CORROSION THEORYDOE-HDBK-1015/1-93CorrosionCH-02Rev. 0Page 2Corrosion of the nuclear fuel cladding may cause the cladding to become brittle and lessductile. The swelling from the generation of fission gases within the fuel may then causethe cladding to crack or blister, and highly radioactive fission products may then bereleased to the coolant.Some of the metallic oxide corrosion products released to the coolant may be depositedon surfaces in the reactor core. The neutron flux in the core produces nuclear reactionsin the corrosion products, and they become highly radioactive. Subsequently, theseactivated corrosion products may be released from the core and redeposited on surfacesoutside the core region. The radiation fields resulting from this redeposited material maythen significantly increase radiation levels, thereby complicating maintenance and accesscapabilities. The corrosion product oxides may also result in fouling of heat transfersurfaces and in the accelerated wear of moving parts by corrosion products trapped inor between them.Currentis the flow of electrons through a medium. An electric current can flow through a metalconductor, and the metal will not show any obvious chemical changes. This type of conductionof electricity is called metallic conduction. Ionizationis the process of adding electrons to or removing electrons from atoms or molecules,creating ions. High temperatures, electrical discharges, and nuclear radiation can causeionization. Many metals have a tendency to lose electrons to atoms or ions that have a tendencyto gain electrons. Current can be conducted by the movement of these ions. The compoundsthat conduct electric current by ion movement are called electrolytes, and this ionic motion is callelectrolytic conduction. Conductivity is a measure of the ability of a substance to allow electronflow. In the context of corrosion, conductivity indicates the amount of ions in solution, whichrelates directly to the potential of corrosion taking place.Corrosionis the deterioration of a material due to interaction with its environment. Corrosioncan have many forms, both wet and dry. Electrolysis is the decomposition by electric current(in the context of corrosion the use of electrical current to bring about chemical change). Thischapter will concentrate on the corrosion of metals in water-based environments (electrolytes).An electrolyte is defined as an electricity-conducting fluid; that is, it has positive and negativeions that can move and constitute an electrical current. Pure water has a limited number ofdissociated H and OH ions and is a relatively poor conductor of electricity. Addition of acids,+ -bases, or salts that dissociate into ions increases the current-carrying capability of the water(electrolyte).
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