Patent Number: 046876278
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 shown several fuel assemblies, in which the central one is generally designated by the numeral 10 while the adjacent assemblies are indicated by 10A and 10B, in combination with an upper core support plate 12 and a spider assembly 14. The upper core support plate 12 is disposed above and extends across the tops of the fuel assemblies 10,10A,10B and, in turn, has the spider assembly 14 disposed above it. The fuel assembly 10 (it being understood that assemblies 10A and 10B are identical thereto), being shown in vertically foreshortened form in FIG. 1, basically includes a lower end structure or bottom nozzle 16 for supporting the assembly on a lower core plate (not shown) in the region of a reactor (not shown), and a number of longitudinally extending guide thimbles 18 which project upwardly from the bottom nozzle 16. The assembly 10 further includes a plurality of transverse grids 20 axially spaced along the guide thimbles 18 and an organized array of elongated fuel rods 22 transversely spaced and supported by the grids 20. Also, the assembly 10 has an instrumentation tube 24 located in the center thereof and an upper end structure or top nozzle 26 attached to the upper ends of the guide thimbles 18. With such an arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conveniently handled without damaging the assembly parts. As mentioned above, the fuel rods 22 in the array thereof in the assembly 10 are held in spaced relationship with one another by the grids 20 being spaced along the fuel assembly length. Each fuel rod 22 includes nuclear fuel pellets (not shown) and the opposite ends of the rod are closed by upper and lower end plugs (not shown). The fuel pellets composed of fissible material are responsible for creating the reactive power of the reactor. 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 heat generated therein for the production of useful work. The upper core support plate 12, being conventional, extends across the top of the fuel assembly 10 as well as across the top of the other fuel assemblies, such as adjacent fuel assemblies 10A,10B arranged within the core. To control the fission process, the core plate 12 has a number of coolant flow openings 28 defined therethrough (only one being seen in FIG. 1) to allow coolant to pass upwardly through the core. At least some of these openings 28 are aligned over several of the guide thimbles 18 such that the improved water displacer rods 30 of the present invention (one of which is seen in FIG. 1) connected at their upper ends to the spider assembly 14 can be moved by the spider assembly at selected times during operation of the reactor down through the core plate 12 and and be inserted into the respective guide thimbes 18 of fuel assemblies 10,10A,10B so as to displace coolant from the thimbles. Improved Water Displacer Rod Generally speaking, water displacer rods, as their name implies, are inserted into guide thimbes in the fuel assemblies to initially displace moderator-coolant water therein and decrease the reactivity of the reactor core. Thus, the purpose of displacer rods is to displace moderator water and not to absorb neutrons. Then, at some later point during the core cycle as reactivity is consumed, the rods are removed from the core so that the amount of moderation and therewith level of reactivity in the core are increased. In order for the displacer rods to carry out their intended purpose satisfactorily, it would seem that a primary goal might be to design the rods so that they will have a high probability of successfully withstanding the extreme pressures generated within the reactor core, which often reach, for example, 2200 psi, so as to avoid rupture and entry of water therein. However, assuming that failure of a displacer rod specifically designed to withstand high external pressure will inevitably occur, it would seem that a secondary goal might then be to design the rods so that little or none of the displaced water will be able to enter the ruptured rod. The improved water displacer rod 30 of the present invention, as illustrated in greater detail in FIG. 2, satisfies both of these primary and secondary design goals. As seen in FIG. 2, the improved water displacer rod 30 basically includes an elongated hollow hermetically-sealed tubular member 32 and a plurality of pellets 34 disposed in a stacked relationship within the tubular member. The tubular member 32 is formed by an elongated thin-walled, metallic cladding or tube 36 having respective upper and lower end plugs 38,40 for sealing the opposite ends of the tube. The upper end plug 38 has an upwardly extending integrally formed stem section 42 with an externally threaded end 44 for connection to the spider assembly 14. The lower end plug 40 is cone-shaped. The pellets 34, tube 36 and end plugs 38,40 are all preferably composed