Patent Number: 050664522
Section: summary

FIELD AND BACKGROUND OF THE INVENTION The present invention relates in general to testing equipment and methods in nuclear power plants, and in particular to a new and useful method and apparatus for detecting and measuring wear on the control rods for the fuel assemblies for nuclear power plants. More and more frequently, control rod assemblies (CRA) are being thought of as components of pressurized water reactor (PWR) systems which require periodic non-destructive evaluation (NDE). In light of recent observations of breach-of-cladding, plant owners are under increasing regulatory pressure to determine the condition of their control components and to insure that shutdown margins are within specifications. Damage mechanisms include vibratory contact with support or guide components and absorber swelling due to neutron irradiation. As a result of either gross diameter increase in the cladding or leeching of the poison from the cladding, the ability to scram the reactor could be jeopardized. During the operation of a PWR, the control rods are suspended above the fuel assembly with the individual pins contained within the brazement guide structure. The tips of the pins are captured in guide tubes located in the fuel assemblies. Coolant flow through the guide tubes causes the pins to vibrate, and the resulting mechanical contact between the control rod pins and the support structure/fuel assembly guide tubes induces localized wear on the outside surface of the individual pins. There are at least two different flaw types in the CRA. The first flaw type is a large volume wear mark caused by contact with the guide tube nut. The second type of defect is a small volume, axial groove caused by contact with the brazement support structure above the fuel assembly. In the past, inspection of control rods, the primary component of rod cluster control assemblies (RCCAs) in pressurized water reactors, has been performed non-destructively, using various eddy current techniques. These eddy current techniques have been used to look for and measure cracks and wear marks that, if large enough, could render an RCCA unusable. The use of eddy current coils has been successful in determining the existence of a breach in cladding or the amount of material left on a control rod. In some situations, measuring the percent of material remaining has provided sufficient information for determining the useability of a control rod. In fact, pass/fail criteria has largely been based upon the kind of data (i.e. location, quantity and accuracy of data points) collected by eddy current inspection equipments. Inherent in the making of eddy current measurements and especially in the interpretation of the measurements is the problem of material variability. Because eddy current measurements are based on the electrical properties of the material of which the control rods are made, a control rod calibration standard must be produced to replicate the actual control rod and damage mechanisms as closely as possible. Any difference in material or defect geometry has the potential to be a source of error in data collected on actual control rods. If a very precise method of measurement that is not dependent upon material properties could be integrated in a system to inspect control rods, the accuracy of measurement data could be improved. SUMMARY OF THE INVENTION The present invention utilizes an ultrasonic non-destructive technique to detect wear on the control rods. The system is comprised of a rotating UT transducer mounting fixture which rests on a storage rack in the spent fuel pool of a nuclear power plant. This fixture houses and rotates a series of transducers while the control rod cluster is run through the fixture. The transducers maintain a constant distance from the control rods at all times as the transducers float to preserve transducer standoff. The system uses high frequency focussed transducers, multichannel ultrasonic thickness gauging instrumentation, a probe/electronics switching network, and a calibration standard. A conventional pulse-echo ultrasonic technique is used by the invention in conjunction with an immersion transducer to measure water path. Taking advantage of the velocity (longitudinal) difference between steel and water, a water path measurement provides a profile of the outside surface of the control rod by gauging the spacing between transducer and the test surface. This ultrasonic profilometry system uses the multichannel ultrasonic thickness gauge and multiple test stations to examine several control rods simultaneously. The transducers are rotated continuously around the control rods, maintaining constant spacing between the transducer and the individual control rods. The control rods are raised and lowered through the inspection fixture by the existing refueling mast in the plant, effecting a helical scan which provides maximum coverage. Both the pulse voltage and reflected signal are transmitted through slip rings, and the resulting data indicative of rod profile are recorded on computer-based oscillographic strip chart via the analog output of the thickness gauge. This ultrasonic technique has advantages over an electromagnetic method for this particular application for several reasons. It is not hindered by variations in the electrical properties of coatings applied to the outside surface of the control rods; it is unaffected by the presence of conductive material inside the control rods; and it is not dependent on representative calibration standard flaw geometry for sizing accuracy. The system can inspect 25% of the rods during one pass of the rod cluster control assembly (RCCA) through the UT transducer mounting fixture. To inspect the remaining rods the RCCA is removed from the fixture and rotated 90.degree.. The RCCA is then lowered back into the fixture and the rods inspected. This is repeated until all control rods have been inspected. The entire process is supported by any additional support tooling which itself, does not form a part of this invention. On-site activities can be performed off the side of the spent fuel pool or off a bridge in the spent fuel pool. Accordingly, an objective of the present invention is to provide an apparatus and method for scanning the outer profile of control rods, in particular, for nuclear fuel assemblies, comprising an ultrasonic transducer for sending an ultrasonic beam to, and for receiving an ultrasonic echo signal from the surface of a control rod, rotation means connected to the ultrasonic transducer for rotating the transducer around the axis of the control rod, and control rod translation means for translating the control rod with respect to the transducer, parallel to the axis of the control rod, for scanning the surface of the control rod along its length. A further object of the present invention is to provide an apparatus which can be used to measure the outer profile of control rods, in particular for detecting wear and surface defects, which is simple in design, rugged in construction and economical to manufacture. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.