Patent Number: 
Section: description

FIG. 1 represents a pressurized water reactor (PWR) nuclear fuel assembly 10 comprising a lower tie plate 12, guide tubes 14, spacer grids 16 spaced along the guide tubes, fuel rods 18 which are spaced radially and supported by spacer grid 16, instrumentation tube 28, an upper tie plate 37 attached to the upper ends of the guide tubes, and tie plate suppression springs 39. Each fuel rod 18 generally includes nuclear fuel pellets 20 composed of fissionable material, and upper end plug 22 and lower end plug 24 which seal the fuel rod. Plenum spring 26 maintains the position of the pellets within the fuel rod. Water as the coolant/moderator is pumped upwardly through the fuel assemblies thereby removing the heat generated by the fuel rods. Control rods 30 which are used to assist in controlling the fission reaction are shown disposed in guide tubes 14. Several control rods are grouped together and each control rod has a radial arm 32 which interconnect with one another at a central cylindrical member 34 to form a control rod cluster control mechanism 36 for vertically lowering and raising the control rods in the cluster into and out of the guide tubes, and hence into and out of the fuel assembly. The upper tie plate 37 of pressurized water reactor nuclear fuel assemblies is designed to allow alignment pins 92 of the upper core support plate 90 to be positioned into the alignment holes of the upper tie plate. FIG. 2 schematically represents the upper portion of a typical PWR nuclear fuel assembly and shows upper tie plate 37 and upper core support plate 90 with the alignment pins 92 disposed through the alignment holes 38 in the upper tie plate with the tie plate suppression springs 39 removed for clarity of illustration. In accordance with the present invention, a Radiation Induced Growth Indication Apparatus is provided in which a device is attached or secured on the top of the upper tie plate and disposed between the upper tie plate and upper core support plate and which when compressed, reduces in height relative to a known fuel assembly upper tie plate height dimension. The device would be compressed between the upper tie plate and the upper core support plate after the assembly has been placed in the core and the upper core support plate has been put in place as a result of radiation induced growth of the nuclear fuel assembly as well as by the differential thermal expansion between the fuel assembly and the surrounding reactor core structure. By providing a device which plastically deforms, or is inelastically compressed by a force, the fuel assembly upper tie plate to upper core plate gap available for expansion of the assembly can be determined at the end of a fuel cycle by a measuring tool to measure the compressed height of the device. In accordance with one embodiment of the present invention, a thin walled or hollow tube is secured to a location of the upper tie plate of the fuel assembly where it can interact with the reactor upper core support plate and which progressively collapses with a decrease of the upper tie plate to upper core plate gap due to thermal expansion and/or radiation induced growth of the fuel assembly. The tube compresses with a relatively moderate force (e.g. 10 pounds or less) and will maintain a plastically deformed shape when the upper core support plate is removed for refueling or other maintenance of the core. The tube could contain a honeycomb pattern of holes through its circumference and extending along its length. In a second embodiment, a rod or roll-type pin is press fit into a hole in the upper surface of the fuel assembly upper tie plate. The height of the rod or pin is selected such as to also contact the underside of the upper core support plate when the fuel assembly is installed in the reactor. As the fuel assembly is irradiation and radiation induced growth occurs, the pin is pressed further into the hole which is predrilled to a depth sufficient to allow pin compression to the full growth potential of the nuclear fuel assembly. Referring to FIG. 3, the Radiation Induced Growth Indication Apparatus 50 comprises a tube 52 positioned between the upper tie plate 37 of the fuel assembly and the upper core support plate 90 (also shown in FIG. 1). Tube 52 can include apertures 54 extending through the wall of the tube at any axial and/or radial position along its circumference. In a second embodiment of the present invention, the Radiation Induced Growth Indication Apparatus 50 which is depicted in FIG. 4 (and also shown in FIG. 1) comprises a rod or roll type pin 56 which is press fit into a hole 40 in upper tie plate 37. As shown in FIG. 3, the height of rod or pin 56 is selected to contact the underside of upper core support plate 90 when the fuel assembly is installed in the reactor. As the fuel assembly is irradiated, the pin or rod 56 is pressed further into hole 40. For a typical nuclear reactor, the Radiation Induced Growth Indication Apparatus requires about one inch, and more typically 0.8 inch of compressible range to provide indication of the upper tie plate to upper core plate gap over the lifetime of the fuel assembly. The compressed height of the device is measured by a depth micrometer or any other similar tool which may be modified as and if necessary for fuel inspection purposes as is known in the art. The present invention provides an accurate assessment of the available nuclear fuel assembly to upper core support plate distance as a function of fuel burnup or exposure for determining the remaining fuel assembly to upper core support plate distance so as to enable a determination of useable fuel assembly life or usage. While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.