Abstract:
The control rod includes an upper absorber section and a lower velocity limiter comprised of a socket, a vane, a transition piece and a set of fins. To adjust the weight of the control rod, lighter and heavier sets of fins, as well as lighter and heavier transition pieces are provided. By suitable selection of the lighter and heavier sets of fins and the lighter and heavier transition pieces, the weight of the velocity limiter can be selectively adjusted in accordance with the weights of differently designed sections to obtain a predetermined control rod weight. Additionally, the heavier transition piece may have material removed therefrom to decrease its weight to provide an infinite adjustment of weight within a predetermined range of weights for the velocity limiter.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to control rods for nuclear reactors having an upper, generally cruciform cross-section and a lower velocity limiter secured to the upper section and particularly relates to apparatus and methods for selectively adjusting the weight of the velocity limiter whereby the overall control rod weight can be adjusted. 
     Control rods in nuclear reactors perform dual functions of power distribution shaping and reactivity control. The rods enter from the bottom of the reactor and are typically connected to bottom-mounted, hydraulically-actuated drive mechanisms to allow axial positioning for reactivity regulation or rapid SCRAM insertion. The control rods are generally cruciform in cross-sectional shape, with each blade or wing of the rod typically containing tubes filled with a nuclear poison such as boron carbide or hafnium. The lower portion of the control rod comprises a velocity limiter disposed in a guide tube. Velocity limiters are conventionally designed to limit the drop velocity to a selected velocity, for example, 3.11 feet per second, minimize drag resistance during a SCRAM event, provide an interface between the control rod and control rod guide tube and provide a coupling between the control rod blade absorber sections and the control rod drive. 
     Historically, a single velocity limiter was developed to satisfy the foregoing design criteria. Newer control blade absorber sections, however, have been designed, each with widely differing load patterns and neutron-absorbing materials. This has resulted in some control rod absorber sections being heavier than the original design and some being lighter. Because control rod weight limits are defined by what the specific plant is licensed to use, two different types of velocity limiters became available for use with any one of the different control rod designs. Specifically, a light and a heavy velocity limiter were designed for use with any one of the control rod designs. The selection of which of the lighter or heavier velocity limiters for a particular control rod design was based on the weight of the control blade absorber section, i.e., a light velocity limiter was used with a heavier absorber section and, conversely, the heavy velocity limiter was used with a lighter absorber section. There has therefore developed a need for a single velocity limiter which enables the weight of the limiter to be modified without affecting the critical external geometric or fluid mechanical properties of the control rod such that the overall weight of the control rod can be closely controlled to meet design and licensing specifications. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, a velocity limiter for a control rod for a nuclear reactor is provided which can be adjusted in weight within a predetermined range such that the velocity limiter with a selectively adjusted weight can be used with any one of the various and different control rod absorber section designs. The specific weight of the velocity limiter is determined by the weight of the neutron absorber section, i.e., with a heavier absorber section, a lighter velocity limiter is provided and, conversely, with a lighter absorber section, a heavier velocity limiter is provided. 
     To provide a substantially infinitely adjustable weight of velocity limiter within a predetermined range of weights, the present invention provides, as part of the velocity limiter, a standard vane, a set of light or heavy transition pieces, a socket and a set of light or heavy fins. By selecting one or the other of the light or heavy transition pieces and combining the selected transition piece with one or the other of a selected one of the light or heavy sets of fins, an approximation of the desired weight for the velocity limiter and, hence, the overall control rod, can be obtained. To provide a further fine adjustment in weight to match a target or originally licensed control rod weight, the heavy transition piece can be modified such that its weight lies within a range from approximately the weight of the light transition piece to the weight of the heavy transition piece without modification. Each transition piece includes an enlarged diameter central section having a bore therethrough. The heavier transition piece has a small bore whereas the light transition piece has a radially enlarged bore. By removing material from within the bore of the heavy transition piece, the weight of the heavy transition piece can be adjusted downwardly within a weight range substantially between the weight of the light and heavy transition pieces. Consequently, by combining the heavier transition piece with the socket and one of the sets of fins, those elements can be assembled with the vane and socket to form a velocity limiter of desired weight such that the overall weight of the control rod approaches the target or licensed weight. 
