Patent Number: 048790885
Section: summary

BACKGROUND OF THE INVENTION By far, the majority of nuclear power reactors are water cooled and moderated reactors, utilizing enriched uranium dioxide as fuel. The core of the reactor is formed by elongated fuel rods which are grouped into bundles which are generally square in cross section. The rods have diameters usually in the range of one-fourth to one half inch and may be ten or twelve feet long. They are held parallel and closely spaced from each other. Each rod is formed of a jacket or "cladding" made of zirconium alloy or stainless steel, which is filled with the uranium dioxide. Most commonly, the uranium dioxide is in the form of pellets which are just enough smaller than the cladding to slide in conveniently. The uranium dioxide may, on the other hand, be in the form of microspheres or granules which are compacted within the cladding. The remainder of the space within the cladding is commonly filled with helium, which has a high thermal conductivity. The helium is frequently under higher than atomospheric pressure. During the operation of the reactor, holes may develop in the cladding due to stress, corrosion, wear, or defective welding to the end plugs which close the ends of the cladding tubes. If this happens, the helium and fission gases will escape into the cooling water of the reactor and the water will enter the cladding tubes. After a given fuel assembly has been exposed in the reactor for a given length of time, it is taken out, checked for defects, repaired if necessary, and either returned to the reactor or sent for reprocessing or permanent storage. If the assembly is to be returned to the reactor, it is almost essential that it be checked for defective fuel rods. These irradiated assemblies are highly radioactive and must be stored and inspected under water in order to remove heat caused by the decay of fission products as well as to protect persons working with them. It is therefore highly desirable to provide a method of testing fuel rods for leaks while they are assembled and underwater. One method of doing so is by ultrasonic testing. Such a method is disclosed in U.S. Pat. No. 4,313,791, granted Feb. 2, 1982 and assigned to the Babcock and Wilcox Company. In this method, a transducer emitting ultrasonic vibrations is placed against a fuel rod and an ultrasonic beam is transmitted into the rod by the transducer. The test is performed on a portion of the fuel rod which does not contain uranium dioxide. An analysis of the waves received by a pulse-echo system, reveals whether or not this portion of the rod is filled with water. It is disclosed as carried out at the lower plenum of a fuel rod and apparently would not be operative in a portion of a rod where uranium dioxide is present. A weakness of this method lies in the fact that many, probably most, fuel rods do not have a lower plenum. SUMMARY OF THE INVENTION We have devised an ultrasonic test for failed fuel rods which is an improvement on prior systems. According to out invention, a transducer is traversed through a fuel assembly, spaced from a row of rods which is to be checked. During the traverse, a series of ultrasonic pulses is emitted from the transducer in the form of a beam. When the beam strikes a fuel rod, it is reflected from the outer surface. If the beam is exactly normal to the surface, it will be reflected back into the tansducer to a maximum degree. By the use of well-known electrical systems, this gives rise to an electrical signal. This method is termed the "pulse-echo" technique. Not all of the ultrasonic beam is reflected at the outer surface, however. A portion continues in the tubing wall and strikes the inner surface of the cladding. In a perfect rod, this is in effect a metal-gas interface. No matter how close the fit between the tube and the uranium dioxide, the contact is not sufficient for efficient transfer of sound energy. Neither is the ultrasound transmitted by the helium gas to any substantial degree. There is, therefore, a reflection from the inner surface of the tubing as well as from the outer surface. In fact, the ultrasound is reflected back and forth between the inner and outer walls of the tubing, producing what is termed "wall ringing". This wall ringing is recorded by the electronic system referred to above. If the tube has filled with water, there will be a transfer of the ultrasonic energy from the tubing wall into the water, where it is effectively dispersed. This greatly attenuates the wall ringing. It is immaterial whether uranium dioxide is present or not. The "coupling" of the cladding to the water within the tube results in the attenuation of the wall ringing and so identifies a defective tube. More specifically, as the transducer is moved along a row of fuel rods, it continuously emits a series of ultrasonic pulses. Typically, the ultrasound has a frequency of 10-30 megahertz and the pulses have a repetition rate of 1-8 kilohertz. When it receives a maximum echo from the outer surface of a fuel rod, a signal is transmitted to a recording medium, e.g., a strip chart. This is done over a pre-selected period of time which may be termed a "time window". After a delay, which is chosen in accordance with the thickness and other characteristics of the cladding, the echo from the "wall ringing" is sampled during another "time window" and, if its amplitude is above a preselected threshold, a second signal is transmitted to the recording medium. Absence of this second signal indicates that water is present within the tube and that the latter is defective. Our invention also includes apparatus for effectively carrying out the method described above. The transducer is mounted on a probe which is so constructed as to provide the proper spacing between the tubes to be tested and the transducer, and also for the proper positioning of the transducer so that the beam will be perpendicular to the fuel rod axes. We have also provided an indexing system which permits the probe to be accurately and rapidly inserted into the fuel assembly while the latter is under water. This indexing system includes a plate having grooves which are parallel to the rows of fuel rods which are to be checked and which are spaced apart the same distance as the rows. A reciprocating system acting along those grooves moves the probe along the rows of fuel elements in the assembly. When the reciprocating element is retracted, a pressure medium autuomatically moves it to the next groove, and in this manner, the assembly can be checked very quickly. This indexing system is claimed in an application of Leo F. Van Swam and Quang D. Ho, Ser. No. 660,787, filed Oct. 15, 1984 and assigned to the assignee of this application, now U.S. Pat. No. 4,689,193, granted Aug. 25, 1987.