Patent Number: 043115608
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the upper portion of a fuel assembly 10 held in place at the top by a fuel assembly alignment plate 12 and at the bottom by a lower fuel support plate (not shown). The assembly 10 includes a plurality of guide tubes 14 extending from the alignment plate 12 to the lower support plate, and a plurality of axially spaced grids 16 connected to the guide tubes 14. The grids 16 define a matrix of support springs (not shown) for spacing and supporting a plurality of fuel elements 18 associated with the assembly 10. The power level of the reactor is usually regulated by the insertion and withdrawal of control rods 20. In modern pressurized water reactors, each fuel assembly 10 has guide tubes 14 adapted to receive control rods 20 over the entire length of the assembly. The control rod 20 is rigidly connected at its upper end to a drive mechanism (not shown) and, because it is quite elongated (15 to 20 feet long and less than 1 inch in diameter), the rod 20 is often not precisely centered within the guide tube 14. When the rod 20 is in the fully withdrawn position 22, the rod tip 24 is still within the guide tube 14. When the control rod 20 is more fully inserted into the reactor core for absorbing neutrons, it will generate heat. The rod 20 is cooled by the upward flow of liquid coolant which enters the bottom of the guide tube 14, flows upward therein over the control rod 20, through the guide tube exit 26 and upward through shroud tubes 28 to be deposited in the upper portion of the reactor vessel (not shown). Inspection of fuel assemblies 10 removed from operating nuclear reactors has shown patterns of wear on the inner surface of the guide tubes 14 containing control rods 20 at precisely the position 22 corresponding to the elevation of the control rod tip 24 when the rod 20 is in the unique withdrawn position. The present invention provides a spring device 30 extending from the tip 24 of the poison-bearing portion of the control rod, for essentially eliminating guide tube wear. FIG. 2 shows the lower portion of the control rod 20 within the guide tube 14 such that the control rod tip 24 is in the unique withdrawn position 22. The control rod tip 24 provides a seal on the lower end of the metal cladding of control rod 20 to isolate the poison-bearing pellets 32 from the liquid flowing within the guide tube. In one embodiment of the invention, the spring device 30 has a base portion 34 which is welded at 35 to the control rod tip 24 so that the device is rigidly connected thereto. In control rods to be built in the future, the base portion 34 may be integral with the material that seals the poison pellets 32 within the rod. A plurality of longitudinally extending projections 36 depend from the base 34, forming cantilever springs. The lower ends of the projection 36 have shoulders 38 formed thereon for contacting the guide tube wall 40 to provide an interference fit between the device 30 and the wall. As can be seen in FIG. 3, the shoulders 38 on the cantilevered extensions 36 are urged against the wall 40 of the guide tube 14 because of the inherent stiffness of the extensions 36. The device 30 is manufactured to have a nominal diameter across shoulders 38 in the relaxed state which is larger than the inner diameter of the guide tube 14. Thus, when the device is inserted into the guide tube 14, a uniform radial biasing force will tend to center the control rod 20 and control rod tip 24 within the guide tube. In a typical embodiment where the guide tube inner diameter is less than one inch, the diametral interference fit is about 0.010 inches with a device made from Inconel X 750. In the configuration shown in FIGS. 2 and 3, each shoulder 38 provides about 2 pounds of radial biasing force. This force is distributed over a relatively large surface area of the guide tube wall 40, and thereby prevents any localized wearing of the tube in the event the device experiences unexpected oscillation. For a tube 14 having an inner diameter d, a satisfactory device 30 of the type described above has four shoulders spanning about 270.degree. of the circumference of the wall 40. The axial extent of the portion of the shoulder 38 that contacts the wall 40 is approximately d/2. It is believed that the device 30 can be made to perform most satisfactorily when the ratio of the total contact area of the shoulders 38 to the cross-sectional area of the guide tube 14 is at least 1/2. Depending on the magnitude of the coolant flow, the weight of the control rod 20, and the surface area of the shoulders 38, the spring constant at each shoulder 38 is believed most effective if in the range of 0.01 to 0.05 inches per pound. It should be understood that the device 30 must be very short in relation to the length of the control rod in order to assure that the poison pellets 32 can be inserted completely within the fuel assembly. If the device 30 is more than a few inches long, the reactivity worth, or neutron absorption power, of the control rod could be