Patent Number: 054523346
Section: description

DETAILED DESCRIPTION OF THE INVENTION 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, and upper tie plate 37 attached to the upper ends of the guide tubes. 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. As is well known in the art, the upper tie plate of pressurized water reactor nuclear fuel assemblies is designed to allow the alignment pins of the upper core support plate to be positioned into the alignment holes of the upper tie plate. These alignment pins may become bent, as discussed above, when the upper core support plate is removed and/or reinserted. If the alignment pins are bent so that they do not realign with the holes in the upper tie plate, they can inadvertently be forcefully jammed into or against the upper tie plate. The next time that the upper core support plate is removed, the stuck fuel assembly could be unintentionally removed with the upper core support plate if the pin is jammed into the fuel assembly alignment hole with sufficient force to hold the fuel assembly against its own weight. If the alignment pin is jammed into the fuel assembly but without the force to retain the fuel assembly and hold it in place, the fuel assembly could drop and the fission products trapped within the fuel rods could be released. According to the present invention, the fuel assembly upper tie plate includes a disengaging upper tie plate corner post which is secured to the upper tie plate. The disengaging upper tie plate corner post is secured to the rest of the upper tie plate by securing pins, bolts, or other mechanical means for securing the post to the upper tie plate. The disengaging corner post will disengage from the upper tie plate when a predetermined force corresponding to less than the buoyance weight of the fuel assembly is applied to the disengaging post. It is preferred that the disengaging corner post would disengage from the upper tie plate when a load of 30% to 45% of the fuel assembly buoyancy weight is transmitted to the securing bolts, pins, or other mechanical securing means. The fuel assembly could then safely be lifted and moved using normal fuel assembly handling apparatus to a location where the upper tie plate would be repaired or replaced. FIG. 2 schematically represents a portion of a typical PWR nuclear fuel assembly and upper core support plate shown in FIG. 1. FIG. 2 shows upper tie plate 37 and upper core support plate 90 with the alignment pins 92 properly disposed through the alignment holes 38 in the upper tie plate. FIG. 3 schematically represents a bent alignment pin 93 which is not disposed through its corresponding alignment hole 38 in the upper tie plate, as well as a second alignment pin 94 which is bent and jammed in the alignment hole 38 of the upper tie plate of the fuel assembly. As the upper core support plate is removed and the weight of the fuel assembly is thereby imposed on the jammed pin (FIG. 4A), the disengaging upper tie plate corner post would disengage from the upper tie plate. The disengaging upper tie plate corner post would disengage from the rest of the fuel assembly thereby leaving the fuel assembly in its proper and intended position in the reactor core. After the disengaging upper tie plate corner post has disengaged from the upper tie plate, and thereby freed the wedged alignment pin from the upper tie plate, the disengaging upper tie plate corner post could remain jammed with the alignment pin and be removed together with the upper core support plate (FIG. 4B), or it could remain with the upper tie plate to be removed at a later time when the fuel assembly is removed. By remaining in its proper core position, the danger of unintentionally moving a fuel assembly stuck to a core support plate would be avoided. FIG. 5 represents in an exploded view the preferred embodiment of the present invention in which, for purposes of clarity, only a portion of the upper tie plate and the disengaging upper tie plate corner piece of the nuclear fuel assembly is shown. Upper tie plate 40 includes upper tie plate side walls 42. Laterally extending from each wall 42 is horizontal grapple flange 44. At each of two diagonal corners of the upper tie plate 40 is a recess 48 formed by edges 43 and shoulders 45. Disengaging upper tie plate corner post 60 which fits into recess 48 of upper tie plate 40 is secured to edge 43 and shoulder 45 by break-away pins 70. Disengaging corner post 60 is shown in this embodiment as having four side walls 62 and top 64 adapted to form guide pin alignment hole 66. The perimeter and shape of disengaging upper tie plate corner post 60 fits into and matches recess 48 formed by edges 43 and shoulders 45. Disengaging upper tie plate corner post 60 includes a tab 68 extending from one of the walls 62. A matching slot 46 to tab 68 is formed in the face of one of upper tie plate sidewalls 42. Disengaging upper tie plate corner post 60 is secured to the upper tie plate 40 by break-away pin 70 which secures tab 68 within matching slot 46 by threading into bore 50, and by break-away pin 70 which secures sidewall 62 to shoulder 45 by threading into bore 52 as shown in FIG. 5. The shape, material of construction, and strength of break-away pins 70 are selected so that each of the two pins 70 fracture when the shear stresses due to loading from the weight of the fuel assembly which is transmitted to the pin 70 due to the wedged alignment pin 94 of the upper core support plate 90 corresponds to less than the buoyance weight of the fuel assembly. In a preferred embodiment, the shear stress at fracture of the breakaway pin(s) 70 is less than about 30% to about 45% of that corresponding to the full buoyancy weight of the fuel assembly. In an alternative embodiment of the present invention which is depicted in FIG. 6, disengaging corner post 60' has a rib 65 which fits into groove 47 of upper tie plate 40'. Break-away pins 72 extend through holes 73 into threaded apertures 54 in edges 43' of upper tie plate sidewalls 42. Caps 74 are installed over break-away pins 72. In a further alternative embodiment depicted in FIG. 7, disengaging upper tie plate corner post 60' has, as in the embodiment shown of FIG. 6, walls 62, alignment hole 66, and break-away pins 72 which extend through holes 73 to secure corner piece 60' to the horizontal grapple flanges 44' of the upper tie plate 40". Horizontal grapple flanges 44' are not adapted to form a recess as in the embodiments shown in FIGS. 5 and 6 into which an upper tie plate corner post fits. Instead, the corner post is secured to the top of flange 44 in which is formed an opening 56 to accommodate the length of the alignment pin. Because of its larger diameter, the alignment pin cannot become wedged in opening 56. Since there are two break-away pins in each of two disengaging upper tie plate corner posts for each fuel assembly, the shape, material of construction, and strength of break-away pins 72 shown in FIGS. 6 and 7 are selected so that they fracture when the tensile stresses due to the loading from the weight of the fuel assembly corresponds to less than the buoyance weight of the fuel assembly. In a preferred embodiment, the tensile stress at fracture of pins 72 is less than about 30% to about 45% of that corresponding to the full buoyancy weight of the assembly. The particular shape and configuration of the disengaging upper tie plate corner post as well as the particular combination of break-away pins, bolts and means to secure the disengaging corner post to the upper tie plate is a matter of design choice which will be apparent to those having ordinary skill in the art. The examples shown and described above are exemplative of the invention and are not intended to limit the true scope of the invention. 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.