In nuclear reactors, shielding against neutron and gamma radiation is essential. When planning the shielding for neutron radiation, it is important to take into con­sideration that the cross section for the fast neutron is rather small (see Figure 6.4). So, for shielding material, those substances should be chosen that efficiently slow down the neutrons and then efficiently absorb the thermal neutrons. The same mod­erators discussed in Section 7.1.1.4 are suitable for this purpose; therefore, the active zone of the reactors is usually surrounded by a mantle consisting of the mod­erator (water, graphite, etc.).

In nuclear reactors, the gamma radiation is very intense. The intensity of the gamma radiation continuously increases because of the increase of the quantity of the fission products. For protection against the gamma radiation, substances with a high atomic number and density are suitable. The stainless steel reactor vessel itself provides some shielding against gamma radiation. The nuclear reactors are sur­rounded by a thick concrete wall.