "critical event".
Most of the effects are on the thyroid and that is attenuated by giving OralIodine immediately to the vitim.
http://www.nrc.gov/reading-rm/doc-coll ... bombs.html
Also
https://en.wikipedia.org/wiki/Dirty_bomb
http://www.forbes.com/sites/stratfor/2 ... f2e8e6134a
Radiological Dispersal Devices
A dirty bomb is a type of radiological dispersal device (RDD), and RDDs are, as the name implies, devices that disperse a radiological isotope. Depending on the motives of those planning the attack, an RDD could be a low-key weapon that surreptitiously releases aerosolized radioactive material, one that dumps out a finely powdered radioactive material or something that dissolves a radioactive material in water. Such methods are intended to slowly expose as many people as possible to the radiation for as long as possible without becoming detected. Unless large amounts of a very strong radioactive material are used, however, the effects of such exposure are limited. To cause adverse effects, radiation exposure must occur either in a very high dose over a short period of time or in smaller doses sustained over a longer period. This is not to say that radiation is not dangerous, but only that small amounts of radiation exposure do not necessarily cause measurable harm. In fact, people are commonly exposed to heightened levels of radiation during activities such as air travel and mountain climbing.
By their very nature, RDDs are prone to be ineffective. To maximize the harmful effects of radiation, victims must be exposed to the highest possible concentration of a radioisotope. But by definition and design, RDDs dilute the radiation source, spreading smaller amounts of the substance over a larger area. Additionally, the use of an explosion to spread the radioisotope alerts the intended victims, who can then evacuate the affected area and be decontaminated. These factors make it very difficult for an attacker to administer a deadly dose of radiation through a dirty bomb.
It is important to note that a dirty bomb is not a nuclear device, and no nuclear reaction occurs. A dirty bomb will not produce an effect like the nuclear devices dropped on Hiroshima or Nagasaki. A dirty bomb is quite simply an RDD that uses explosives to disperse a radioactive isotope; the only blast effect or damage produced is from conventional explosives and not from the radioactive material itself. In a dirty bomb attack, radioactive material is spread in an obvious manner, causing mass panic. In other words, the RDD is a weapon intended to create fear and terror.
The radioisotopes that can be used to construct an RDD are fairly common. Those materials considered most likely to be used in an RDD, such as cobalt-60 and cesium-137, have legitimate medical, commercial and industrial uses. Organizations such as the International Atomic Energy Agency warn that such radioisotopes are readily available to virtually any country in the world, and they are almost certainly not beyond the reach of even moderately capable non-state actors. Indeed, given the ease of obtaining radiological isotopes and the simplicity of constructing a dirty bomb, it is surprising that we have not yet seen one successfully used in a terror attack, especially considering jihadist groups in Iraq, Syria and Libya have captured cities that likely contain radioactive sources. In light of this, let’s examine what effectively employing a dirty bomb would entail.
Creating An Effective Dirty Bomb
Like nonexplosive RDDs, unless a dirty bomb contains a large amount of very strong radioactive material, the radiological effects of the device are not likely to be immediate or dramatic. In fact, the explosive effect of the RDD is likely to kill more people than the device’s radiological effect. Moreover, the need for a large quantity of a radioisotope not only creates challenges for obtaining the material but also means the resulting device would be large and unwieldy — and therefore difficult to smuggle into a target such as a subway or stadium.
In practical terms, a dirty bomb can produce a wide range of effects depending on the size of the improvised explosive device (IED) and the amount and type of radioactive material involved. (Powdered radioisotopes are easier to disperse than materials in solid form.) Environmental factors such as terrain, weather conditions and population density also play an important role in determining the effects of such a device.
Significantly, while the radiological effects of a dirty bomb may not be instantly lethal, the radiological impact of an RDD would likely affect an area larger than the kill radius of the IED itself and persist far longer. The explosion from a conventional IED is over in an instant, but radiation released by an RDD can remain for decades unless the area is decontaminated. While the radiation level may not be strong enough to affect people exposed briefly during the initial explosion, the cumulative effects of such radiation could prove very hazardous. Again, the area contaminated and the ease of decontamination depends on the type and quantity of the radioactive material used. Materials in a fine powdered form are easier to disperse and harder to clean up than solid blocks of material. In any case, it would be necessary to evacuate people from the contaminated area, and people would need to stay out of the area until it could be decontaminated, a process that could prove inconvenient and expensive.
Though dirty bombs are not truly weapons of mass destruction like nuclear devices are, they are frequently referred to as “weapons of mass disruption” or “weapons of mass dislocation” because they can temporarily render areas uninhabitable. The expense of decontaminating a large, densely populated area, such as a section of London or Washington, would be quite high. This cost also makes a dirty bomb a type of economic weapon.
