Patent Number: 048790885
Section: description

DETAILED DESCRIPTION Referring to the drawings, FIG. 1 shows the basic probe structure 2 used in our test. It includes a torsionally flexible probe handle 3, near one end of which is a transducer 4. The transducer is mounted in an alignment tab 6, which is spaced from a leaf spring 8 (FIG. 4). At the other end of probe handle 3 is a mounting block 10 which serves as the connector for the electrical cable 12. Probe handle 3 is fabricated from two strips of stainless steel which are welded together. Signal wires 13 connect transducer 4 to cable 12. FIGS. 2, 3, and 3A show the operating means by which the probe is inserted into the fuel assembly. The mounting block 10 is carried on a support means 16, which rides on rails 18, 18', which provide for longitudinal movement, and on rails 20, which provide for transverse movement. A guide pin 22 slides in grooves 24 of index plate 26. The grooves 24 are open at one end, shown at the left in FIGS. 2 and 3 and are faced by deflection plate 28 which is provided with multiple deflecting surfaces 30, each of which faces one of the grooves 24. The actuating means, which are not shown on FIG. 2, are indicated diagrammatically on FIG. 3. A reciprocating hydraulic cyclinder 32 moves carrier 16 and probe 2 longitudinally of the latter so that the transducer is moved along a row of fuel rods 34 in fuel assembly 36. At the same time, another cylinder 38 exerts a continuous pressure laterally. Under the influence of these cylinders, the pin 22 moves longitudinally along a groove 24 to the right in FIG. 2, then returns. When it reaches the lefthand end of the groove, the force of pressure cylinder 38 forces it laterally along the deflecting surface 30 to the next groove 24, as best shown in FIG. 3A. These grooves are spaced apart the same distance as the spaces between the rows of fuel elements 34. The transducer, therefore, passes successively along the rows of fuel elements in the fuel assembly 36. When the pin 22 has moved in both directions along the last groove 24, indicated as 24', the operator reverses the direction of pressure exerted by cylinder 38. The pin 22 then moves back along the groove 40 at the end of index plate 26 to the starting position, carrying with it the carrier 16 and the probe 2. Members 37 and 39 are guide members, made in the same form as the fuel tubes 34. One of these members is open at both ends. When the system is immersed in water, it fills, so that it simulates a defective fuel tube. This provides a check on the operation of the system during actual testing. FIG. 4 shows the position of the transducer 4 relative to a tube 34 when a test is made. As the probe is inserted between the rows of tubes 34, the transducer 4 continuously emits a series of pulses. When the transducer is in most positions of its travel, no reflection from a tube is returned to it. However, when it is in the position shown in FIG. 4, the ultrasound waves follow the paths shown by the arrows, resulting in echoes received and recorded by the transducer. As can be seen in FIGS. 1 and 4 the transducer 4 is recessed within the alignment tab 6 which is pressed against the rod 34 which is being tested. This results in a "water path", indicated by arrows in FIG. 4, between the transducer and the outer surface of the rod 34. This water path is necessary to provide a suitable time interval between the transmission of the pulse and the reception of the echoes which will now be described. FIG. 5 shows typical examples of the form of the echo as recorded on an oscilloscope. The horizontal axis of the graph measures time while the vertical axis measures the amplitude of the echoes received by the transducer. FIG. 5 shows, in solid lines, the signals characteristic of a tube which contains no water, and, in dotted lines, those characteristic of a tube containing water. The transducer, as it travels past the rods, emits a series of pulses, one of which is shown at 36. The remaining peaks show various reflections which are received by the transducer when it is aligned so that the emitted beam is radial to the tube. The first peak, 38, is the reflection from the outer surface of the rod. It will be noted that this is received by the transducer about 2.0 microseconds after the transmitted pulse. During the next 1.7 (approximately) microseconds there is a series of closely spaced peaks 40a, 40b, 40c, and 40d. They are from the inner surface of the tube nearest the transducer and are the result of reflection of the ultrasound back and forth between the inner and the outer surface of the tube wall nearest to the transducer. This is termed "wall ringing". Finally, there is another pulse 42 which results from the reflection of the ultrasound from the outer wall, back to the transducer, again to the outer wall, and again to the transducer. This is termed the "second surface echo". The curves connecting the peaks show the decay of the "wall ringing" with time. It will be noted that the rate of decay is much greater for a tube containing water than for one free from water. This is because there is a relatively high degree of "coupling", i.e., transfer of energy, between the metal and water, and almost no coupling between the metal and a gas, such as helium. The effect is the same whether of not the portion of the tube being tested contains fuel. The reflected sound energy is a function of boundary condition on the inside of the cladding. As long as the water layer is thick relative to the wave length of the sound being used then material beyond this water layer will have no affect on the measurement. The amount reflected from the fuel, if present, will be small as compared to that reflected from the inner wall of the tube, perhaps 2 percent of it. In making the test, the instrumentation is so designed that when the transducer is centered on a rod, there is a recordation over a "time window" 44, which includes the peak 38. After a specified lapse of time, chosen to exclude the second surface echo 42, there is again a recording of the signal over the "time window" 46, which includes the fourth reflection from the inner surface, provided the signal at this time is above a predetermined amplitude. If the signal is below that level, no recording is made. A sample of such a record is shown in FIG. 6. In this figure, the upper row records the echo received in the time windows 44 from each rod traversed by the transducer, while the lower row indicates the signals received during the time windows 46. It will be noted that in some instances there is no recording 46 corresponding to a record 44. The relationship is shown in larger scale and in relation to the tubes in FIG. 7. This figure shows a row of tubes 34 including one defective tube 34' which contains water. The upper signal trace shows the echoes received during the time windows 44. The lower shows echoes received during the time windows 46. It will be noted that no signal 46 appears opposite the defective tube 34'. The entire traverse of the transducer along a row of tubes 34 (FIG. 2) requires only a very few seconds; hence, an assembly can be checked very quickly with this system. While we have described in detail one embodiment of our invention, it will be apparent to those skilled in the art that various changes can be made, we therefore wish our patent coverage to be limited solely by the scope of the appended claims.