Issues for Nuclear Power Plants Steam Generators

Abstract

Steam Generators (SGs) are one of the big metal components in nuclear power plants. The function of Steam Generators is the removal of the thermal power generated into reactor core and its transfer to the secondary side of the tubes where secondary coolant flows. The preservation of the complete separation between the primary and secondary fluid is of capital importance in order to avoid radioactive contamination of secondary fluid and small loss of coolant also. Mainly two designs have been developed for nuclear thermal power plants: vertical U-tubes with upstream and downstream flow for primary water, and horizontal steam generators. Also once through design can be found, but vertical U-tubes steam generators prevail. A brief focus on operating life, of inspections and mainteinance, and of water conditions is presented. Water conditions both at the primary and secondary side of the steam generator are of primary importance. During operating life, cooling radioactive contamination occurs, but water conditions must be mantained inside specific ranges. A typical PWR isotopic composition of contamination inside primary circuit components is presented. To avoid unwanted degradation of water conditions and of thermal exchange, specific ispections and interventions are applied. Problems associated with Steam Generators in nuclear power plants are tube denting, wastage, thinning, corrosion, flow-induced vibrations, cracking and deformation of U-tube bend, or of support plates, tube leakage, fractures. We consider the case of a leakage, a rupture which would compromise the integrity of separation between radioactive and notradioactive fluid, and the main symptoms and interventions. The periodical inspections of SG have shown localized corrosion and mechanical wear problems on some SG tubes and degradation of SG lifetime (Bezdkian 2009). A replacement management program is necessary to achieve 40 years lifetime. According to Wade (1995) in the United States 35 steam generators had been replaced in 12 plants and a list of 23 units are identified as potential candidates for steam generators replacement or shutdown. When approaching end of life, decontamination and decommissioning of Steam Generators must be planned. Decommissioning is the end of life of a facility. It implies many issues: strategic, technological and scientific, measurement, environmental, legislation, and economic issues.