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biological shield[¦bī·ə¦läj·ə·kəl ′shēld]
in nuclear power engineering, a complex of structures and materials surrounding a nuclear reactor and its units, the purpose of which is to reduce radioactive emissions to a biologically safe level. A biological shield is designed to absorb neutron and gamma radiation. Water, concrete, graphite, and other materials are used to lessen neutron radiation, and lead and steel to lessen gamma radiation. Since secondary (capture) gamma rays arise during neutron absorption, the materials used in a biological shield are arranged in a definite order: materials with light elements are closest to the radiation source, followed by those with heavy elements. If there are no limitations on the mass and size of a biological shield, only one kind of material is used—that is, the most convenient and cheapest (usually concrete or water).
IU. I. KORIAKIN