Patent Number: 
Section: description

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 20a and 20b 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 20a 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 20a is secured, for example, by welding to the upper end of the socket 24. In FIG. 5B, there is illustrated a similar transition piece 20b which is heavier than the transition piece 20a of FIG. 5A. Particularly, transition piece 20b is similar in external configuration to the transition piece 20a. However, transition piece 20b 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 20a is illustrated in the heavier transition piece 20b to indicate the extent of the increase in material and, hence, weight of the heavier transition piece 20b as compared with the lighter transition piece 20a. Transition pieces are preferably formed of a stainless steel material. Referring to FIGS. 7A, 7B, 8A and 8B, there are illustrated two sets of fins 18. The first set of fins 18a 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 18b 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 18a 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 18b are heavier than the first set of fins 18a. 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 18b as compared with the lighter fins 18a, 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 20a, and the lightest of the fin sets 18, i.e., fin set 18a, 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 20b may be selected and combined with the heavier set of fins 18b. 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 20b. 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 20b can be removed, down to the weight of the lightest transition piece 20a. 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 20a to render it heavier. For example, weld material could be suitably secured along the interior wall of the transition piece 20a 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.