Hafnium stainless steel absorber rod for control rod

Abstract: In a control rod construction for moderating and controlling a reaction of a nuclear reactor, a control rod structure is disclosed in which hafnium (46) is friction-welded to stainless steel (44). Typically, the control rod is confined for movement (52) axially along its length into and out of the reactor. The hafnium is surrounded in the control rod structure by a sheath of stainless steel (40) to inhibit movement relative to the control rod structure other than in the axial direction of control rod movement. The weld is thus stressed only in either tension or compression due to dynamic loading occur­ ring from control rod movement. The stainless steel to which the hafnium is welded is in turn convention­ ally welded (49) or mechanically connected to the remaining steel components of the control rod. There results a secure control rod structure having welded hafnium that can withstand the dynamic loadings of fast insertion of control rods such as those loadings which commonly occur under rapid control rod accelera­ tion and deceleration due to SCRAM of a reactor.

Description: This invention relates to control rods utilized with nuclear reactors. Specifically, a control rod construction is disclosed in which hafnium is friction welded to stainless steel in a control rod construction where the weld is stressed in tension or compression only by the dynamics of control rod movement. Hafnium is a metallic element of atomic number 72 having a high specific gravity (13.1) and an extremely high thermal neutron cross-section (115 barns). Hafnium is commonly used in control rods for nuclear reactors in that it is a long lived neutron absorber. Needless to say, hafnium is heavy. Its specific gravity of 13.1 is heavier than lead (specific gravity 11.35). While it is desirable from the neutron standpoint to include hafnium as part of the control rod of a nuclear reactor, hafnium is most undesirable as a portion of a control rod because of its weight and its inability to be joined intricately to the stainless steel of conventional control rods. Stainless steel is the substance from which the structural components of most control rods are fabricated. Hafnium is not suited for conventional welding to stainless steel. In short, during the high heat of conventional welding in which the metals to be welded are heated to a molten state, hafnium forms a brittle intermetallic media with stainless steels. This brittle intermetallic lacks the ductility and general tension and compression properties required in welds. Control rods are typically fabricated from stainless steel. The stainless steel is used to encase neutron absorbers, such as boron carbide (B₄C). The boron carbide is utilized for the control of nuclear reactions. Unfortunately, when boron carbide undergoes the decomposition due to the presence of high neutron flux and other radiation, gases are liberated. Typically, the control rods of nuclear reactors are subjected to simultaneous high heat and high pressure due to the gases created when boron carbide is subjected to radiation. Moreover, boron carbide is not a long lived absorber. Adding hafnium tips as long lived absorbers to control rod having boron carbide absorbers is known. The hafnium tips provide the composite control rods with optimum life at reasonable cost. Unfortunately, because of the inability of hafnium to be joined to stainless steel and because of its high density, the joining of hafnium to the otherwise stainless steel structure of control rods is difficult. This difficulty arises from the current practice of surrounding the hafnium with stainless steel for containment purposes. When a nuclear reactor is SCRAMed, control rods are inserted on a relatively instantaneous basis. Control rods of length in the order of 3.66m (12-feet) are fully inserted within a reactor in less than 2 seconds. Hafnium tips (specific gravity 13.1), being considerably more dense than the stainless steel (specific gravity 7.9), subject any containing stainless steel to dynamic loadings during changes of control rod acceleration. By way of example, during initial accelerations, forces of up to 30 gravitational fields can be imposed. Upon full insertion of the control rod and sudden stopping against the reactor structure, forces up to at least 200 gravitational fields can be present. Therefore, virtually any freedom of contained hafnium to move relative to the containing stainless steel parts of a reactor control rod can cause breakage in the stainless steel control rod containment structure. This breakage can include rupture of the stainless steel tubing containing pressurized boron carbide. Any such breakage is an unacceptable result that can cause transport of radioactive particles throughout a nuclear reactor in the steam cycle. US-A-4655999 discloses a control rod for a nuclear reactor in which neutron absorbing rods are surrounded with a second neutron absorber which differs in material from the neutron absorbing rods. When the neutron absorbing rod is composed of boron carbide powder and a cladding for receiving the powder therein, the second neutron absorber is a sheet of a material having a neutron irradiation lifetime longer than that of boron carbide, for