significantly reduced. Inconel X750 has been found to provide the desired spring rates in a device about two inches long, and this material does not relax after being irradiated as do other materials such as Zircaloy. Another consideration in the satisfactory performance of the device 30 is a provision for allowing the upward flowing coolant to pass over the device without experiencing a significant pressure drop. As shown in FIGS. 2 and 3, the extensions 36 extend longitudinally from the outer circumference of the base 34, and as a group form an elongated member having an open-ended bottom and a hollow inner region 42. Coolant flowing upward through the guide tube 14 enters the open end 44, enters the hollow region 42 and exits the device through holes 46 formed between the individual extensions 36. In addition, the lateral spacing of the extensions 36 forms passages 48 which also permit the liquid coolant to bypass the shoulders 38. One important feature of the illustrated embodiment having cantilever springs 36 is the ability of the device 30 to be severely displaced to one side of the guide tube without exceeding the elastic limit of the spring 36. Given the requirement that the device be only a few inches long, it is not a simple matter to provide springs that have the required force supplied over a distributed area, without the spring being subject to unwanted buckling or inelastic deformation which could effect the spring rate and destroy the uniformity of the interference fit. Each cantilevered spring 36 shown in FIG. 2 has an unrestrained end 44 which is free to move axially to accommodate a large radial distortion. Another requirement on the device is that the interference fit not be so tight as to significantly retard the scram time of the control rod. The scram time could also be affected if the pressure drop across the device were too large. The present invention satisfies these requirements, and, in addition, is compatible with current fuel assembly designs wherein the control rod buffering associated with a scram occurs at the bottom of the guide tube. The flexible cantilevered shoulders 38 can slide into the dash pot (not shown) at the bottom of the guide tube 14 and thereby assure that the poison material 32 will not be stopped above the lower end of the fuel. Referring now to FIGS. 4a and b, there is shown in graphical form a summary of the mode shapes for the relative acceleration amplitudes of a standard control rod and a control rod having the invention described above. The data appearing on these graphs was generated in a laboratory mock-up of a guide tube having an inner diameter of 0.900 inches and a rod having an outer diameter of 0.816 inches containing a distributed mass equivalent to an actual control rod. The control rod was rigidly held at its upper end and the control rod tip was located 22 inches below the guide tube exit 26 (FIG. 1). Acceleration measurements were made at one foot intervals starting at the control rod tip (zero on the ordinate) over the entire length of the control rod (18 feet). The relative acceleration (g's) of the control rod at different elevations, as a function of the frequency of the vibrating tip (Hz) are plotted. In FIG. 4a, it can be seen that at the control rod tip significant accelerations occur at less than 1 at 4.5, 9.5, 14, and 28.5 Hz. The severity of the rod oscillation at a given elevation on the control rod is related to the sum of the acceleration amplitudes at the various frequencies at that elevation. With the standard control rod, the total oscillation at the control rod tip has contributions from less than 1, 4.5, 9.5 and 14 Hz. FIG. 4b shows the same test performed with a control rod having the inventive device 30. For the same liquid coolant flow rate (4500 pounds per hour), and, therefore, about the same driving force, the improved control rod displayed measurable accelerations only at 1 Hz or less, and at the control rod tip there was no measureable acceleration. Furthermore, the accelerations at higher elevations in the control rod were all small as compared with the total acceleration at a given elevation as shown in FIG. 4a. Thus, the invention not only eliminates vibrations at the control rod tip, but also reduces vibrations over the full length of the control rod. Referring now to FIG. 5, an alternate embodiment of the invention is shown wherein the lower end of the extensions 36' are connected to a common ring 50 shown in section in FIG. 6. The shoulders 38' are intermediate the ends of the elongated members 36'. This embodiment may be used where a different spring constant or spring pressure is required that can be provided with the embodiment shown in FIGS. 2 and 3. A severe radial distortion will still not permanently deform the cantilever springs 36' even though these are connected to a common ring 50 because the ring can move axially to accommodate such a distortion. As shown in FIG. 7, the shoulders 38' deflect inward a few mils when the device is inserted in the guide tube 14 so that the interference fit is provided.