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Materials and components safety
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The structural integrity of aging nuclear reactor pressure vessels is assessed by fracture mechanical analyses. A key task is to assure that a postulated or detected flaw would not grow larger on the application of the load. Beside flaw size and stress (loading conditions) the material's fracture toughness and other fracture mechanical properties need to be known. These material properties are quantified by destructive and non-destructive testing in static and cyclic loading.
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The aim of investigating irradiation induced defects in construction materials is to identify and clarify the mechanisms which lead to decisive modifications of material properties under given operation conditions of an equipment. To explore these mechanism small angle neutron scattering and other experimental methods are applied in combination with modelling and simulation techniques. One research emphasis lies on the irradiation induced embrittlement of pressure vessel steels in modern fusion or fission reactors.
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By means of coupled thermal and mechanical Finite-Element-Simulations the behaviour of the reactor pressure vessel of light water reactors during an assumed core melt down accident is investigated. The numerical models are validated against medium scaled experiments. The aim is the evaluation of failure modes and times and their impact to the further accident progression or an assessment of possible safety margins against failure. What are possible conclusions for severe accident management measures?
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This page describes the analytical work done by modelling and evaluating a thermal shock in a VVER-440 reactor pressure vessel due to an emergency case. An axial oriented semi-elliptical underclad/surface crack is assumed to be located in the core weld line. Three-dimensional finite element models are used to compute the global transient temperature and stress-strain fields. By using a three-dimensional submodel, which includes the crack, the local crack stress-strain field is obtained. The determination of the stress intensity factor, which is a measure for the loading of the crack, follows the VERLIFE code. With a subsequent postprocessing using the J-integral technique the stress intensity factors along the crack front are obtained. The results for the underclad and surface crack are provided and compared, together with a critical discussion of the VERLIFE code.
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The small punch test is a miniaturized testing method, using small disk shaped specimen to determine mechanical material properties. It is comparable with a deep drawing procedure, where the specimen is deformend until failure. Due to the small amount of material needed, specimen can be taken out without a significant damage of the structure. The application of this method regards to problems, where material aging, embrittlement as well as mechanical, thermal or chemical operating conditions cause a changing of the mechanical material behaviour.
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