example, hafnium. Where the neutron absorbing rod is a round hafnium rod, the second neutron absorber having a neutron absorption worth higher than that of hafnium is located so as to surround the round hafnium rods. Thus, the neutron absorbing rods are effectively used, and the lifetime of the control rod can be prolonged. US-A-4676948 discloses a control rod for a nuclear reactor wherein plates of neutron absorbing material (e.g. hafnium) are used in the upper portion thereof and tubes containing a different neutron absorbing material (e.g. boron) are positioned beneath the plates. To relieve the tubes of the weight of the plates, the plates are attached to support members of the control rod frame. To prevent or reduce the gap between the plates and the tubes, joints therebetween are provided. So-called friction welding is known. Friction welding comprises relatively rotating and simultaneously forcing two pieces of metal together. The rotating and forcing is typically continued until friction causes plastic metallic flow extrusion of metal at the metallic interface between the two relatively rotating metals. As a part of such welding, plastic flow at the weld interface occurs. Hot material flows from the weld interface. Upon extrusion of hot plastic metallic material out of the weld interface, the relative rotation between the two pieces to be welded is stopped. The force bringing the two pieces together is maintained. During the cooling process, welding occurs in the two joined materials. We are unaware of any attempt ever being made to weld hafnium to stainless steel. It is known that hafnium and stainless steel form a brittle intermetallic which has poor tension, compression, ductile and elastic properties. We have discovered that the friction welding of hafnium to stainless steel produces a weld having high tensile strength. This weld, however, generally lacks accepted ductility for conventional weld stresses. Utilizing this discovery, we confine hafnium within a control rod so that loadings experienced by the hafnium at the weld will be under tension or compression only. We therefore, disclose for the first time a control rod structure for a nuclear reactor in which direct hafnium welds to stainless steel occur. We suspect that the success of the disclosed welds is due to the presence of an extremely thin metallic bond between the hafnium and the stainless steel. This bond occurs at a relatively thin interface between the two metals. The process of friction welding of these two metals has been discovered by us to form a small thin intermetallic region having relatively high tensile strength. The present invention provides a control rod for controlling a nuclear reaction, by movement into and out of a nuclear reactor along an axis of movement comprising a stainless steel body having a hollow interior; neutron absorbing poisons contained within said hollow interior; a piece of hafnium contained within and joined to the stainless steel body of said control rod; characterized by a piece of stainless steel joined to said piece of hafnium by a thin diffusion interface created by friction welding, said pieces of hafnium and stainless steel oriented serially in said axial direction of control rod movement with said thin diffusion interface disposed normal to the axial direction of said control rod movement; means for confining said piece of hafnium as attached to said piece of stainless steel to movement along the axial direction with said control rod whereby said thin diffusion interface is stressed in tension or compression only upon dynamic loading of said control rod by movement in said axial direction; and means for attaching said joined pieces of hafnium and stainless steel to the stainless steel body. In a control rod construction for moderating and controlling a reaction of a nuclear reactor, a control rod structure is disclosed in which hafnium is welded to stainless steel. Typically, the control rod is confined for movement axially along its length into and out of the reactor. The hafnium is surrounded in the control rod structure by a sheath of stainless steel to inhibit movement relative to the control rod structure other than in the axial direction of control rod movement. The hafnium is friction welded to the stainless steel of the control rod so as to stress the weld in either tension or compression due to dynamic loading occurring from control rod movement. The stainless steel to which the hafnium is welded is in turn conventionally welded or mechanically connected to the remaining steel components of the control rod. There results a secure control rod structure having welded hafnium that can withstand the dynamic loadings of fast insertion of control rods such as those loadings which commonly occur under rapid control rod acceleration and deceleration due to SCRAM of a reactor. As an embodiment of the invention there is disclosed a nuclear absorber rod constructed by the welding of hafnium to stainless steel as follows. A rod of hafnium, in the order of 4.76mm (3/16-inch) diameter is relatively rotated with respect to a rod of stainless steel having approximately the same diameter. During the relative rotation, a force bringing the two pieces one toward another, is applied. The force of the rods towards one another