Method of detecting defective fuel pins with the aid of ultrasonics

The invention relates to a method for identifying defective fuel rod cladding combined within to complete fuel fuel assemblies, in which between the fuel rod ultrasonic probes therethrough are moved so that the respective cladding tube is placed between two ultrasonic probes for testing, said a probe (transmitting test head) sonicated the cladding tube and the other probe (Empfangsprüfkopf) receives the entering of the jacket tube ultrasonic signals, and in which subsequently in an evaluating those signals are evaluated by means of an amplitude discriminator which fall into a temporal expectation range of predetermined width.

A method of the type mentioned above is, for example, from the article by G. Bäro include "locating defective fuel rods in irradiated fuel" s, Proceedings of the Annual Meeting on Nuclear Technology 8₀, Reactor Meeting 1980 Berlin, 25.-27.03.1980, Published by German Atomic Forum Bonn, S. 827 ff known. is, however, with the help of temporal Erwartungsbereichesder in the aforementioned items not described in detail - to the entering of the jacket tube in the Empfangsprüfkopf ultrasonic signals (circulation Echos) by entering directly from the transmitting test head in the Empfangsprüfkopf signals (sound transmission echoes) are separated. Only by this measure it is possible to automatically determine the terms of circulation echoes without distortion effected by the ultrasonic transmission echoes.

In the method used to date, however, was working with a constant expectation range. This has proved to be disadvantageous because the distance between the probes during their movement to the individual sheaths can pass relatively strong fluctuations. Thus it is not ruled out that sound transmission echoes falling into the aperture and defective fuel rods are classified as error-free per se.

From DE-A-24 22 43g, it is already known to track the expected range for flaw echoes when changing the thickness of the test piece. A change in the thickness of the test piece is indicated in this known method by constant control of the light reflected from the rear wall of the test echoes. A transfer of this known method, namely to control the thickness of the test piece from time to time and readjust the expected range accordingly when testing of cladding tubes would be possible only with extremely great effort. Furthermore, would such a method all those changes of the mutual distance of transmitting and Empfangsprüfkopf that are not due to changes in thickness of the cladding tubes are not included.

The present invention is therefore based on the object of specifying a method of the aforementioned type, wherein the expected range is tracked according to the change in distance between the transmitting and Empfangsprüfkopf during movement of these probes through the streets of the fuel assembly in a simple manner.

This object is achieved in that the beginning of the expected range is recalculated for each cladding tube to be tested before this cladding tube is between the transmitting and Empfangsprüfkopf. For this purpose, the term of the ultrasonic signals between transmitting and Empfangsprüfkopf measured in the gap between the last audited cladding tube and the new test duct. a constant value is then subsequently by this measured transit time value subtracted, which is selected such that the received at the time of flight measurements Ultraschallsingal not fall straight in the expected range.

Further details and advantages of the invention will become apparent from the hereinafter described with reference to figures embodiments.

Show it:<ul><li>Figure 1 is a plan view of a section of a fuel assembly with attached to the finger-shaped elements transmitting and Empfangsprüfköpfen at 4 different locations.</li><li>Fig 2a shows the arrangement of transmit and Empfangsprüfkopf in determining a new expected range.</li><li>FIG. 2b shows the pulse representation in the arrangement of FIG. 2a;</li><li>Figure 3a shows the arrangement of transmitting and Empfangsprüfkopf when testing a fuel rod cladding tube.</li><li>Fig. 3b the momentum representation in the arrangement of Fig. 3a; and</li><li>Fig. 4 is a circuit device for implementing the method according to the invention.</li></ul>

are with reference to FIG. 1 again, the difficulties encountered when measuring with festeingestelltem expected range, are explained.

