Patent Number: 056617666
Section: summary

INTRODUCTION 1. Field of the Invention The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly to an apparatus for measuring fuel assembly bow and twist. 2. Background In nuclear power reactors, the reactor core is comprised of a number of fuel assemblies. Depending on the size of the reactor, there can be 200 or more fuel assemblies in the core at any time. Since the fuel assemblies are densely packed in predetermined positions in the reactor core, dimensional standards of each fuel assembly must be met within very close tolerances. Pre-service quality assurance inspections are performed to ascertain any deviations in the formation and assembly of new fuel assemblies from accepted tolerances. The fuel assembly is also inspected to determine whether it is straight, unbowed, and without any twist. During reactor operation, the fuel assemblies become irradiated and can become bowed and twisted due to differential growth resulting from thermal gradients and neutron flux gradients. During each reactor refueling cycle, fuel assemblies are moved to different locations in the reactor core, with some of the fuel assemblies being replaced by new fuel assemblies. Certain fuel assemblies become spent and are removed from the reactor. Since at least a portion of the irradiated fuel assemblies are moved from one location in the reactor core and eventually to another location in the core during refueling operations, there is a need to verify the continued serviceability of these fuel assemblies. Thus, the accurate and efficient determination of deviations from dimensional standards of the fuel assembly as well as whether the fuel assembly is straight, unbowed and without twist is particularly important for irradiated fuel assemblies which have to be inspected and remotely manipulated to protect personnel against exposure. Pressurized water reactor nuclear fuel assemblies are positioned at their lower ends in predetermined positions in the lower core support plate of the reactor. Depending upon the particular design, each fuel assembly typically has two alignment pins extending downward from the lower tie plate and which are received in alignment holes in the lower core support plate. The upper core support plate which is positioned on top of all the fuel assemblies serves to align the upper portion of the fuel assemblies. Extending downward from the underside of the upper core plate are fuel assembly alignment pins. Typically, two alignment pins insert into two alignment holes in the upper tie plate of each of the fuel assemblies. As the upper core support plate is aligned over the top of the reactor core support and lowered onto the fuel assemblies, the alignment pins slide into their respective alignment holes and align the upper portion of the fuel assemblies into their predetermined positions. Bowing or twisting of the fuel assembly can prevent the accurate alignment of the fuel assembly with the upper and/or lower core support plate, cause interference with adjacent components, and in extreme cases, significantly increase the insertion force of control rods. Moreover, the bow and twist of the fuel assembly in its freestanding or unconstrained condition usually bears little resemblance to its constrained position within the reactor core between the upper and lower core support plates. In addition, the bow and twist of the fuel assembly is frequently not perceived until the fuel assembly is attempted to be placed within its constrained position within the reactor core between the upper and lower core support plates. In some prior art designs, assembly bow is measured by very simple measurement techniques which however have the disadvantage of not being accurate. The most common places a long straight edge suspended from the upper tie plate of the fuel assembly to the bottom tie plate and the distance from the straight edge to the assembly body is measured. This type of device and all such similar devices require viewing and thereby suffers from errors of observation such as parallax, which is exacerbated for irradiated fuel assemblies which are inspected underwater usually with a television camera. Other prior art designs involve devices which position a component in direct physical contact with the fuel assembly. Some of such prior art designs utilize linear variable differential transformer (LVDT) which is an electromechanical transducer which produces an electric output which is proportional to the movement or displacement of an interrelated component positioned to physically engage or contact a portion of the fuel assembly. However, LVDT systems have several disadvantages which impact the accuracy of the measurements. First, LVDT devices must be in constant contact with the fuel assembly with the possibility that the measuring device affects the position of the measured feature (e.g. a fuel rod) of the fuel assembly. Secondly, the surface area of the measuring device (i.e. the end of the LVDT) must be sufficiently large to always measure the same feature of the fuel assembly irrespective of the amount of bow and twist, otherwise the LVDT may cause interaction with a feature or features adjacent to the measured feature. These errors introduced by measurement system interaction with the fuel assembly by the use of LVDT's can therefore be significant. Some of the prior art designs utilize proximity sensors which produce an inductive field which generates eddy currents in the portion of the fuel assembly within its range. These eddy currents change the state of the field which can be translated into an output signal that is proportional to the distance from the sensor to the portion of the fuel assembly being examined or measured. Such eddy current sensors suffer from the disadvantage of