Patent Number: 041586026
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates the arrangement of components in a conventional liquid metal fast breeder reactor. The reactor includes a vessel 4 that contains a thermal shield 6, a plurality of fuel carrying subassemblies 10 and the primary coolant flowing through the reactor. The reactor uses partially enriched uranium (U-235) or plutonium (Pu-239) for fuel and the primary coolant is typically sodium at atmospheric pressure. The fuel is located in the core indicated by reference numeral 8 and is carried by those subassemblies 10 that pass through that area. Surrounding the core is a blanket of depleted uranium. The liquid sodium is pumped into the reactor 4, FIG. 1, through the inlet nozzles 12, 12'. The sodium entering through the inlet nozzle 12 passes into a lower plenum 13 and flows through the subassemblies 10 that penetrate the core area 8. The sodium entering through inlet nozzle 12' passes through the radial blanket subassemblies 10'. All of the subassemblies 10, 10' discharge the sodium into an upper plenum 14 where it thereafter flows out of the reactor through an outlet nozzle 15. The liquid sodium is maintained at essentially atmospheric pressure in the reactor by a blanket of inert gas 16 located in the upper portion of the reactor vessel. Typically, ten percent of the subassemblies in a liquid metal fast breeder reactor contain control rods and the other subassemblies contain either fuel or radial blanket elements. FIG. 2 illustrates one of the subassemblies that contains a control rod 22. This subassembly also includes a can 20 of hexagonal shape and a control rod drive shaft 24 that raises and lowers the control rod 22 with respect to the core 8. The rod drive shaft is connected to a rod drive mechanism (not shown) of conventional construction. The can is typically fabricated from stainless steel sheet stock and forms a conduit through which the sodium flows. The subassembly is terminated by an alignment stud 26 that maintains the lateral relationship of the subassembly with respect to a horizontal support plate (not shown). Referring to FIGS. 2-4, the control rod 22 is generally hexagonal in cross section and is freely movable within the can 20. The control rod consists of a plurality of elongate circular poison containing rods 28. Each rod is fabricated from boron carbide (B.sub.4 C) and is rigidly mounted by pins 29 between an upper and a lower support member 30, 32. The support members are rigidly mounted with respect to each other by a vertical support tube 34 that is surrounded by the poison rods 29. The upper support member 30, FIG. 4, has an upwardly projecting ring that forms a sealing surface 48 for the control rod as described in detail below. Referring to FIG. 4, the subassembly can 20 includes a separator plate 40 that forms the top of the subassembly. The separator plate contains an orifice 41 through which the control rod drive shaft 24 penetrates. To maintain alignment between the control rod 22 and the inside of the can 20, the drive shaft 24 has a plurality of centering vanes 42 that aid in seating the control rod 22 against the separator plate 40. The separator plate 40 further includes a downwardly projecting ring that forms a sealing surface 46. This sealing surface is engaged by a complementary sealing surface 48 located on the upper support member 30. When the two sealing surfaces 46, 48 are brought into contact as illustrated in FIG. 4, a fluid-tight seal is made. The complementary sealing surfaces are slightly rounded in order to prevent lateral displacement of the control rod with respect to the separator plate due to ordinary vibration. Such lateral displacement could break the seal and cause the control rod to drop as described below. The control rod 22, FIG. 4, is raised and lowered within the can 20 by a plurality of laterally disposed lifting bosses 38. The bosses are circular in cross section and engage a cam located on the inner surface of the side wall of the vertical support tube 34. FIG. 5 is an illustration of this cam and is a projection of the inner cylindrical surface of the support tube. The motion of the lifting bosses along the cam is described in detail below. OPERATION When sodium is being pumped through the reactor of FIG. 1, it enters the vessel 4 through one of the inlet nozzles 12, 12'. The sodium entering through nozzle 12 passes into a lower plenum 13 and flows through the subassemblies 10 containing fuel. The sodium entering through nozzle 12' flows through the subassemblies 10' containing radial blanket material. All of the subassemblies 10, 10' discharge into the upper plenum 14 and from there the sodium flows out of the reactor through the exit nozzle 15. The flow of sodium through the subassemblies causes a drop in pressure and in FIG. 1 the pressure P1 in the lower plenum 13 is substantially larger than the pressure P2 in the upper plenum 14. In a conventional liquid metal fast breeder reactor the differential pressure P1/P2 is typically about 100 PSI. The sodium that flows through the subassembly 10 containing the control rod 22, FIG. 2, enters the can 20 around the alignment stud 26. The flow of directed by the can, around the control rod and out through the orifice 41, FIG. 4. Orifice 41 leads directly to the upper plenum 14. If the control rod is positioned against the separator plate 40, a fluid-tight seal is made between the sealing surfaces 46, 48 and the differential pressure P1/P2 across the subassemblies is sufficient to retain the control rod in place. FIGS. 6-12 are schematic diagrams illustrating the operation of the preferred embodiment. Each figure depicts three adjacent subassemblies that pass through the core area 8, FIG. 1. The two outer subassemblies contain the fuel elements 8 and the inner subassembly houses the control rod 22. In each of the outer subassemblies the blanket areas are those areas that are located above and below the core 8 in FIG. 1. In all three subassemblies the primary coolant passes from the lower plenum 13, FIG. 1, into the bottom of the respective subassembly and flows out its top into the upper plenum 14. The presence of this flow is indicated by the arrows in the figures. In particular, FIG. 6 illustrates the reactor in a shut-down condition. The control rod 22 is positioned opposite the fuel and in a position to absorb the maximum number of neutrons. The control rod is supported by the control rod drive shaft 24 and the lifting bosses 38. There is a flow of primary coolant through the control rod subassembly that enters at the bottom and exits through the orifice 41. The control rod is maintained in position by a rod drive mechanism (not shown) that stops any further downward motion of the rod drive shaft 24. FIG. 7 diagrams the operation of the control rod 22 in regulating the power and the neutron flux in the reactor. The control rod 22 can be moved with respect to the fuel by raising and lowering the rod drive shaft 24. The position of the control rod with respect to the fuel controls the neutron flux and hence the power level. The control rod is supported in the same manner as FIG. 6 and coolant flows through the subassembly 10 and out the orifice 41. In FIG. 8 the procedure for locking the control rod 22 against the separator plate 40 is shown. The control rod drive shaft 24 and the lifting bosses 38 raise the control rod until the sealing surfaces 46, 48 come into contract. These sealing surfaces form a fluid-tight boundry and there is no flow of coolant out of the orifice 41. The differential pressure P1/P2 across the separator plate caused by the flow of primary coolant through the reactor locks the control rod in the position shown in FIG. 8. After the control rod 22 is locked against the separator plate 40, the control rod drive shaft 24 can be lowered to the position shown in FIG. 9. FIG. 9 illustrates the normal mode of operation of the preferred embodiment. The control rod subassembly can 20 is sealed and the flow of primary coolant through the orifice 41 is blocked. The differential pressure P1/P2 across the reactor maintains the sealing surfaces 46, 48 together. The control rod 22 thus remains up and out of the core. FIGS. 10-12 depict three of the ways a scram can be initiated by the preferred embodiment. In FIG. 10 a loss of primary coolant flow through the reactor initiates the scram. Prior to the loss of flow the control rod 22 and the control rod drive shaft 24 were positioned as shown in FIG. 9. That is to say, the lifting bosses 38 were positioned so as not to restrict the downward motion of the control rod. When a loss of flow occurs, the differential pressure P1/P2 across the reactor automatically decreases. Since it is merely the pressure drop across the separator plate that is holding the control up, the control rod 22 falls by gravity as indicated in FIG. 10. Motion of the rod drive shaft is not required. The control rod falls until the lifting bosses 38 engage the upper support member 30, FIG. 4, of the control rod or until the control rod comes to rest on a conventional support. The lifting bosses are positioned and the control rod is dimensioned so that the control rod 22 comes to rest opposite the fuel as illustrated in FIG. 6. This self-actuated motion shuts down the reactor. In FIG. 11 the scram is initiated when one of the reactor safety circuits or the reactor operator commands the rod drive mechanism (not shown) to scram the reactor. Upon receiving this command the rod drive mechanism releases the rod drive shaft 24 to drop by gravity or to descend under the force of a spring. This is the conventional mode of initiating a scram. When the control rod drive shaft 24 is either dropped or driven downward, the lifting bosses 38 engage the lower support member 32 of the control rod and the sealing surfaces 46, 48 are separated. This situation is illustrated in FIG. 11. When the sealing surfaces are separated, the differential pressure across the separator plate 40 is removed and the flow of coolant through the subassembly is restored. The control rod 22 then falls by gravity and comes to rest opposite the fuel as illustrated in FIG. 6. This type of scram can also be initiated by having the rod drive mechanism drive the rod drive shaft downward and thereby separate the sealing surfaces. FIG. 12 illustrates how the reactor can be scrammed when subjected to a severe lateral acceleration or impulse such as experienced during an earthquake. Typically, a severe lateral acceleration will cause the control rod to rock over or move laterally and force the sealing surfaces 46, 48 to separate slightly. The flow of primary coolant out of the orifice 41 is then reestablished and the differential pressure removed. The inside diameter of the vertical support tube 34 and the rod drive shaft are dimensioned to permit this type of movement. It should be noted that the preferred