The lifetime extension of ageing nuclear power plants for electricity production is an economical way to reduce electricity generating costs for the benefit of the customers. Extending lifetime of existing installations requires the development of innovative reliable techniques for the inspection of critical components. Such techniques will detect changes in the materials and will allow to plan actions foe failure prevention. One of the components under consideration is the Reactor Pressure Vessel (RPV). For the characterisation of RPV-steels the innovative Magnetoinductive Harmonic Analysis (MHA) is being used. This indirect test method is based on the relation between magnetic and mechanical properties of ferromagnetic materials. For the MHA is of particular interest the shape of the magnetic hysteresis loop and its variation due to change in material properties which is essential for degradation studies. The shape of the hysteresis loop is obtained by the Fourier Analysis of higher harmonics from the measuring signal which, in turn, is generated by a sinusoidal magnetic field from a driving coil and transmitted into the steel under investigation. Due to the fact that the base material of RPV's is ferritic steel the MHA is well suited to characterise these types of steel. Experiments have been performed primarily with JRQ-steel (ASTM A-533-BCl. 1) because of its international use for analysis and comparison of results. JRQ-samples were investigated in the as-received-state as well as after thermal ageing in order to detect changes in the microstructure by MHA. Encircling coils as well as surface coils have been used for the analysis. The characteristic magnetic measured values of MHA (amplitude and phase of the harmonics) were correlated with hardness and upper-shelf-energy values found from destructive tests. There is a very good correlation between the magnetic and mechanical values and samples of different microstructure are well distinguished. It is al so shown that MHA is capable to penetrate through austenitic cladding material of 6 or more mm thickness. Furthermore MHA is a robust technique and easy applicable at components and due to its electromagnetic nature results are obtained in a couple of seconds.
Publication Source: 2nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, May 24-26, 2000, New Orleans, Louisiana USA. Publisher: EPRI - [Homepage]
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