Patent Number: 039895890
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the description which follows, like reference characters indicate like elements in the various figures of the drawings. FIG. 1 illustrates a nuclear reactor incorporating the hydraulic control rod drive mechanism provided by this invention. A reactor vessel 10 is shown which forms a hermetically sealed pressurized container when sealed by closure head 11. The reactor vessel 10 has coolant flow inlet nozzles (not shown) and coolant flow outlet nozzles 12 in and through the cylindrical wall thereof. The closure head 11 has a plurality of head penetration adaptors 13 extending through a substantially hemispherical wall and sealingly affixed thereto. A nuclear core comprising a plurality of fuel assemblies 14 is contained within the reactor vessel 10. For purposes of simplicity only two fuel assemblies 14 are shown. Each fuel assembly 14 includes a plurality of fuel rods 15 and a plurality of guide tubes 16 symmetrically interspersed therebetween. The fuel rods 15 and guide tubes 16 are held in a fixed relationship with respect to each other by an egg crate type of grid structure 17. This type of so-called canless fuel assembly 14 is well known in the art. The guide tubes 16 serve as guidance channels or receptacles for full length or part length control rods 18. In FIG. 1, again for purposes of simplicity, four control rods 18 are shown whereas as many as 20 individual control rods are used within a single fuel assembly 14 in a typical large nuclear reactor. This invention however is not thereby restricted to a particular number of control rods in any given fuel assembly. The worth of a single control rod 18 is so calibrated that the insertion of one or two such rods will not greatly change the power distribution of the entire core as would the insertion, for example, of a complete cluster of part length control rods. However, a single control rod 18 would have enough control rod worth to axially trim the power distribution in its immediate vicinity. The distribution of control rods 18 within a fuel assembly 14 is shown in FIG. 5. The part length control rods 18 are joined in pairs at their upper ends to thin gusset plates 19 to which drive shafts 20 are also attached. This can be seen more clearly in FIG. 6. It is to be realized, that although the figures show two control rods 18 attached to one drive shaft 20, either one or, in the alternative, more than two control rods 18 might be used with a single drive shaft 20. Control rod assemblies 21 are not used in all of the fuel assemblies 14; a large nuclear reactor will typically have eight fuel assemblies 14 equipped with part length control rod assemblies 21. In FIG. 1 it is seen that the plurality of drive shafts 20 associated with a fuel assembly 14 pass through a guide tube structure assembly 22. Guide tube structure 22 guides individual drive shafts 20 on passage through a reactor coolant outlet plenum 23 which is positioned above the nuclear core. The drive shaft guide structure 22 consists of a generally square vertical column 24 having guide plates 25 axially disposed therein. The guide tube support structure 22 extends between an upper core plate 26 and an upper support plate 27. The control rod drive shafts 20 pass within and through the reactor vessel head penetration adaptors 13 and terminate within hydraulic control rod drive mechanisms 28 as provided by this invention. The exact manner by which the control rod drive shafts 20 are guided and supported within the head penetration adaptor 13 is shown in FIG. 2. The drive shafts 20 are supported above the upper support plate 26 by a flange 29 to which is attached a vertical central shaft 30. Circular guide plates 31 are attached to the central shaft 30 such as by welding and are provided with holes 32 to support and guide the drive shafts 20. The funnel-shaped member 33 and the slots 34 in guide blocks 31 serve to facilitate assembly. The upper portion of the head penetration adaptor 13 is shown in detail in FIG. 3. A guide block 40 provides final guidance for the control rod drive shaft 20 just prior to exiting from the head penetration adaptor 13. A cylindrical lifting fixture 41 rests on top of guide block 40 and is oriented therewith by means of pin 42. The lifting fixture 41 is provided with vertical holes 43 with ledges 44 into which pistons 45 attached to the drive shafts 20 nest when the control rod assemblies 21 are in their full inserted position in the core. Any anticipated misalignment between the pistons 45 and their guide channels will be compensated for by a slight rotation of the lifting fixture 40. The control rod drive mechanism 28 is attached to the head penetration adaptor 13 as shown in FIGS. 1 and 3. The details of the control rod drive mechanism 28 are shown in FIGS. 3, 4, 7, 8, 9 and 10. Referring now to FIGS. 3 and 4, it is seen that the drive mechanism 28 comprises a pressure bearing member which includes a