Patent Number: 045171521
Section: summary

BRIEF SUMMARY OF THE INVENTION This invention relates to an ultrasonic method for testing fuel element tubes for defects, such tubes being assembled in bundles to form a complete reactor core, using ultrasonic transducers. One transducer, forming the transmit transducer, transmits ultrasonic search pulses into the tube or sheath of the fuel element while an opposite transducer, the receive transducer, receives ultrasonic signals from the tube. The received signals, if falling within a predetermined time range, i.e. a time gate, are evaluated by an evaluation means which includes an amplitude discriminator. A method of this type is known, having been described for example in an article by G. Baro et al entitled "Ortung defekter Brennstabe in bestrahlten Brennelementen", Tagungsbericht der Jahrestagung Kerntechnik 80, Reaktortagung 1980, Berlin, 25-27 March 1980; published by Deutsches Atomforum e.V. Bonn, pp. 827 et seq. The purpose of the time gate, although the latter is not described in detail in that article, is to separate the ultrasonic signals which reach the receive transducer from the tube, known as the revolving or circumferential echo, from the signal transmitted by the transmit transducer and passing directly to the receive transducer, the transmitted signal. Only by this feature is it possible to determine automatically the transit time of the revolving echo signal without obtaining a misreading due to the interference of the transmitted signal. In the methods known heretofore a constant time gate setting has been used. This arrangement, however, has been proven disadvantageous since the distance between the two transducers varies greatly as the transducers pass through the array of tubes. Therefore, it is possible that transmitted signals may also arrive within the gated time interval and, hence, defective fuel tubes may become classified as being defect-free. From German OS No. 24 22 439 it is known to adjust the time gate when the thickness of the workpiece changes. In that described arrangement a change of the workpiece thickness is indicated by a reference signal responsive to the occurrence of the workpiece rear wall echo signal. Utilizing that method, that is, periodically monitoring the thickness of the workpiece and adjusting the time gate accordingly, is possible only with the greatest circuit complexity when testing fuel tubes. Moreover, in such an arrangement changes arising from the spacing between the transducers, i.e. those not caused by a change in thickness of the tubes, are not taken into consideration. The present invention discloses a simplified method for changing the setting of the time gate in response to the spacing between the transmit transducer and receive transducer as both transducers pass through the spaces between the tubes. To this end, the time gate setting is redetermined for each tube to be tested prior to such tube becoming disposed between the transmit transducer and the receive transducer. Accordingly, in the space between the last tested tube and the next to be measured tube the transit time of the echo signal between the transmit transducer and receive transducer is measured. Thereafter, a constant value is subtracted from the transit time, such constant value being selected so that the transmitted signal indication obtained during the transit time measurement no longer falls within the gated time interval. Other particulars and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings.