Patent Number: 053612795
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates a boiling water reactor pressure vessel suitable for housing the internal control rod drive of the present invention. The pressure vessel 20 has a top head 21 and a bottom head 22. The nuclear core 25 is positioned on a core plate 26. A matrix of fuel bundles 28 is arranged within the core. The bundles are spaced sufficiently so that a cruciform shaped control rod blade 32 of control rod 31 can be slid back and forth in the region between the channels to control the reactor output. A top guide plate 35 is positioned near the top end of the fuel bundles to help position the channels. The channels may extend well above the fuel bundles to form a chimney 37. Of course, the invention can be used with other conventional reactors designs, as for example, designs wherein the chimneys are eliminated and/or larger channels are provided that contain a plurality of fuel bundles and/or in conjunction with a cluster type core wherein finger type control rods are used. An open control rod drive grid 40 is positioned a distance above the top guide plate 35. The spacing between the tops of the fuel bundles and the grid 40 is sufficient so that substantially the entire control rod blade 32 can be lifted above the core 25. A multiplicity of control rod drives (CRDs) 30 are mounted on the control rod drive grid as best seen in FIG. 2. Specifically, one CRD 30 is positioned above each control rod 31. In the embodiment of the invention shown in FIG. 1, a multiplicity of standpipes 42 are positioned somewhat above the control rod drive grid 40 with each standpipe being arranged to receive the jack rod 34 of an associated control rod drive 30. The standpipes 42 are provided with an internal guide tube to contain the jack rods and to control flow, induced vibration in the jack rods. In alternative embodiments, these guide tubes could be arranged between the standpipes. Conventional steam separators 44 are positioned above the stand pipes 42 and a conventional steam dryer 45 is positioned above the steam separators and typically within the top head 21. Referring next to FIG. 2, the construction of a first embodiment of the control rod drives 30 will be described. Each control rod drive 30 includes a jack rod 34, a connector 36 and a hydraulic jack 38. The connector 36 is arranged to couple the jack rod 34 to the control rod 31. In alternative embodiments, the jack rod could be formed integrally with the control rod. The jack rod 34 takes the form of a notched shaft which is designed to cooperate with the latches of the hydraulic jack 38 to create a ratchet type mechanism. The hydraulic jack 38 includes a holding mechanism 50 for holding the jack rod 34 in a stationary position and a lifting mechanism 60 for lifting and lowering the jack rod 34. Both the lifting mechanism and the holding mechanism are housed within a casing 59 and are hydraulically operated. The holding mechanism 50 includes a pair of pivotally mounted latch fingers 51 that are respectively coupled to sliding members 53 by pivotal linkages 52 and to a fixed support 57 by pivots 58. The latch fingers 51 each include a holder 54 that is adapted to engage the notches in the jack rod 34. In the described embodiment, the holders 54 take the form of holding pins, but it should be appreciated that latches, pins or other suitable mechanism may all be used as holders within the scope of this invention. The sliding members 53 are biased in a downward direction by biasing spring 55. A hold piston 56 is positioned under the sliding members 53 for pushing the sliding members upward. The hold piston 56 in turn is supplied by a hydraulic holding line 71. Thus, the sliding members 53 are biased downward by the biasing spring 55, and forced upward by the hold piston 56 when the holding line 71 is pressurized. When no pressure is exerted by the hold piston 56 against the sliding members 53, then the biasing spring 55 will push the sliding members downward. This movement will release the latch fingers 51 which in turn release the jack rod. When a significant pressure is applied in the hydraulic holding line 71, then the hold piston 56 pushes the sliding members 53 upward against the force of the biasing spring 55, thereby causing the latch fingers 51 to pivot into the engaging position which firmly holds the jack rod in place. The lifting mechanism 60 has a pair of latch fingers 61 that are respectively pivotally mounted to a pair of sliders 63 by pivots 62. The latch fingers 61 each have a lifter 69 thereon that is adapted to engage the notches in the jack rod 34. Like the holders 54, the lifters 69 in the described embodiment are lifting pins but may take the form of latches or other suitable mechanisms. The sliders 63 are free to move a limited distance within casing 59. The free end of each latch finger 61 is coupled to a sliding member 65 by a linkage 64 that is pivotal on each end. The sliders 63 are biased in a downward direction by biasing spring 66. The sliding members 65 are each biased in a downward direction by a biasing spring 67 positioned between the housing and the lifting mechanism. A lift piston 68 is positioned under the sliding