Gamma radiation is the most difficult to shield against and, therefore, presents the biggest problem in the reactor plant. The penetrating power of the gamma is due, in part, to the fact that it has no charge or mass. Therefore, it does not interact as frequently as do the other types of radiation per given material.

Gamma rays are attenuated by processes which are functions of atomic number and mass (that is they all involve interactions near the nucleus or interactions with the electrons around the nucleus). Gamma shielding is therefore more effectively performed by materials with high atomic mass number and high density. One such material is lead. Lead is dense and has about 82 electrons for each nucleus. Thus, a gamma would interact more times in passing through eight inches of lead then passing through the same thickness of a lighter material, such as water. As the gamma interacts with the shielding material, it loses energy and eventually disappears. Lead and lead alloys have been used to some extent in nuclear reactor shields and have an added advantage of ease of fabrication. Because of its low melting point, lead can be used only where the temperatures do not exceed its melting point.

Iron, although a medium weight element, also functions well as a gamma attenuator. For gamma rays with energies of 2 MeV, roughly the same mass of iron as of lead is required to remove a specific fraction of the radiation. At higher and lower energies, however, the mass-attenuation efficiency of lead is appreciably greater than that of iron. In many cases, the selection of iron is based on structural, temperature, and economic considerations.

Water is a poor material for shielding gamma rays; however, large amounts will serve to attenuate gamma radiation.

Concrete, as discussed previously, is also a good attenuator of gamma rays and is superior to water. This is mainly a result of the presence of moderately high mass number elements, such as calcium and silicon. As a general shield material, there is much to recommend about concrete; it is strong, inexpensive, and adaptable to both block and monolithic types of construction.

Alpha and Beta Radiation

Alpha particles, being the largest particles of radiation and having a +2 charge, interact with matter more readily than other types of radiation. Each interaction results in a loss of energy. This is why the alpha has the shortest range of all the types of radiation. Alpha particles generally are stopped by a thin sheet of paper. As a comparison, a 4 MeV alpha particle will travel about I inch in air, whereas a 4 MeV beta particle will travel about 630 inches in air. Because it deposits all of its energy in a very small area, the alpha particle travels only a short distance.

The beta particle is more penetrating than the alpha. However, because of the -I charge, the beta particle interacts more readily than a non-charged particle. For this reason, it is less penetrating than uncharged types of radiation such as the gamma or neutron. The beta particle can generally be stopped by a sheet of aluminum. Because the beta travels farther than the alpha, it deposits its energy over a greater area and is, therefore, less harmful than the alpha if taken internally. All materials described under neutron and gamma radiation are also effective at attenuating beta radiation.

Since alpha and beta particles can be easily shielded against, they do not present a major problem in the nuclear reactor plant.

Summary

The important information in this chapter is summarized below.

Shielding Materials Summary

Neutron Radiation

Low mass number and high cross section (preferably hydrogenous material) for low energies. Water ranks high due to advantage of low cost, ready means for removing heat.

Good inelastic scattering properties (high energies). Iron is used due to the large change in neutron energy after collision but it has little effect on lower energy neutrons.

Gamma Radiation

High atomic mass number and high density are required to attenuate y radiation. Lead has advantage of ease of fabrication. The disadvantage of lead is its low melting point. Iron is used for higher and lower energies. Iron is selected based on structural, temperature, and economic considerations. Water can be used but requires large amounts because water is a poor absorber of gamma radiation. Concrete is a good gamma attenuator as a general shield material. Concrete is strong, inexpensive, and adaptable to different types of construction.

Alpha and Beta Radiation

No particular shielding material is required to guard against alphas and betas.