What Determines Whether Radiation Passes Through Materials?

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I've read that nuclear waste is stored in Olympic-sized swimming pools, but what is it about water that so effectively stops radiation? And apparently, this shielding effect of water is so powerful that one could literally stand right next to the pool (and according to some things I've read, they could even accidentally fall in) without suffering any ill effects whatsoever. But why? Water, chemically is h2o of course, but what is it about this compound that would be able to block such powerful rays that are capable of traveling through solid objects? Because X-rays (which are also emitted by nuclear waste if I'm not mistaken) are capable of traveling through the body and even through cargo in airports. These objects are solid, yet water is a liquid and stops them. Also, why is it that visible light is able to extend through water, but not the higher spectrum stuff (e.g. x rays and gamma rays)? Because one can see the waste at the bottom of the pool clearly, yet the radiation cannot reach the top of the pool in any substantial quantities?

Also, I'm aware that ozone (in trace amounts in the upper atomsphere) blocks a large amount of high frequency radiation (UV light) despite being present in the thin levels of the stratosphere. What is it about ozone that has protective qualities? And in general, what determines if a chemical is likely to block high frequency radiation or if it is likely to pass through it?
 

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  • #2
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The human body absorbs some part of the x-rays, that's how the x-ray images are created. Human bodies filling the same volume would give about the same shielding properties (especially as humans are mainly water anyway) but they can't be used in nuclear power plants for obvious reasons.
Concrete would give similar shielding, steel and lead would give better shielding. Mercury would give better shielding. But all these materials would make handling the material more difficult. Water is cheap, it is not toxic and it makes handling the material inside easy.

> Also, why is it that visible light is able to extend through water, but not the higher spectrum stuff (e.g. x rays and gamma rays)?

There are no excitations of water molecules with transition energies in the energy range of visible light, and visible light doesn't have the energy to ionize water molecules either.

> And in general, what determines if a chemical is likely to block high frequency radiation or if it is likely to pass through it?

Available transitions between energy levels and the ability to ionize the molecule.
 
  • #3
I've read that nuclear waste is stored in Olympic-sized swimming pools, but what is it about water that so effectively stops radiation?
It is not particularly efficient. You need about 10 cm layer of water of reduce gamma radiation by half. To the first approximation, gamma shielding is proportional to mass. Denser materials have more mass per unit of thickness. Thus, denser materials are "more efficient" if efficiency is measured by thickness.

Water shielding in spent fuel pools is achieved by having sufficient thickness of water. 1 meter gives you about ~1000 fold attenuation. 2 meters - one million. 3 meters - one billion. IIRC fuel pools have ~7 meters of water above fuel assemblies. That gives attenuation of ~10^21.
 
  • #4
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That gives attenuation of ~10^21.
Be careful with extrapolations. The attenuation is not completely independent of the energy, and the energies with the weakest attenuation will dominate after a meter. Just continuing the exponential approach will massively overestimate the attenuation.
 
  • #5
Be careful with extrapolations. The attenuation is not completely independent of the energy
Right. It's actually very much energy-dependent. IIRC the worst are ~2-5MeV gammas.
 
  • #6
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it is cheap and free
 

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