members 65 such that when actuated, it can push the sliding members upward against the force of biasing spring 67. This action serves to pivot the latch fingers from a release position into an engaging position. When no pressure is exerted by the lift piston 68 against the sliding members 65, then the biasing spring 67 will push the sliding members 65 downward. This movement will release the latch fingers 61 which in turn will release the jack rod. When a significant pressure is applied in the hydraulic lifting line 73, then the lift piston 68 pushes the sliding members 65 upward against the force of the biasing spring 67, thereby causing the latch fingers 61 to pivot into the engaging position. When the holding pins 54 are released and the lifting pins 69 are engaged, then the movement of the jack rod can be controlled by the pressure within lifting line 73. Specifically, if a high pressure is applied against the lift piston 68, then the piston will stroke upward with the lifting pins 69 engaged, thereby moving the sliders upwards against the springs 66, which lifts the jack rod by a notch. On the other hand, if only a moderate pressure is applied against the lift piston 68, then the weight of the control rod combined with the force of spring 66 will push the sliders 63 downward and thus the lift piston will stroke downward. In this manner the jack rod can be lowered by a notch. The described embodiment requires two hydraulic lines to operate each jack mechanism. By way of example, in a reactor that employs two hundred cruciform shaped control rods, two hundred control rod drives may be used. In such an embodiment, four hundred hydraulic lines would be necessary. An alternative embodiment of the invention which uses an addressing system to reduce the number of hydraulic lines required will next be described referring to FIG. 3. In this embodiment, modified holding and lifting mechanisms are used and a hold control valve 180 is added to each drive. Specifically, the hold control valve 180 has a piston 182 that is biased in a first direction by biasing spring 184. The side of the piston opposite the biasing spring has a plunger 186 that has a much narrower diameter then the piston 182. The position of the piston is influenced by three factors. They include the biasing spring 184, the pressure in lifting line 73 which acts against the piston 182 in a direction opposite the biasing spring, and the pressure of the holding line 71 which also acts against the plunger 186 in a direction opposite to the biasing spring. The piston is moveable between open and closed positions. In the open position, a communication path is formed between the holding line 71 and a control line 175. The piston 156 in holding mechanism 150 is influenced by three forces as well. These forces include a biasing spring 155, the pressure in control line 175 and the pressure in holding line 71. The biasing spring 155 and the pressure in control line 175 urge the hold piston 156 towards a disengaged position while the pressure in holding line 71 urges the piston towards a closed position. The surface area of the piston 156 that is influenced by the control line 175 is somewhat larger than the area influenced by holding line 71. The pressure in control line 175 will be very similar to the pressure in holding line 71 when the hold control valve 180 is opened. Therefore, the position of the hold piston 156 will effectively be determined by whether the hold control valve 180 is open, as long as there is pressure in holding line 71. If all pressure is removed from the holding line, the biasing spring 155 will move the piston to the withdrawn position. The lifting mechanism 160 effectively includes a lifting piston 165 and a lift cylinder 168. The position of each of these components is influenced by the pressure in lifting line 73. The pin positioning piston 165 is influenced by a biasing spring 164 and the pressure in lifting line 73. Specifically, as long as there is a significant pressure within the lifting line, the pin positioning piston will remain in the engaged position. When pressure is removed from the lifting line, the lifting pins 169 will move to the disengaged position under the influence of biasing spring 164. The lift cylinder 168 is influenced by a biasing spring 166 and the pressure in lifting line 73, as well as the engagement of the holding pins 154. Specifically, when the holding pins 154 are engaged, the jack rod will remain in place regardless of the pressure exerted in the lifting line 73. However, when the holding pins 154 are disengaged and the lifting pins 169 are engaged, then the full weight of the jack rod 34 and the control rod will be borne by the lift cylinder 168. In this case, if a high pressure is applied in lifting line 73, then the lift cylinder will stroke upward and the jack rod 34 will be lifted a notch. On the other hand, if a medium pressure is applied to the lifting line 73, then the weight of the control rod would cause the lift cylinder 168 to stroke downward, thereby lowering the control rod a notch. With the described logic, if a high pressure is applied to both the lifting and holding lines, the jack rod 34 will be lifted one notch. Specifically, the