Assessment of actual loading conditions of steam generators and heat exchangers according to probabilistic criteria

Abstract

Classical design codes are based on nominal conditions, e.g. pressure, temperature, material strength and tube diameter for tubes under pressure; local loads, hot spots, deviations in strength and geometry, etc. are compensated by the use of safety factors. More recent methods — in particular those for the assessment of steam generators and heat exchangers for nuclear applications — call for designs to meet actual loading conditions. Thus, detailed investigations are required to determine realistic operational characteristics of the components to be assessed. However, a worst case treatment on the basis of the above criteria would yield an over-conservative design which in special cases might even be worse than the classical one, e.g. in the case of heated tubes due to increased thermal stresses. As a consequence, with all the data or the spectra of data available, the assessment has to be executed according to probabilistic criteria resulting in a design with an acceptably low and, above all, known failure rate adjusted to fit into the overall plant concept. In components to which the above criteria have been applied before — mainly pressure vessels — the design variables pressure and temperature are generally considered uncorrelated and the temperature is in most cases assumed to be constant. For heat exchangers and steam generators these simplications are not acceptable on principle. On the contrary, even deviations in geometry (like tube tolerances) result in flow rate deviations with corresponding deviations in temperature and pressure. Thus a new procedure for assessment according to actual loading conditions, e.g. by the application of probabilistic criteria, is proposed. This procedure can be used for heat exchangers and steam generators. With this new method, components of nuclear power plants like steam generators and heat exchangers may be designed to meet the low failure probability required for satisfying the overall reliability concept of the plant.