is maintained until plastic flow occurs at the intended weld interface. An annulus of extruded plastic material forms about the relatively rotating pieces. When this annulus has formed, the relative rotation is stopped while the force of the two pieces is maintained one towards another. This force is maintained during the cooling which follows. It has been found that a weld between the two pieces occurs with a thin bimetallic interface. An advantage of the described process is that it gives for the first time a positive welded connection of stainless steel and hafnium. This welded connection, while possessing low ductility, has relatively high tensile strength. According to another aspect of the invention, a friction weld is utilized in a control rod. According to this aspect of the invention, the welded piece of hafnium is confined so as to be loaded in tension or compression upon dynamic movement of the control rod. This confinement is preferably accomplished by surrounding the hafnium with stainless steel so that the hafnium is only free in the direction of control rod movement. The weld is placed relative to the confined hafnium so that it will only be subjected to tension or compression upon dynamic control rod movement. It is not oriented so that forces of shear or bending will be present on the weld. An advantage of this aspect of the invention is that the relatively brittle but strong tensile friction weld of hafnium to stainless steel is oriented so that it may be utilized within a control rod structure. Other features and advantages of the invention will become more apparent from the following description given with reference to the accompanying drawings in which: Figs. 1A, 1B and 1C are a cartoon series of the process of producing a friction weld; Fig. 2 is a side elevation section of a control rod absorber rod hollow cylinder having a hafnium plug held in the interior of the cylinder by the preferred apparatus and process of this invention; Fig. 3 is an alternate embodiment of a hafnium plug inserted in a control rod; and, Fig. 4 is an alternate embodiment of hafnium being mounted to the tip of a control rod, the mechanism here illustrated including a mechanical lock between the upper and lower portions of the control rod. Fig. 1A is a schematic illustrating a rod of stainless steel 14 being friction welded to a piece of hafnium 16. Stainless steel rod 14 is placed within a rotating chuck 18 which rotates in the direction of arrow 20. Hafnium rod 16 is held stationary in jig 17. During the relative rotation between the two members, a force (symbolized by vectors 22, 24) is placed across the weld interface. Preferably, the diameters of the two rods 14 and 16 are between 0.12 and 0.30 ins. (3.0 and 7.6 mm) Relative rotation of the jig 18 relative to the stationary hafnium rod 16 occurs. The force between the jig and the rotating chuck is sufficient for an interface to form containing the bimetallic metal formed diffusion. It is preferable to provide a point to each of the rods 14, 16. That is to say, the rods each have a conical surface. This conical surface is the point of first contact during the so-called "friction" weld. Referring to Fig. 1B, relative rotation between the two rods 14, 16 has occurred until an annulus 26 develops between the two rods. This annulus comprises a layer of plastically flowing material. This plastically flowing material flows out of the interface between the two metals to expose unoxidized and pure metallic interfaces. This permits a clean weld to be made. Referring to Fig. 1C, rotation of the chuck member 18 relative to the jig member 17 has ceased. At this point, there is no relative movement between rods 14, 16. At the same time, the force between rods 14, 16 is maintained. Due to the high heat present, a metallic bond is formed. This metallic bond consists of a narrow diffusion interface. Examination of the resultant bond when hafnium and stainless steel are welded together has revealed that a relatively high tensile strength in the order exceeding 345 MPa (50,000 lbs./inch²) is developed. The weld, however, lacks ductility. Specifically, when one of the two rods 14, 16 is held in a vise, it is possible to break the other rod free by bending at the weld. Destructive testing of the weld illustrates that some of the welds are weaker in tensile strength than either of the component metals. Breakage occasionally occurs at the weld, revealing the thin metallic fusion bond there created. A tensile strength greater than 345 MPa (50,000 pounds per square inch) is obtained. The reader will understand that the friction weld herein illustrated is conventional -- except for the specification of the two materials to be welded, stainless steel and hafnium. Referring to Fig. 2, an absorber element of a control rod is illustrated. The element includes a stainless steel outer sheath tube 40 having a bottom end plug 42 and a top end plug 44. Top end plug 44 is friction welded to a rod of hafnium 46. Hafnium rod 46 is dimensioned to fit within the inside diameter of the stainless steel tube 40. Sealing of the tube (shown broken away in Fig. 2) is easily understood. Both bottom plug 42 and top plug 44 are conventionally welded at respective welds 48 and 49 to the top and bottom of the tube. The friction weld at 50 (constructed by the process of Figs. 1A, 1C) forms a thin metallic bond or interface of welded material. It will be understood that the axis of control rod movement is schematically illustrated in direction 52. Preferably, control rod insertion will be upward. It will be seen that hafnium plug 46 is confined to exert a tension or compression force only on weld 50. Shear and/or bending will not be present on the weld 50. It is this ability to constrain the load on the resultant friction weld which allows its utility in the disclosed application. It will be understood that the control rod structure here illustrated will admit of other embodiments. Referring to fig. 3, a control rod 60 is shown having a hafnium plug 62 welded to an end cap 67, by the friction welding process hereinbefore set forth, at a weld interface 66. Welding again occurs by the process previously illustrated with respect to Figs. 1A-1C. Cap 67 is welded to the exterior of the rod at welds 69. Thus, conventional boron carbide 63 is confined within the interior of the control rod. Again, the movement of the control rod is constrained in the axial direction of the rod illustrated in Fig. 3. Therefore, hafnium plug 62 loads the weld 66 in either tension or compression. Bending or shear is not present at the weld interface 66 to an appreciable degree. Finally, and referring to Fig. 4, an alternate embodiment is illustrated. Specifically, a stainless steel control rod 70 containing boron carbide 72 is capped at an end plug 74 by conventional stainless steel weld 75. The top of the end plug has previously been friction welded to a hafnium rod 78. Hafnium rod 78 has a connection 79 configured at the upper portion thereof with an L shape which cooperates with a mating L 80 from the upper part of the control rod: by the expedient of engaging the thus-formed opposing teeth 79, 80, a mechanical connection occurs at the top of hafnium rod 78. It can thus be seen that the cross-section includes a hafnium rod 78 welded at one end to plug 74 of the absorber rod and mechanically fastened at the opposite end of the control rod. Realizing that the hafnium is approximately twice the density of steel, the importance of maintaining the hafnium stationary with respect to the typically pressurized and very highly heated stainless steel rod containing boron carbide can be understood. Simply stated, if the hafnium is allowed to undergo appreciable relative movement, over stressing of the control rod can occur. Excessive over stressing can cause breakage which can liberate particles of boron carbide or other neutron absorber materials within the water of a boiling water reactor with resultant contamination throughout the plant. Those having skill in the art will appreciate that there can be numerous other configurations of the weld herein illustrated. It is important that the welded hafnium and stainless steel be confined for loading in tension or compression only.

Claims: A control rod for controlling a nuclear reaction by movement into and out of a nuclear reactor along an axis of movement, comprising : a stainless steel body (40,60,70) having a hollow interior; neutron absorbing poisons (63,72) contained within said hollow interior; a piece of hafnium (46) contained within and joined to the stainless steel body (40,60,70) of said control rod; characterized by a piece of stainless steel (44) joined to said piece (46) of hafnium by a thin diffusion interface (50) created by friction welding, said pieces (44,46) of hafnium and stainless steel oriented serially in said axial direction of control rod movement (52) with said thin diffusion interface (50) disposed normal to the axial direction; means for confining said piece (46) of hafnium as attached to said piece (44) of stainless steel to movement along the axial direction with said control rod whereby said thin diffusion interface (50) is stressed in tension or compression only upon dynamic loading of said control rod by movement in said axial direction; and means (49) for attaching said joined pieces of hafnium and stainless steel to the stainless steel body (40,60,70) The control rod according to Claim 1 wherein said means for attaching said joined pieces of hafnium and stainless steel to the stainless steel body comprises a welded connection. The control rod of Claim 1 wherein said means for attaching said joined pieces of hafnium and stainless steel to the stainless steel body comprises a weld. The control rod of Claim 1 wherein said means for confining said piece (46) of hafnium to movement along said axial direction with said control rod comprises a tube surrounding said piece (46) of hafnium. The control of Claim 4 wherein said piece (46) of hafnium is cylindrical and fits within said tube. The control rod of Claim 1 wherein the means for attaching said joined pieces of hafnium and stainless steel to the stainless steel body includes a mechanical interlock (79,80) therebetween.

Inventors: AIELLO LOUIS LEONARD, CEARLEY JAMES ELWOOD, CHARNLEY JAMES EDWARD, DIXON ROBERT CARL, IZZO KENNETH ROY, AIELLO, LOUIS LEONARD, CEARLEY, JAMES ELWOOD, CHARNLEY, JAMES EDWARD, DIXON, ROBERT CARL, IZZO, KENNETH ROY
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1989
Main classification: G21C7
IPC codes: G21C7