hold control valve 180 is opened against the force of biasing spring 184, which introduces a high pressure in the region behind the holding piston 156. Thus, the holding piston 156 will stroke towards the disengaged position, which serves to release the holding pin 154 from the jack rod 34. On the other hand, the high pressure on the lifting piston 165 and the lift cylinder 168 causes the lifting pins 169 to engage the jack rod and the lifting cylinder to stroke upwards, thereby moving the jack rod upwards. The jack rod 34 may be lowered a notch by keeping the pressure in the holding line 71 high while asserting a medium pressure in the lifting line 73. In this condition, the hold control valve is again opened by the combined forces of the pressures within the holding and lifting lines acting against the biasing spring 184. However since a medium pressure is asserted against the lifting piston, the weight of the control rod will stroke the lift cylinder downwards, which will step the jack rod down a notch. If no pressure is applied to the lifting line, then the lifting piston 165 will disengage. At the same time, the hold control valve 180 will close and the pressure in the holding line will stroke the hold piston 156 which serves to lock the holding pin 154 in place. This is true regardless of whether a high or medium pressure is applied in the holding line 73. On the other hand, if no significant pressure is present on either the holding line or the lifting line, then the force of biasing spring 155 will stroke the holding piston to the right (as seen in FIG. 3) which releases the holding pin 154. Thus, when no pressure is applied to either line, both pins will release and gravity will cause the control rod to fall into the core. Thus, the system is failsafe in that in the event of a power loss or other control failure, the control rods will automatically descend into the core. Any time that a medium pressure is applied to the holding line, the hold control valve 180 will be closed. Thus, the pressure of the holding line 71 will stroke the holding piston in a manner that moves the holding pin 154 into the engaged position. In this condition, regardless of the pressure on the lifting line, the jack rod will be firmly held in place. Any time that the holding line does not have any pressure, the holding pin will disengage from the jack rod. In this condition the jack rod could be lifted or lowered by varying the pressure in the lifting line 73 between the high and medium pressures. However, in the current embodiment, this type of control is not used. With the described arrangement, it can be seen that an addressing system can be arranged that would provide individual control of the control rod drives, but eliminate the need to use dedicated holding and lifting lines. Specifically, in such an addressing grid arrangement, a plurality of holding line are provided with each holding line being connected to a row of drives. Similarly, a plurality of lifting lines are provided with each lifting line being connected to a column of drives. In the steady state, the holding lines would be pressurized with a medium pressure, while the lifting lines would not be pressurized at all. As explained above, with this arrangement, all of the drives would hold their associated jack rod firmly in place. When a particular control rod is to be lifted or lowered, the pressure in its associated holding line is increased to a high pressure. The remaining holding lines remain at the medium pressure. The pressure in the selected control rods lifting line is then adjusted to either lift the control rod (by applying a high pressure) or to lower the control rod (by applying a medium pressure. The remaining lifting lines will remain unpressurized. It is noted that since all of the holding lines other than the selected holding line are at a medium pressure, their associated drives in the selected column will not move, regardless of the pressure in the selected lifting line. Similarly, since all of the lifting lines other than the selected lifting line are unpressurized, the unselected drives in the selected row will not move. Thus, with the described arrangement, true addressing can be used to control the control rod drives. With this arrangement, a total of 30 hydraulic lines (15 lifting lines and 15 holding lines) can be used to control a system having over 200 drives. It will be appreciated by those skilled in the art that this is a significant improvement and would substantially simplify the system's plumbing. Referring next to FIGS. 4-6, a variety of control rod drive mounting arrangements will be described. The first described embodiment is suitable for use in a reactor that employs cruciform shaped control rods. As seen in FIG. 4, the grid 40 is an open matrix of beams 81 having mounting surfaces 80 formed at each beam intersection. The mounting surfaces are designed to correspond in shape to the shape of the bottom end of the control rod drive. A cruciform slot 83 is formed in each mounting surface 80 for receiving an associated control rod blade 32. Each control rod drive 30 also has a corresponding cruciform slot 84 through which the jack rod and the control rod blade may pass. In