Radiation Shielding
Criteria for the Selection of a Shield Material
Theoretically any material can be used for radiation shielding as long as it is thick enough to attenuate the radiation to safe limits.
The choice of the shield material is dependent upon many varied factors. All of these need to be considered:
The final desired attenuated radiation levels,
The ease of heat dissipation
Resistance to radiation damage,
Required thickness and weight,
Multiple use considerations (e.g., shield and/or structural),
Uniformity of shielding capability,
Permanence of shielding, and
Availability and cost.
Gamma Ray Shielding
Awith a higher atomic number is a better attenuator of high frequency electromagnetic radiation than a less dense one. Lead enjoys the advantage of being the densest of any commonly available material. Where space is at a premium and radiation protection is important, lead is often prescribed. It is recognized that lead is not the densest element (e.g., tantalum, tungsten, and thorium are higher on the density scale), but lead is readily available, easily fabricated and the lowest cost of these materials.
Less dense materials need to be thicker than a lead shield will be. A concrete shield will therefore be much thicker than a lead shield of the same attenuating property. Lead is cheaper but not easy to move around.
Neutron Shielding
Nuclear power plants (and nuclear-powered aircraft carriers and subs) all employ heavy shielding. Nuclear power plants generally have three layers of shielding in addition to the containment structure, which is made of concrete several feet thick and houses the reactor.
reinforced concrete as the shield.
If the shield material has a high rate of neutron capture (a big cross section for neutrons), it will in time become radioactive, sharply reducing its effectiveness as a shield material. See artificial transmutation.
Pure lead itself does not become highly radioactive under bombardment by neutrons. Therefore, lead shielding, even after long periods of neutron exposure, emits only insignificant amounts of radiation due to activation. But any impurities in the lead may cause it to absorb neutrons - undergo artificial transmutation and have radioactive particles within it as a result.
Lead has an advantage over various aggregate materials, such as concrete, being more uniform in density throughout. In addition, because commonly used forms of lead exhibit smooth surfaces, lead is less likely to become contaminated with dirt or other materials which, in turn, may become radioactive.
But lead alone is not suitable for radiation shielding in a nuclear power station. It is expensive and not suitable for large structure and does not withstand the high temperatures produced.
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