Final answer:
Secondary radiation barriers typically use high-density materials like a steel shell or high-density concrete, ranging from 3-20 centimeters for the steel to 1-3 meters for concrete. Lead is also used for its efficiency in thinner layers, with the exact thickness depending on the specific radioactive source and desired level of protection.
Step-by-step explanation:
Secondary radiation barriers are designed to protect against gamma rays and X-rays which are very penetrating and can pass through a thick layer of most materials. The typical thickness for secondary radiation barriers involves the use of high-density materials such as a steel shell around the reactor vessel that can be anywhere from 3-20 centimeters thick, effectively absorbing much of the radiation produced. Additional shielding can include a main shield composed of 1-3 meters of high-density concrete, and a personnel shield of lighter materials for further protection. Lead, being a dense, high atomic number element, is particularly efficient at attenuating gamma radiation and can be used in thinner layers compared to other materials. The exact thickness required for a lead shield would depend on the radiation source, but as an example, one half of the gamma rays from Technetium-99m are absorbed by a 0.170-mm-thick lead shielding. To protect operators fully and absorb all but one in 1000 of these gamma rays, multiple layers of this thickness would be needed.