embodiment overcomes the problem of scramming the reactor during a severe earthquake when the upper reactor structure is displaced relative to the core. Such a displacement could prevent the control rod drive shaft from dropping and/or the rod drive mechanism from unlatching a conventional control rod and allowing it to drop. The preferred embodiment overcomes this problem because movement of the control rod drive shaft 24 is not required to initiate a scram. The scram is directly and inherently initiated by the action of the earthquake itself. Thus, the response of the control rod is an inherent reaction to the accident itself. Referring to FIG. 5, a cam 50 may be employed in order to enhance flexibility of operation by providing a means of breaking the seal without reducing the flow of sodium. FIG. 5 is a projection of the inner surface of the vertical support tube 34 and illustrates the raised surface of the cam as one of the lifting bosses 38 is sequenced by the various surfaces of the cam. In particular, when the control rod drive shaft 34 supports the control rod 22 as shown in FIG. 6 or raises the control rod as illustrated in FIG. 7, the lifting boss 38 engages the cam at point 52. If the control rod is sealed against the separator plate and the drive shaft 24 is lowered to the position illustrated in FIG. 9, the lifting boss moves from point 52 to point 54. When the lifting boss is located at point 54, it is out of vertical engagement with the control rod. The differential pressure P1/P2 across the separator plate 40 is pushing the control rod against the separator plate 40 and maintaining the two sealing surfaces 46, 48, FIG. 4, together. When a loss of flow accident occurs, the differential pressure P1/P2 decreases to effectively zero and the control rod drops by gravity, FIG. 10. The lifting boss in FIG. 5 remains at one elevation and the cam moves downward until the boss engages the cam at point 56. The vertical distance between points 54 and 56 is such that the control rod will come to rest opposite the fuel as shown in FIG. 6. If a scram is commanded by the reactor or by one of the reactor safety systems, FIG. 4, the seal between the separator plate 40 and the control rod 22 is broken by the downward motion of the rod drive shaft 24. In FIG. 5 the lifting boss moves downward from position 54 to position 58 where it engages the cam. The seal between the separator plate 40 and the control rod 22 can also be broken without flow interruptions and without having the control rod drop. That is to say, the mode of reactor control depicted in FIG. 9 can be shifted to that shown in FIG. 7. To effect this change, control rod drive shaft 24 can be lifted from point 54 to point 56 and then lowered to point 60. The motion of the boss is indicated by arrows in FIG. 5. When the lifting boss engages the cam surface at point 60, the seal can be broken by further downward motion of the control rod drive shaft. The control rod then drops and engages the boss at point 62 where it remains suspended by the rod drive shaft. The horizontal distance between points 60 and 62 is small and the motion of the control rod does not substantially affect the power level. Besides the slightly rounded sealing surfaces 46, 48, FIG. 4 used in the preferred embodiment, the present invention also contemplates varying the contour of the sealing surfaces to satisfy other design criteria. Contours having both concave and convex cross sections, triangular cross sections and knife edges can be used. In addition, the contour can be excluded and a flat surface used for sealing. It should also be noted that although the sealing surfaces 46, 48 in the preferred embodiment form a tight seal, the present invention does not required that the seal be fluid-tight. For example, the poison rods 28, FIG. 2, may require some flow of coolant in order to remove self-generated heat. Thus, a small aperture may be necessary in the control rod assembly 22 in order to allow coolant to reach the poison rods when the control rod is sealed against the separator plate, FIG. 9. Although this small aperture permits a flow of coolant to effectively flow across the separator plate, the aperture is dimensioned small enough that the differential pressure P1/P2 is not substantially reduced. The present invention also contemplates providing a follower attached below the control rod 22, FIG. 2, to fill the void in the reactor caused by the withdrawal of the control rod. The follower has the same shape as the control rod and is raised into the core area 8, FIG. 1, as the control rod is pulled out of the core. The follower is contructed of the same material as the blanket, thereby increasing breeding game and the "worth" or effectiveness of the control rod. The addition of the follower does not affect the sealing between the surfaces 46, 48 because the differential pressure P1/P2 across the separator plate is sufficiently large to retain both the follower and the control rod against the separator plate. Although the preferred embodiment is described in connection with a liquid metal fast breeder reactor, it is contemplated that this invention can be used on any comparable nuclear reactor. Although the best mode contemplated for carrying out the present invention has been herein shown and described, it will be apparent that modification and variation may be made without departing from what is regarded to be the subject matter of the invention.