having very limited range and must therefore be close to the fuel assembly. In addition, with large bowing or large twisting, there is the possibility of interaction with an adjacent feature. Furthermore, the accuracy of eddy current sensors is limited by material conductivity changes as well as oxide and crud build-up. The prior art discloses devices in which the fuel assembly which is to be measured forstraightness, bow and twist, is simply supported and held in a position in such a way as not to impart any loads to it by twisting or tilting. Thus, the fuel assembly is not rigidly constrained as if the assembly was actually positioned within the reactor core. In one prior art design, the fuel assembly is suspended from its upper end on a support having a profile similar to the fuel assembly, and a measuring device is moved along the support. The measuring devices are typically moved on a carriage along the support at a predetermined distance from the fuel assembly by a guide system. Even if the guide system, typically guide rails of some sort, is made to be as straight as possible, the guide system or guide rails which is not perfectly straight will usually develop further imperfections or flaws during operating which will cause them not to remain straight. Since these guide systems do not generally compensate for any deviation from its correct path of travel, the error introduced by the deviation produces inaccuracies in the measurements of the fuel assembly. Efforts of the prior art to correct this error include the use of positioning sensing apparatus which detects when the guide system moves beyond an acceptable tolerance relative to its correct position. However, the complexities of these positioning sensing apparatus can introduce further errors or inaccuracies in addition to the inaccuracies introduced by the guidance system. As stated above, further or other errors can be introduced in the determination of the amount of bow and twist of the fuel assembly as positioned within the core by not placing the fuel assembly in a rigid fixture to constrain the fuel assembly as if it was actually positioned within the reactor core. Thus, the prior art devices are inaccurate in the measurement of bow and twist due to errors of observation (such as parallax), measurement system/fuel assembly interaction errors, guiding system inaccuracies, positioning/sensing systems inaccuracies, and inaccuracies due to the failure to measure the fuel assembly when constrained in a rigid fixture as if the fuel assembly was actually positioned within the reactor core. OBJECT OF THE INVENTION It is a general object of the invention to provide a system for measuring nuclear fuel assembly bow and twist which avoids the disadvantages of the prior art teachings while affording greater accuracy and facility of operation. SUMMARY OF THE INVENTION In accordance with one aspect of the invention, an apparatus is provided for measuring dimensional characteristics of a nuclear fuel assembly having at its lower end a lower alignment pin being extending downward from a lower tie plate, the lower alignment pin being positioned and oriented for engaging a corresponding alignment hole in a lower core support plate of a nuclear reactor, the fuel assembly having at its upper end an upper alignment hole located in the upper tie plate, the upper alignment hole being positioned and oriented for receiving an alignment pin extending downward from the underside of an upper core support plate. The apparatus comprises: (a) an elongated fixture defining an internal volume for a nuclear fuel assembly, the fixture mounted in an upright position and having an opening disposed towards an upper end of the fixture, the opening adapted to receive the fuel assembly therethrough, the fixture further including a removable top adapted to fit into the opening and having a locating pin with a longitudinal axis extending from its underside for engaging the upper alignment hole in the upper tie plate of the fuel assembly, the fixture further including at a lower end a bottom reference plate adapted to form a locating hole having a second longitudinal axis for receiving the lower alignment pin of the fuel assembly, the locating pin and the locating hole being adapted to engage the fuel assembly lower alignment pin and the fuel assembly upper alignment hole so as to constrain the fuel assembly as if the fuel assembly was positioned in the reactor, and at least one of the first longitudinal axis and the second longitudinal axis defines a predetermined longitudinal axis of the fixture. The apparatus further includes (b) at least one reference wire extending from the upper end of the fixture to the lower end of the fixture, the at least one reference wire being disposed parallel to the predetermined longitudinal axis of the fixture, and (c) an ultrasonic measuring device comprising: a transponder for (I) producing an ultrasonic signal toward the at least one reference wire and the fuel assembly, and (II) receiving a first reflected wave from the ultrasonic signal reflected from the at least one reference wire back to the transponder and (III) receiving a second reflected wave from the ultrasonic signal reflected from the fuel assembly back to the transponder. The ultrasonic measuring device further includes an ultrasonic flaw detector for receiving a signal from the transponder of the first reflected wave and the second reflected wave where the time difference between the first reflected wave and the second reflected wave is a measure of the distance from the reference wire to the fuel assembly. The apparatus further includes (d) transmission means which transmits from the transponder the signal of the first reflected wave and signal of the second reflected wave to the ultrasonic flaw detector.