the illustrated embodiment, the cross section of the control rod drive casings 59, and the mounting surface are substantially cruciform in shape as seen in the drawings. The lifting and holding mechanisms are positioned in the region 85 formed between adjacent arms 86 of the cruciform casing. The mounting surface 80 includes a pair of raised hydraulic ports 90 and a pair of raised mounting guides 92. The hydraulic ports 90 are designed to fit into matching female ports on the bottom surface of the control rod drive. Similarly, the mounting guides are received by matching indentations in the bottom surface of the control rod drive. Thus, the raised ports 90 and the raised mounting guides 92 cooperate to position the drive 30 on the mounting surface. Then a plurality of bolts or other fasteners (not shown) are used to secure the drive to the mounting surface. The hydraulic supply lines 93 (which include holding lines 71 and lifting lines 73) may be strung along the sides of the beams 81. An alternative embodiment of the grid 40 is shown in FIG. 5. This embodiment also has an open matrix structure and is adapted for use in a system having large channels with chimneys extending therefrom and wherein each channel houses a group of four fuel bundles. The control rods used in this embodiment are designed to extend into the spaces between adjacent fuel channels, as seen in FIG. 5. In this embodiment, the grid 40 takes the form of a top beam grid having a multiplicity of mounting surfaces 80 that are similar to those described above with respect to the previous embodiment. A plurality of side beams 95 extend downward from the top beams 94. The side beams extend outward in pairs that run in the direction of each arm 86 of the cruciform drive casing. The parallel side beams are spaced apart a distance that is slightly wider than the walls of chimney 37 and have flared lowered ends 97 that insure an easy fit over the top end of the chimneys. The grid is positioned such that the top of the chimneys are positioned slightly below the top beams 94. With this arrangement, during installation the grid can simply be lowered into place over the top edge of the chimney walls. This permits replacement of the chimneys upon removal of the top beam grid. Another alternative grid structure is shown in FIGS. 6 and 7. This embodiment is particularly well adapted for use in cluster type core configurations. In such configurations, it is advantageous if each jack rod 34 carries a plurality of finger type control rods 100 using a spider 102 positioned below the grid 40, as best seen in FIG. 7. The holes in the center of the mounting surface 80 and the drive 30 are each cylindrical in nature. As such, they only pass the jack rod itself. As best seen in FIG. 7, a hydraulic coupling 105 can be used to connect the jack rod 34 to the spider 102. The grid takes the form of an open matrix formed by I-beams. The positioning and mounting of the drive is accomplished in the same manner as the previously described grids wherein raised mounting guides 92 and raised hydraulic ports 90 cooperate with matching recesses and recessed ports on the drive to position the drive and bolts or other fasteners are used to secure the drive. The hydraulic lines 93 are strung along the web portion of the I-beam. In other respects, the grid is similar to those previously described. The described hydraulic jack can be very compact in size. By way of example, a suitable overall drive height is about two feet. The reduced size of the CRD permits a substantial decrease in the required pressure vessel and containment building heights. By way of example, the described control rod drive system has the potential of reducing the containment height by three times the core height (i.e. approximately 36 feet) and the pressure vessel height by one core length when compared to conventional BWR control rod drive systems. The use of internal jacks also has the advantage of significantly reducing the vessel penetrations in both size and number. By way of example, four penetrations of less than 10 inches in diameter around the periphery of the pressure vessel would be sufficient to handle all of the required hydraulic lines for an array of 200 control rod drives even if two lines are provided for each drive. This must be compared with the 200 six-inch-diameter penetrations that would be required for conventional CRDs. This reduces vessel fabrication costs and significantly reduces the amount of in service inspection required. The size of the openings is even further reduced if the described addressing system is used. By way of example, just two four-inch-diameter penetrations would be sufficient to control 200 control rod drives if the addressing system were used. The described control rod drive is failsafe in operation. Thus, in the event of a rupture of one of the lines or a loss of pressure, the control rods would automatically fall into the core. Since the drive is internal to the reactor pressure vessel, none of the components need to be pressure retaining, which serves to substantially reduce the cost of the drives themselves. Although only a few embodiments of the invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.