Thermal shields:

Iron, as carbon steel or stainless steel, has been generally used as the material for thermal shields. Like shields could absorb a considerable proportion of the energy of fast neutrons and gamma rays escaping from the reactor core. It is possible to utilize the properties of both of these materials through making shields composed of iron and water. PWRs utilize two or three layers of steel along with water among them as an extremely effective shield for neutrons and gamma both rays. The interaction  (inelastic  scattering)  of high energy  neutrons  occurs commonly along  with  iron, that degrades  the neutron to a much lower energy, whereas the water is more effectual for slowing down  (elastic  scattering)  neutrons. At one the neutron is slowed down to thermal energy,  it diffuses by the shield medium for a small distance and is captured through the shielding material, resulting in a neutron-gamma (n, γ) reaction.  Those gamma rays represent a secondary source of radiation.

Iron  punchings  or turnings and iron oxide have been incorporated into heavy concrete  for shielding purposes also. Existing along with seven weights percent or greater of water appears to be adequate for neutron attenuation. Therefore, a rise in the water content has the drawback of decreasing both the density and structural strength of ordinary concrete. Along heavy concretes, a produce amount of attenuation of both neutrons and gamma rays could be achieved through means of a thinner shield than is possible with ordinary concrete. Several categories of heavy concretes used for shielding involve barytes concrete, iron concrete, and ferrophosphorus concrete along with several modified   concretes and associated   mixture. Boron  compounds   (for  instance,  the  mineral colemanite)  have also  been added to concretes  to increase  the possibility  of neutron  capture without high-energy gamma-ray production.