     In a preferred embodiment according to the present invention, there is provided a method of adjusting the weight of a control rod for use in a nuclear reactor, the control rod having an upper, generally cruciform absorber section and a lower velocity limiter attached to the section and including a vane, a transition piece and fins, comprising the steps of providing first and second sets of fins with the second set of fins being heavier than the first set of fins, providing first and second transition pieces, with the second transition piece being heavier than the first transition piece, selecting one of the first and second sets of fins and one of the first and second sets of transition pieces and forming a velocity limiter for the control rod containing the vane and the selected ones of the fins and the transition pieces thereby selectively adjusting the weight of the control rod. 
     In a further preferred embodiment according to the present invention, here is provided apparatus for selectively adjusting the weight of a control rod for use in a nuclear reactor comprising an upper control rod body, a lower velocity limiter including a vane, a first set of fins, a second set of fins heavier than the first set of fins, a first transition piece, a second transition piece heavier than the first transition piece, the control rod being formed with one of the first and second sets of fins secured to the vane and one of the first and second transition pieces secured to the vane whereby a control rod of selected adjusted weight is formed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a control rod constructed in accordance with the present invention; 
     FIGS. 2 and 3 are enlarged portions of light and heavy velocity limiters, respectively, parts being broken out and in cross-section for ease of illustration; 
     FIG. 4 is an enlarged side elevational view of a vane employed in the velocity limiter hereof with parts broken out and in cross-section; 
     FIGS. 5A and 5B are fragmentary side elevational views with parts broken out and in cross-section of light and heavy transition pieces, respectively, for selective use in the velocity limiter hereof; 
     FIG. 6 is a cross-sectional view of a socket employed in the present invention; and 
     FIGS. 7A and 8A are side elevational views of light and heavy sets of fins, respectively, with FIGS. 7B and 8B illustrating corresponding respective halves of those fins. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is illustrated a control rod constructed in accordance with the present invention and including an upper cruciform absorber section or body  12  and a lower velocity limiter section  14 . The blades or wings  16  of the upper section  12  typically include tubes filled with a neutron poison such as boron carbide, the tubes conventionally extending vertically within each of the blades  16 . The lower velocity limiter is typically mounted in a control rod guide tube, not shown, for movement in a vertical direction between the correspondingly-shaped cruciform portions in the interstices of the core among the fuel bundle assemblies. The control rod is typically under hydraulic fluid control for movement vertically to allow axial positioning of the absorber section adjacent to the fuel bundle assemblies in the event of a SCRAM or for reactivity regulation. 
     Referring to FIGS. 2 and 3, the component parts of the velocity limiter  14  include a set of fins  18  aligned vertically with the blade  16  of the upper section of the control rod and transition piece  20  and a vane  22 , the transition piece mounting a socket  24  at its lower end. The socket  24  is adapted for connection to a control rod drive, not shown, whereby the control rod can be vertically positioned relative to the fuel bundle assemblies. 
     The vane  22  is illustrated in FIG.  4  and includes a generally frustoconical section having a hollow interior  26  open at its bottom for receiving the coolant/moderator. An annular turning vane  28  is disposed in the vane  22  and defines an annular flow channel  30  between the vane  28  and the upper portion of vane  22 . The vane  28  mounts a plurality of lugs  32  which mount rollers, not shown, for engaging along the interior surfaces of the control rod guide tube whereby the control rod can be displaced vertically. 
     Referring to FIGS. 5A and 5B, two separate transition pieces  20   a  and  20   b  are provided, only one of which is employed in the formation of the velocity limiter for a particular control rod. In FIG. 5A, the transition piece  20   a  is generally in the form of a hollow cylinder having a sleeve  34  projecting from its upper end. The cylinder of the transition piece is of a relatively thin wall. The lower end of the transition piece  20   a  is secured, for example, by welding to the upper end of the socket  24 . In FIG. 5B, there is illustrated a similar transition piece  20   b  which is heavier than the transition piece  20   a  of FIG.  5 A. Particularly, transition piece  20   b  is similar in external configuration to the transition piece  20   a . However, transition piece  20   b  is formed of a substantially solid interior with a small axial passageway  36 . An imaginary outline  38  of the interior wall of the lighter transition piece  20   a  is illustrated in the heavier transition piece  20   b  to indicate the extent of the increase in material and, hence, weight of the heavier transition piece  20   b  as compared with the lighter transition piece  20   a . Transition pieces are preferably formed of a stainless steel material. 
