Patent Number: 052157067
Section: summary

BACKGROUND OF THE PRESENT INVENTION 1. Field of the Present Invention The present invention relates to the field of testing of nuclear fuel rods and, in particular, to improvements in the testing of nuclear fuel rods by ultrasonic techniques. 2. Background Prior Art The use of ultrasonic energy for detecting defects in nuclear fuel rods is known. U.S. Pat. Nos. 4,174,255 and 4,517,152 are two patents disclosing such use of ultrasonic energy for testing of nuclear fuel rods. In U.S. Pat. No. 4,879,088, assigned to the same assignee as that of the present invention, nuclear fuel rods are checked for defects by ultrasonic pulses emitted from a transducer. Such fuel rods, which form the core of the reactor, are elongated elements. The rods typically have diameters of about 3/8 to 1/2 inch, are about 10 to 12 feet long and are grouped in bundles which are generally square. Each rod is formed of cladding of zirconium alloy or stainless steel, which is filled with uranium dioxide, typically in the form of pellets. The remainder of the space within the cladding is commonly filled with helium. 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 occurs, the helium and fusion gases will escape into the cooling water of the reactor and the water will enter the cladding tubes. According to the approach of U.S. Pat. No. 4,879,088, a transducer is caused to traverse a fuel assembly consisting of a group of fuel rods. 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 transducer to a maximum degree which will give rise to an electrical signal ("pulse-echo" technique). Part of the ultrasonic energy penetrates the cladding but the normal rod does not pass ultrasonic energy (the internal gas and uranium dioxide are nonconductive) resulting in "wall ringing" between inner and outer walls of the rod. The wall ringing is recorded. If the rod is 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. This will occur whether uranium dioxide is present or not. The attenuation of the wall ringing identifies a defective fuel rod. In practice, test results employing the ultrasonic approach of fuel rod testing have indicated erroneous readings under certain circumstances. Such erroneous readings lead to conclusions that specific fuel rods indicate failure, when in fact they were properly functioning. Further study of the fuel rod assembly suggests that differences in fuel rod and guide tube diameters (in the case of PWR fuel assemblies) and fuel rod bow, will cause alignment problems of the rods leading to the erroneous ultrasound readings. While U.S. Pat. No. 4,847,037 to Scharpenburg does recognize that bowing of fuel rods occurs, there is no suggestion for providing pre-alignment to all fuel rods of an assembly to correct for such bowing. SUMMARY OF THE PRESENT INVENTION It is an object of the present invention to avoid erroneous readings in ultrasonic testing of nuclear fuel rods. It is a further object of the present invention to provide apparatus to pre-align fuel rod assemblies for avoiding erroneous readings in ultrasonic testing of nuclear fuel rods. It is another object of the present invention to overcome misalignment in fuel rod assemblies caused by differences in fuel rod and guide tube diameters and fuel rod bow which cause erroneous readings in ultrasonic testing of nuclear fuel rods. In accordance with the invention, a method is provided for the ultrasonic testing of a nuclear fuel rod array with attendant suppression of erroneous readings comprises the steps of aligning the individual rods of a fuel rod array with respect to one another, providing a source of ultrasonic energy and applying said source to a fuel rod of an aligned fuel rod array and determining whether a fuel rod of an aligned assembly is operationally acceptable by developing a signal representing the conduction of ultrasonic energy within the rod. In a particular form of the method, the providing and determining steps of an aligned fuel rod array are repeated for each rod of the array. Further in accordance with the invention, an arrangement for the ultrasonic testing of a nuclear fuel rod array with attendant suppression of erroneous readings comprises means for aligning the individual rods of a fuel rod array with respect to one another prior to ultrasonic testing of the fuel rod array, means for supplying ultrasonic energy to a fuel rod of a fuel rod assembly for testing whether the rod is operationally acceptable, means for developing a signal representative of whether a fuel rod is operationally acceptable upon application of ultrasonic energy to the fuel rod and means for causing the ultrasonic energy supplying means to traverse the fuel rod array to permit testing of each rod of the array. Accordingly, alignment of the fuel rod array prior to ultrasonic testing suppresses tendencies to produce erroneous test readings caused by mechanical deviations of individual fuel rods. For a better understanding of the present invention, reference is made to the following description and accompanying drawings while the scope of the invention will be pointed out in the appended claims.