cylindrical base 50 having an outwardly extending flange 51 at its lower extremity and an upper cylindrical head cap 52 joined by welding to an intermediate heavy walled tube 53. The upper cylindrical head cap 52 is sealed by a threaded and steel welded plug 54 or other suitable means. Flange 51 of the mechanism is sealingly connected to flange 54 of the head penetration adaptor 13 thereby maintaining the pressure containing integrity of the reactor vessel 10. A plurality of cylinders are provided within the control rod drive mechanism; one cylinder being provided for each control rod assembly 21 being actuated by the drive mechanism 28. The cylinders comprise the channel formed by a machined hole 55 in base 50, a long tube 57 within intermediate tube 53 and a machined hole 56 in the head cap 52. The long tube 57 is fixedly supported in tight counter-bores in openings of holes 55 and 56 as shown in FIGS. 3 and 4. Between the counter-bore supporting points, the long tubes 57 are laterally supported by appropriate guide blocks (not shown) axially disposed with intermediate tube 53. As mentioned above, the control rod assemblies 21 are withdrawn from the core by differential hydraulic pressure acting on piston 45. The manner in which this is accomplished is shown in detail in FIG. 3. Referring now to that figure, the central space 60 directly below the flange 51 of the mechanism 28 is opened to the reactor vessel interior and is thereby pressurized to the pressure of the reactor coolant, which in the example shown is approximately 2000 psi. Space 60 is separated from a surrounding annular space 61 having a pressure of approximately 1700 psi. Pressurized areas 60 and 61 are conventionally sealed from each other by means of an O-ring gasket 62 acting in conjunction with tubular member 61'. Space 61 is connected to an exterior adjustable pressure source (not shown) by flow channels 63 machined within flange 54 of the head penetration adaptor 13 and tubing 64 connected thereto. Flow channel 65 within the flange 51 of the control rod drive mechanism 28 is connected at one side to space 61 and at the other side to one port 66 of an electromagnetically operated valve 69. As can be seen, valve 69 is attached to and supported by the lower portion or base 50 of the control rod drive mechanism 28. One valve 69 is provided for each control rod assembly 18. A second port 71 of valve 69 is flow connected to channel 70 which is machined in base 50. Ports 66 and 71 are sealingly separated by a seal connected valve stem 72 of the magnetic valve 69. Flow channel 70 is flow connected to the top of piston 45 by connecting tube 73 and holes 74 and 75, within the upper cylindrical head cap 52 of the mechanism (FIG. 4, FIG. 10). Withdrawal of the control rod assembly from the core is accomplished by energizing the coil of the magnetic valve 69 thereby lifting valve stem 72 from its seat. This has the effect of connecting the 1700 psi hydraulic pressure within space 61 to the top of piston 45. Since approximately 2000 psi hydraulic pressure acts on the lower part of piston 45, a net hydraulic pressure differential of approximately 300 psi acts on the control rod piston 45. This pressure differential causes the control rod assembly 21 to move at a rather high speed toward its fully withdrawn position. So as to lessen the shock of the control rod assembly reaching this final position, a self-damping decelerating mechanism is provided in the upper cylindrical head cap 52 as shown in FIG. 4. The decelerating device comprises a restriction 80 located in the upper part of the hydraulic cylinder 56 dimensioned to give a rather close diametrical clearance between the piston 45 and cylinder 56. A typical diametrical clearance might be of the order of ten to twenty-five thousandths of an inch. When the piston 45 reaches restriction 80, the water trapped above piston 45 is forced through the clearance gap thereby creating a considerable overpressure within the trapped water. This effectively and slowly decelerates the motion of the control rod assembly 21 as it reaches its fully withdrawn position. It is to be observed that restriction 80 has, of course, the effect of increasing the time required to fully insert the control rod assembly by gravity. However, this is a distinct advantage for part length control rods since as previously explained, full insertion of part length control rods has the effect of increasing reactivity of the core. On the other hand, the increased drop time of full length control rods may possibly be a disadvantage especially during reactor scramming operations when rapid insertion of these control rods is required. After a piston 45 has reached its upper limit of travel, it is held that this position by an internal mechanical latching arrangement 81, the magnetic valve 69 is then deenergized so as to conserve the pumping potential of the system. The latching assembly 81, which is located within the upper portion of the cylindrical