     Referring to FIGS. 7A,  7 B,  8 A and  8 B, there are illustrated two sets of fins  18 . The first set of fins  18 a comprise four fin halves which have short, vertically extending side edges  39  followed by long, downwardly inclined outer edges  40  terminating at their lower ends in contoured edges  42  for connection to the transition piece  20 . In FIGS. 8A and 8B, the fins  18 b are similarly formed except that the outside vertical edge  41  of each fin extends vertically downwardly to a greater extent than the outside vertical edge  39  of each of fins  18 a of FIGS. 7A and 7B. Also, the inclined edge  44 , which transitions between the vertical edge and the contoured edge  42  for seating on the transition piece is at a greater downwardly inclined angle to the vertical. Furthermore, the thickness of the fins shown in FIGS. 8A and 8B is substantially greater than the thickness of the set of fins shown in FIGS. 7A and 7B. It will be appreciated that upon comparing the configurations of the fin sets of FIGS. 7A and 7B with those of FIGS. 8A and 8B, the second sets of fins  18   b  are heavier than the first set of fins  18   a . The increase in weight is due solely to the increase in material forming the fins. While there is an increased surface area for the heavier fins  18   b  as compared with the lighter fins  18   a , the external geometry is not altered sufficiently to have significant extent on the fluid mechanical properties of the velocity limiter. It will also be appreciated that four fin halves of the selected set are used at right angles to one another for each control rod. 
     To provide a control rod of selectively adjustable weight corresponding to a target or licensed weight, the weight of the velocity limiter is determined by the weight of the neutron absorber section, i.e., upper section  12 . The total weight of the control rod will be the combined weight of the upper section  12  and the velocity limiter  14 . Given the weight of the upper section which can vary substantially with respect to the design of the upper control rod section, the total control rod weight can be adjusted to meet the target or licensing weight by adjusting the weight of the velocity limiter. To accomplish this for a given known weight of the control rod absorber section, a combination of the selected transition pieces and fins will be made to approximate the desired weight. For example, if the upper section of the control rod is heavy, the lightest of the transition pieces, i.e., transition piece  20   a , and the lightest of the fin sets  18 , i.e., fin set  18   a , may be selected for combination with the vane  22  and socket  24 . This affords the lightest control rod for the given heavy weight of absorber section. Conversely, for a control rod design having an extremely light absorber section, the heavier velocity limiter  20   b  may be selected and combined with the heavier set of fins  18   b . This affords the heaviest control rod for the given light weight of absorber section. By forming two sets each of heavier and lighter transition pieces and fins interchangeable for use with the vane and socket, it will be appreciated that the weight of the velocity limiter can be varied between a maximum and minimum range and with a mix of the light and heavier fin sets and transition pieces. That is, to meet weights in-between the maximum and minimum weights of the velocity limiter, a different combination of fin sets and transition pieces can be utilized, i.e., a heavy transition piece with a set of light fins or a light transition piece with a set of heavy fins. 
     Additionally, once a fin set and the heavier transition piece are selected from the light and heavier fin sets and transition pieces, respectively, the weight of the heaviest transition piece can be adjusted, i.e., lowered. Thus, if the heavier transition piece is selected, a weight between it and the weight of the lighter transition piece can be provided by removing material from within the bore of the transition piece  20   b . From a review of FIG. 5B, it will be appreciated that more or less of the material within the bore  36  of the heaviest transition piece  20   b  can be removed, down to the weight of the lightest transition piece  20   a . The extent of passable removal of the material is illustrated in FIG. 5B by the material volume between bore  36  and the dashed lines  38 . Consequently, greater control over the total control rod weight is achieved by using selected transition pieces and sets of fins and further adjustment of the weight of the transition piece should the heavier transition piece be selected. It will also be appreciated that additional material can be provided the lighter transition piece  20   a  to render it heavier. For example, weld material could be suitably secured along the interior wall of the transition piece  20   a  to increase its weight. Accordingly, there has been provided a system for adjusting the weight of a control rod within predetermined maximum and minimum limits to arrive at a target weight. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.