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07 November 2009

Answer to Question #3905 Submitted to "Ask the Experts"

Category: Alpha Emitters — Uranium

The following question was answered by an expert in the appropriate field:

In movies, when characters are around nuclear reactor rods on submarines, nondetonated nuclear bombs, etc. they are killed through radiation exposure. If uranium, the reactor fuel, is not dangerously radioactive by itself, how is it made that way? Is there some other factor or radioactive compound that makes the objects dangerous to be around, or is it just another example of dramatic license in Hollywood?

You're correct that uranium, itself, is not terribly radioactive. However, when uranium fissions, the atoms are split into fission fragments, and these fission fragments (also called fission products) are highly radioactive. So the radiation exposure comes from the fission fragments, not from the uranium itself.

In addition, nuclear fission causes neutrons to be emitted and, when neutrons are absorbed by other atoms, they can cause these atoms to become radioactive. As one example, non-radioactive cobalt (cobalt-59) absorbs a neutron to become radioactive cobalt (cobalt-60). So the neutron activation products can also contribute radioactivity and radiation dose to the movie character.

Finally, the process of fission also produces high levels of radioactivity because, in addition to producing neutrons and fission products, gamma rays are emitted during the fission process. This means that, if the poor actor is playing with the fuel rods while the reactor is operating, he or she will also be affected by the radiation from the fission process itself.

All of this means that a lump of uranium sitting there as a nuclear weapon or as new reactor fuel is not going to produce a lot of radiation. So reactor fuel might be very radioactive, but the nuclear weapon will not be. Also, you have to bear in mind that nuclear reactors are designed so that nobody is going to be able to reach in and pull out a fuel rod - you have to go to a LOT of trouble to gain access to the fuel, and this is not going to happen without a lot of people knowing and helping out. So these scenarios are pretty unrealistic.

So...the bottom line is that there can be danger from reactor fuel rods, not much danger from a nuclear weapon (unless it explodes or falls on you), but that it's quite difficult to find yourself within arm's reach of either of these. Which means that there is also (as you suspected) a fair amount of dramatic license as well.

Andrew Karam, CHP, PhD

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