Development of magnetostrictive and piezoelectric high-temperature-resistant sensors - A condition for reliable reactor diagnosis

Acoustic measuring methods are most suitable for monitoring nuclear power stations. Boiling and cavitation, leakages at steam generators and at heat exchangers as well as rotating elements such as pumps, can be monitored and loose parts can be detected. To meet these requirements, high-temperature-resistant, medium-resistant and radiation-resistant sensors are needed. These requirements are particularly important if the acoustic source is to be localized and if the measurement directly occurs in the medium. The requirements for the measurement apply to the transmitter properties and to a sufficiently high sensitivity. Sensors are described which are based on two different physical methods. In the first case the magnetostrictive sensor is to be applied at very high temperatures and is most reliable owing to its simple design. Moreover, the susceptibility of the magnetostrictive measuring system to electromagnetic interferences is very low. The second case concerns the piezoelectric method, for which sensors have been developed which use high-temperature-resistant piezocrystals equipped with an oxygen barrier, so that they can be applied in a nuclear power station without requiring additional equipment. The advantages of the piezoelectric sensors are the broad transmission characteristic and an excellent signal-to-noise ratio. Besides the development problems and a description of the sensors, the measuring methods for determining the characteristic parameters of the sensors are described. The technical properties of the magnetostrictive and piezoelectric sensors are compared and the results regarding the response behaviour under temperature load are assessed. (author)