1 with a fuel assembly is referred to, which is composed both of the fuel rods 2 as well as from the guide thimbles. 3 The fuel rods consist essentially of a tubular jacket, the fuel rod cladding, and the cladding tube located therein radioactive material. There is water between the fuel rods 2 and the cladding tubes. 3

To test the fuel rod cladding, the ultrasonic probes are 4 and5 pushed against finger-shaped holders 6 and 7 in such a way between the fuel rods through that one test head, the transmitting test head 4, so sonicated cladding tube to be tested that a circulation echo arises (position I). This echo is received with the Empfangsprüfkopf 5 and evaluated the corresponding electrical signal in an ultrasonic flaw detector 8 and can be displayed on the screen 9 of a CRT. Via lines 10 and 11, the ultrasonic flaw detector 8 with the transmission and Empfangsprüfkopf 4 or 5 is electrically connected.

With I to IV show different positions of the probes 4 and 5 and the corresponding positions of these echoes displayed on the screen are in g. In position I usually obtained in addition to the transmission pulse SI the circulation Echo UE and a transmission echo DE, resulting from the direct from the transmitting test head 4 coming into the Empfangsprüfkopf 5 ultrasonic wave. Particularly in the dynamic test, in which the Sendeprüfköpfe 4 and 5 Empfangsprüfköpfe be continuously pushed past the fuel rods 2, disrupts this transmission echo DE because it is located near the circulation Echo UE. In particular, if a defective fuel rod is present and decreases the amplitude of the circulation echoes by scattering in the fuel rod interior, it can easily lead to false alarms in the presence of a sound transmission echoes. Man forms for this reason an expected range, falling only circulation echoes in the possible. This expectation range 12 has been shown by dashed lines in the echoes displayed.

As already mentioned, results in the use of an expected range of constant temporal position incorrect measurement results. Because due to the change of the mutual distance of the probes along its path between the fuel therethrough (see FIG. Dashed path of finger-shaped brackets 6 and 7), the through-transmission echoes also fall within the expected range (position 111) and the expected range is not fully utilized, so that circulation echoes can fall out of the expected range (position IV).

According to the invention, therefore, the expected range is tracked through the streets of the fuel during the movement of the probes. 4 and 5 For this purpose, as shown in Figure 2a, in the gap between the most recently audited and each new test cladding tube the position of the through-transmission and thus echoes the term T_L the ultrasonic pulses between transmitting and Empfangsprüfkopf 4 and 5 determined. The beginning of the expected range then results by T from the measured transit time value_L a constant C is subtracted. This constant is slightly larger than the likewise constant width B of the expected range 12 and is selected such that the received at the time of flight measurement transmission echo DE no longer falls straight in the expected range. With the thus determined expected range 12 is then - as shown in Fig. 3 a - the casing tube 2 next examined. This is then followed in turn - as above beschriebendie determining the next expected range etc.

Fig. 2b and 3b show the echo images that match the in 2a and 3a shown Prüfkopfstellungen. The expected range 12 was again indicated by dashed lines.

In FIG. 4, a device for performing the method described above is illustrated. A trigger 13 causes the transmitter 14 to generate electrical pulses which are fed to the transmitting test head. Corresponding to the ultrasonic echo electrical signals pass through an amplifier 20 both to a visual display unit 14 and a gate circuit 15 and to a Laufzeitmeßeinrichtung 16. If the transducers 4 and 5, the position shown in Figure 2a hold, causes a control unit 17 that the Laufzeitmeßeinrichtung 16 is connected via the switch S to the memory 18th In this memory 18 is the beginning of the expected range, ie the measured time value T_L minus an adjustable constant C stored. The constant C can be adjusted for example by means of a code switch, not shown. The gate 15 remains closed during the entire expected range determination.

Have now the test heads for testing of the cladding tube 2 in the in Fig. 3a is moved further reproduced position, it switches the control unit 17, the run-time unit 16 via the switch to the control unit 19. At the same time Torschaltungseinheit 15 is released, so that echoes which in the fall expected range 12, go to the evaluation 19th The constant width B of the expected range can also be adjusted by means of a code switch, not shown. As usually gate, an AND gate is used.

The proposed process diaphragm position is not limited to the examination of fuel rods and fuel rod, but can also be used in the examination of other, closely abutting tubular specimens (eg tubes of a heat exchanger mouse).