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- Axial cracks (1)
- Cellular polypropylene (1)
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Steam generator (SG) tubes are part of the primary circuit of pressurised water reactors and therefore also part of the pressure retaining boundary components. Furthermore, steam generator tubes are the components which guarantee the separation between the primary and secondary circuits. Therefore the knowledge of the loss of tube integrity as soon as possible is an essential. Optimised nondestructive testing methods based on the eddy current array technique are used to find material degradation, especially intergranular stress corrosion cracks during the periodical in-service inspection. These methods are supported by the measurement of the leakage rate during operation based on radioactivity and Tritium measurement regarding a high crack grows. Shutdown criteria depending on leakage rates which are described in the operation procedures but there are no specifications about the conditions for the measured leakage. Is the leakage coming from a single crack or are there multiple cracks or is this the leakage of a critical crack. With the help of FE calculations answer about the uncertainties of leakage rate versus critical crack grows were found. Shutdown criteria must guarantee to avoid critical crack sizes, i.e. between the measured tritium activity and the critical crack size must be a safety gap.
Electromechanical response of polypropylene ferroelectret transducers under application of high-voltage pulses was measured by laser Doppler vibrometry and compared with results of ultrasonic through-air transmission between two ferroelectret transducers. The electromechanical response was completely explained by piezoelectric and electrostrictive effects. The electrostrictive effect dominates at high voltages and provides significant enlargement of the transducer constant, up to factor of 2.5. The induced strain of 1.7% was achieved at – 2000 V. The nonlinear ultrasonic transmission was shown to be well described by the piezoelectric and electrostrictive response of transmitter, except in the range of high negative exciting voltages where some limitation of the transmitted signal was observed. This limitation seems not to be a fundamental one and does not abolish the advantages of high-voltage excitation of polypropylene ferroelectret transducers.