head cap 52, has a hook shape lower end 82 which engages with a similarly shaped end 83 on the piston 45. The latch assembly 81 fits within an extension 84 of the cylinder 56. Extension 84, however, has a slightly larger diameter than cylinder 56. Extension 84 is closed at its top by a cover plate 85 which is secured to the upper cylindrical head cap 52 by a central bolt 86. The cover plate 85 and the central bolt 86 are located immediately below the threaded and welded seal plug 54. Thus, the pressure containing integrity of the control rod drive mechanism 28 is maintained. A radial projection 87 of the latch 89 fits into a narrow slot 88 (FIG. 7) machined radially in cylinder extension 84. As more clearly shown in FIGS. 7 and 8, the projection 87 serves as a seat and fulcrum for the latch 89 and maintains the correct rotational location of the latch 89 with respect to the bore of cylinder extension 84. A spring 90 which is fitted within a respective hole in the latch 89 tends to rotate the latch 89 around its fulcrum point until the edge of the ledge 91 contacts the wall of the cylinder extension 84. This is the normal position of the latch assembly 81 when the control rod assembly is inserted in the core. When the piston 45 and hence the control rod assembly 21 is lifted by application of the differential hydraulic pressure, the latch 89 will be pushed aside momentarily but will be returned immediately to the latched position (as indicated by the dash and dot lines) by latch spring 90 when the piston 45 reaches its uppermost position which is somewhat higher than that shown in FIG. 4. When the hydraulic differential pressure is removed, by deenergizing the magnet valve 69, the piston 45 will drop slowly until its movement is arrested when the hook shaped ends of the piston and the latch mechanism 82 and 83 respectively, firmly engage thereby holding the control rod assembly 21 in a fully withdrawn position. An electromagnet 100 is provided to permit insertion of the control rod assembly 21 from a withdrawn and latched position. The electromagnet 100 consists of two pole pieces 101 which are attached by welding to a nonmagnetic support tube 102 surrounding the upper cylindrical head cap 52 and resting on top of the seal plug 54. As can be seen most clearly in FIG. 9, the inner faces of the pole pieces 101 are machined to obtain a close fit with the head cap 52. A magnetic coil 103 is conventionally mounted to the pole pieces 101. A magnetic path to the latch 89 which is made of magnetic stainless steel or other suitable material, is completed by plugs 104 made of the same material. The plugs 104 may be threaded into the non-magnetic head cap 52 and may be pressure sealed by welding. When the coil 103 of the electromagnet 100 is energized, a magnetic force of sufficient magnitude is exerted on the latch 89 to disengage the latch mechanism 81 from the piston 45 permitting insertion by gravity of the associated control rod assembly 21. Position indicating electromagnets 110 and 111 are provided for each control rod assembly 21 to ascertain a withdrawn or inserted position, respectively. Electromagnets 110 and 111 are mounted to the non-magnetic support tube 102 in similar fashion as electromagnet 100. To ascertain the position of the piston 45 an AC signal is imposed on the coil of electromagnets 110 and 111. The presence or absence of the piston 45 in the vicinity of the electromagnets will change the AC coil current thereby indicating the position of the control rod assembly. For this purpose, it is necessary that the entire drive shaft 45 be made of a magnetic steel. With this arrangement, it is also possible to ascertain whether any control rod assembly 21 is stuck between its fully inserted end and fully withdrawn position and also which position it is in. Referring again to FIG. 4, it is to be noted that proper design dimensioning of the diametrical clearance between piston 45 and cylinder 56 at restriction 80 is necessary along with proper design dimensioning of a distance given the reference character X, in order to stop the upward piston movement short of impact with the latch 89 but still safely above the latched or engaged position. It will therefore be apparent that there has been disclosed nuclear reactor having drive mechanisms for control rod assemblies which are: capable of positively and mechanically holding a plurality of control rod assemblies in a fully withdrawn position; are capable of hydraulically decelerating a withdrawn control rod thereby preventing possible damage due to impact; and, are capable of positively controlling either a single control rod assembly or a plurality of control rod assemblies which are designed to be either completely inserted or completely withdrawn from a core of a nuclear reactor. Since numerous changes may be made in the above described apparatus, and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all the matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.