Answer to Question #4548 Submitted to "Ask the Experts"

Time, distance, and shielding are all still very relevant to radiation protection. However, the most effective and relevant of the three depends entirely on the situation causing the exposure.  
"Duck and cover" (along with designated bomb shelters, etc.) [i.e., shielding] was the radiation protection strategy of the civil defense approach in the '50s and '60s when the greatest concern for radiation protection of the public was from a detonation of a nuclear bomb delivered by an enemy country. This approach was based on a scenario of a well-designed nuclear weapon with a large yield exploding over a urban or strategic city with a few (10-20) minutes warning. The "duck and cover" approach was appropriate because the first concern for protection for this scenario was protection from the physical blast and then from the immediate radiation dose  from the detonation (hence "duck and cover" under the structural material of your desk or in the school halls with thick walls away from glass windows). The nuclear weapon blast would have been so rapid and immense there was no way to run from it (distance) or to reduce the time you would be exposed to the short-lived immediate radiation dose (time) from the blast. The follow-up action of "hunkering down" in a bomb shelter that was prestocked with food and water for the ensuing days was based on the belief there would be wide-spread contamination with unusable transportation means or routes. So, it continued to be "shelter" from the outside contamination because it would take a long time to get any distance between you and the contamination, not to mention an uncertainty of vital supplies like food and water.
 
With the passing of the threat of a well-designed nuclear weapon delivered by an organized country in the '50s and '60s, in 1979 Three Mile Island response focused the need for radiation protection for the public on the possibility of an accident in a nuclear power plant. In this case, the most likely scenario involved a series of miscues in a nuclear plant that would take hours, if not days or a week, to develop into a "meltdown" with a release of a radioactive cloud that could last for days or weeks. (Of course, the Three Mile Island accident never developed into this, but it put the focus on the possibility). For this scenario, the radiation protection strategy became to evacuate the area where the cloud would pass (i.e., time and distance). Evacuation was the first protective action of choice, with sheltering the next protective action if it was shown that it was more protective than evacuation, because there would most likely be time to evacuate before the radioactive cloud started or because it would last long enough that getting out of the cloud would reduce the dose more than staying sheltered in the cloud. (When there is a persistent cloud of radioactive material, a home or building loses its "sheltering effectiveness" due to the air exchange with outside air that exists in every home or building.)
 
Now, with the latest threat of radiological terrorism, there are two general scenarios of concern. One is that a "homemade" nuclear explosive device, called an Improvised Nuclear Device or IND, would be exploded without warning in an urban area, and the second is the detonation of a "dirty bomb." 
 
For an IND, the concern again is to protect the public from the blast and the immediate radiation dose, or to "duck and cover." The problem is there may be no warning so the opportunity to shelter before the blast may be lost. After the blast, like in the case of a nuclear weapon, the next concern is the contamination from the fallout of the bomb. However, in the case of an IND it is not expected the area will be nearly as widespread and it is expected emergency services and responders will be available to help, so the protective action is to stay sheltered until emergency responders/managers give directions on evacuation from the area.
 
For a "dirty bomb," that is, a high-explosive device (bomb) laced with radioactive material, the immediate danger is the bomb blast itself. Then the radiation protection approach is to get out of, or stay out of, the area where the radioactive material has been spread by the bomb. Although there have been many different "scenarios" reported on how far and wide radioactive material can be spread with a dirty bomb, the most likely expectation is that it will be a relatively small area. That is, about the area covered by the bomb fragments. So, for someone in the area of the blast, the best action is to get inside to be out of the contaminated area until emergency responders/managers direct an orderly evacuation that reduces or eliminates the spreading of the contamination beyond its original area.
 
In summary, the same principles of radiation protection—time, distance, and shielding—apply now as before but the most effective one depends on the manner of exposure.
  
Keith H. Dinger, CHP