Patent Number: 052001385
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIG. 1, there is illustrated a nuclear fuel assembly, represented in vertically foreshortened form and being generally designated by the numeral 10, that can incorporate a spectral shift-producing subassembly, generally designated 12, of the present invention which will be described later on. The fuel assembly 10 is the type used in a pressurized water reactor (PWR) and basically includes a lower end structure or bottom nozzle 14 for supporting the fuel assembly on the lower core plate (not shown) in the core region of a nuclear reactor (not shown), and a number of longitudinally extending guide tubes or thimbles 16 which project upwardly from the bottom nozzle 14. The fuel assembly 10 further includes a plurality of transverse grids 18 axially spaced along the guide thimbles 16, and an organized array of elongated nuclear fuel rods 20 transversely spaced and supported by the grids 18. Also, the fuel assembly 10 has an instrumentation tube 22 located in the center thereof and an upper end structure or top nozzle 24 attached to the upper ends of the guide thimbles 16. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conventionally handled without damaging the assembly parts. As mentioned above, the fuel rods 20 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 18 spaced along the fuel assembly length. Each fuel rod 20 includes nuclear fuel pellets (not shown) and the opposite ends of the fuel rod 20 are closed by upper and lower end plugs 26, 28. The fuel pellets composed of fissile material are responsible for creating the reactive power of the PWR. A liquid moderator/coolant such as water, or water containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract some of the heat generated therein for the production of useful work. Spectral Shift-Producing Subassembly In the operation of a PWR it is desirable to prolong the life of the reactor core as long as feasible to better utilize the uranium fuel and thereby reduce fuel costs. To attain this objective, it is common practice to provide an excess of reactivity initially in the reactor core and, at the same time, provide means to maintain the reactivity relatively constant over its lifetime. The present invention relates to such means in the form of the spectral shift-producing subassembly 12 having a plurality of hermetically-sealed hollow tubular empty rodlets 30 inserted in the guide thimbles 16 of the fuel assembly 10, as shown in FIG. 1. The sealed empty rodlets 30 of the subassembly 12 are stationarily supported in the fuel assembly 10 by a mounting means in the form of a holddown mechanism 32. The sealed empty rodlets 30 are capable of displacing moderator water while in their sealed condition within the fuel assembly 10. The holddown mechanism 32 supports the rodlets 30 in the guide thimbles 16 of some of the fuel assemblies 10 in the core to assist in controlling reactivity over the life of the core. As best seen in FIGS. 2-4, the holddown mechanism 32 of the spectral shift-producing subassembly 12 supports the water displacement rodlets 30 in generally parallel, spaced side-by-side relationship. The holddown mechanism 32 includes a lower flat perforated support plate 34 which fits within the fuel assembly top nozzle 24 and rests on a lower adapter plate 36 of the top nozzle 24. The holddown mechanism 32 also includes a central sleeve 38, being attached at its lower end within a central opening 40 in the support plate 34 and extending upwardly therefrom, and an upper holddown plate 42 which receives the central sleeve 38 and is slidable vertically along it. Further, a holddown coil spring 44 is disposed about the central sleeve 46 and extends between the lower support plate 34 and the upper holddown plate 42. Thus, the support plate 34 is held down against the top nozzle lower adapter plate 36 by the coil spring 44 which is compressed by the upper core plate (not shown) acting through the upper holddown plate 42 which abuts the upper core plate. This arrangement assures that the water displacement rodlets 30 which are attached to the holddown support plate 34 cannot be ejected from the reactor core by coolant flow forces while any thermal growth of the rodlets 30 is accommodated. Turning next to FIG. 4 as well as FIG. 2, there is shown one embodiment of the water displacement rodlet 30 of the spectral shift-producing subassembly 12 of the present invention. The rodlet 30 is hermetically sealed at its opposite ends. The rodlet 30 basically includes an elongated hollow empty tube 46 having an elongated annular (preferably, cylindrical) wall 48, and opposite upper and lower end plugs 50, 52 hermetically sealed to the opposite ends of the tube 46. The upper end plug 50 includes a threaded upper end 50A and an enlarged diameter annular collar 50B spaced below it for facilitating attachment of the rodlet 30 to the holddown support plate 34 of the holddown mechanism 32. The tube 46 and end plugs 50, 52 can be composed of any suitable material, such as zirconium-based alloy. The tube wall 48 of the rodlet 30 has a weakened section in the form of an axially-extending annular wall section 54 of reduced thickness compared to the thickness of the remainder of the wall 48 of the tube 46. The reduction in the thickness of the wall section 54 of the rodlet 30 adapts it to creep collapse and rupture after a desired extended period of use in the nuclear reactor permitting moderator to enter and fill the empty rodlet. In such manner, the displacement of water by the rodlet 30 is produced resulting in an increase in the water/fuel ratio and thereby an increase in reactivity through the occurrence of a spectral shift. Referring to FIGS. 5-7, the respective reduced thicknesses of the axial wall sections 54A, 54B of first and second groups of the rodlets 30A, 30B are different to adapt the rodlets 30A, 30B to rupture at different times and permit water to enter the rodlets at different times to produce an incremental increase in the water/fuel ratio and thereby an incremental increase in reactivity. The rodlets 30A, 30B can also have different levels of internal pressurization, such as by helium gas, to initiate rupture at different times. The axial length of the thinned wall sections is small in comparison to the overall length of the rodlet 30. As an example, the rodlet 30 can be the same length as a nuclear fuel assembly, whereas the wall section 54 can be only six inches in length. The outside diameter of the rodlets 30 is the maximum amount possible to permit the rodlets to fit in the guide thimbles 16 and still not interfere with the guide thimble so as to create problems during removal of the subassembly 12 during refueling. The reduced thicknesses and pressurization of the rodlets 30 would be preset such that the ruptures occur sometime between the middle and the end of the reactor fuel cycle. The increase in water in the guide thimbles 16 after the rodlets 30 fail results in about a 6% change in the H/U ratio which corresponds to a 3% decrease in fuel cycle costs. The 3% decrease in fuel cycle cost is equivalent to $45-$60 kg U fabrication cost improvement. It should be noted that the present invention is very tolerant to some of the spectral shift-producing rodlets not failing during the cycle or failing somewhat sooner or later than desired since the overall gross effect is what is desired and small changes do not significantly affect either safety or economics. It is thought that the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.