of Zircaloy material. The hollow tube 36 is thin-walled to minimize its moment of inertia and thereby increase its lateral flexibility which adapts the dispacer rod incorporating the tube for insertion into fuel assemblies being in a variety of structural conditions, such as ones which are warped or bowed do toextended use in the extreme environments found in nuclear reactor cores. The pellets 34 which fill the tube 36 between its end plugs 38,40 bolster its resistance to collapse due to the high pressures imposed upon the exterior of the tube while resident in the reactor core. At least a substantial proportion of the pellets 34 of each dispacer rod 30 have the construction seen in FIGS. 2-5. The pellet 34 has a body 46 which fits concentrically within the interior surface of the tube 36 and has a hollow annular cross-sectional shape defining a central void 48 through the pellet. The use of hollow pellets minimizes the mass added to the weight of the rod 30 by the addition of the pellets. Also, a pair of solid webs 50,52 extend across and close the void 48 at the opposite ends of the pellet body 46 so as to encapsulate or seal the void of each individual pellet. Therefore, although the stack of pellets 34 augments the collapse resistance of the tube 36, should any breach of the tube still occur the central voids 48 of the pellets cannot fill with reactor coolant. Consequently, little or no water will enter the failed tube because the water has no empty space therein which is accessible to it. In effect, the rod 30 contains a double barrier against entry of moderator water into the rod interior. The body 46 and one end web 50 of the Zircaloy pellet 34 can be fabricated by drilling the central bore of void 48 in ground bar stock having the desired outside diameter. Then the other end web 52 in the form of a cap could be stamped from Zircaloy sheet material and coined to form the weld projections 54 shown in FIG. 6. The body 46 and cap 52 can then be resistance welded together using conventional techniques, such as by using welding heads 56 which apply force and electric current to both ends of the pellet 34, as diagrammatically depicted in FIG. 6. Other techniques could also be used to attach the cap 52 to body 46. By using the end webs 50,52, the pellet 34 can be thinner walled but yet more resistant to external pressure than a purely annular pellet without solid ends due to the radial support provided by the webs at each end of the pellet 34. In summary, the annular pellet body 46 within its ends sealed by webs 50,52 encapsulates the central void 48, minimizes the mass of material in the displacer rod 30 and assures that the rod 30 will not be flooded in the event of tube rupture. FIGS. 7-11 illustrate alternative forms of the pellet 34. In FIG. 7, a pellet 58 is composed of a pair of identical halves which each include a short body portion 60 and a solid web 62 integrally formed on one end of the body portion. Then the two body portions 60 are welded together at their respective open ends 63 so as to seal the void 64 defined by the welded pellet halves. FIG. 8 illustrates a pellet 66 substantially identical to the pellet 34 of FIG. 4, but having the added feature of its void 67 being prepressurized with an inert gas through a passage 68 which is then welded shut at 70. This would allow the pellet 66 to have an even thinner wall and still withstand the same leve of external pressure as pellet 34. FIG. 9 shows a pellet 72 in which each of the webs is a cap 74 similar to the cap 52 of the pellet 34. As in the case of cap 52, each cap 74 is attached, such as by electric welding, to an opposite end of the hollow body 76 to form the pellet 72. FIG. 10 illustrates a pellet 78 having the configuration of the pellet 58 of FIG. 7, but being formed of two identical pieces each of which is composed of a hollow annular body 80 with an integrally connected end solid web 82 which are generally similar to the body 46 and integral web 52 of the pellet 34. The two pieces are then welded together at their respective open ends to seal the void 83 formed by the pieces. The pellet 78 is approximately twice the length of the pellet 58 shown in FIG. 7. Finally, FIG. 11 shows a pellet 84 which is substantially identical to pellet 34 absent its end cap 52. The pellet 84 does not encapsulate its central void 86 but its integral web 88 does provide radial support. Also, when the pellets 84 are disposed in a stacked relationship within a displacer rod 80 with each pellet 84 oriented in the same way in the stack with respect to adjacent pellets, the end solid web 88 of each pellet 84 contacts the open end 90 of the next pellet 84 in the stack thereof such that, in effect, the respective voids 86 are individually enclosed, one from the next. It is thought that the improved water displacer rod of 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 examplary embodiment thereof.