Control rod for boiling water reactor and method for manufacturing the same

A tie rod having a cruciform cross section is provided with steps for fixing sheaths at tips of cruciform arms of the tie rod; the tips of each of sheaths are fitted onto the steps of the tie rod, each of the sheaths having a U-shaped cross section; and each of the sheaths is fixed to the tie rod by performing a laser welding using a YAG laser beam or a CO2 laser beam with the sheath being fitted onto the tie rod to achieve a continuous weld of at least a part of the tie rod in a longitudinal direction thereof. An axial center position of the beam is shifted from an end face position of the step of the tie rod at least toward an axis center of the tie rod.

BACKGROUND OF THE INVENTION

The present invention relates to a control rod for controlling the power of a boiling water reactor and a method for manufacturing the same.

The control rod typically has a structure wherein a handle is attached to an axially upper part of a tie rod having a substantially cruciform cross section; a lower part support member (or velocity limiter) is attached to at an axially lower part of the tie rod; and four sheaths, each of which incorporates a reaction rate controlling material, are fixed at a lower end of the handle, an upper end of the lower part support member and ends of the substantially cruciform of the tie rod by welding. In this case, a perfect weld penetration by the TIG (tungsten inert gas) welding has been performed for welding the sheaths to the ends of the handle, the lower part support member and the tie rod.

The control rod moves upward and downward in the narrow gap secured among the fuel assemblies during operation of the reactor. Therefore, a high degree of machining precision is required in manufacturing the control rod.

However, the conventional TIG welding has such drawbacks that it requires a large amount of heat input and tends to increase deformation due to welding. Thus, in order to suppress the deformation caused by welding, a method employing a laser welding, which requires a less amount of heat input, has been proposed in Japanese Patent Laid-open No. 2000-329885.

In the aforementioned prior art literature, the perfect weld penetration is carried out in the following manner. Steps are provided on each of tips of arms of the tie rod to fit a U-shaped tip of each of the sheaths thereonto, and each of tips of the sheath are directly irradiated with a laser beam in such a manner that the axial center position of the beam is shifted from an end face of the step of the tie rod to a side opposite to the axis center of the tie rod by 0.1 to 2.0 mm.

The above-described prior art has the following problems. Specifically, since a width of the step at the tip of each of the arms of the tie rod is typically about 0.5 mm, an overlap of the step of the tie rod with the tip of the sheath is about 0.5 mm. Therefore, if an error occurs in the axial center position of the laser beam and the laser beam is deviated from the very narrow overlap portion, the sheath is heated to melt down due to a slow heat transfer of the laser beam to the tie rod, to thereby cause a welding failure.

Further, even if the irradiation position of the laser beam is located within the overlap, a contact area of the step of the tie rod with the tip of the sheath must be sufficiently large to satisfactorily transfer the heat of the laser beam to the tie rod. Therefore, the step of the tie rod must be machined to achieve a precise rectangular shape. More specifically, if an R shape (round portion) is formed at a corner of the step of the tie rod, the contact area of the tie rod with the sheath becomes small to fail to provide the satisfactory heat transfer, and such imprecision may cause the melt-down of the sheath, resulting in the welding failure.

In order to prevent the above problems, the prior art requires a high precision control of the laser beam for the prevention of the deviation of the laser beam irradiating position from the overlap and a high precision machining of the step of the tie rod. Thus, it has been difficult to simplify the manufacturing process of the control rod, and, also, the production cost has been undesirably increased in the prior art.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a control rod for boiling water reactor and a method for manufacturing the control rod for boiling water reactor, whereby the manufacturing process can be simplified and the production cost can be decreased.

(1) According to an aspect of the present invention, a tie rod having a cruciform cross section is provided with steps for fixing sheaths at tips of cruciform arms of the tie rod; the tips of each of sheaths are fitted onto the steps of the tie rod, each of the sheaths having a U-shaped cross section; and each of the sheaths is fixed to the tie rod by performing a laser welding using a YAG (yttrium aluminum garnet) laser beam or a CO2laser beam with the sheath being fitted onto the tie rod to achieve a continuous weld of at least a part of the tie rod in a longitudinal direction thereof. An axial center position of the beam is shifted from an end face position of the step of the tie rod at least toward an axis center of the tie rod.

In this case, the laser beam is not irradiated directly on the sheath, but firstly on a surface of the tie rod so that heat is transferred from the surface of the tie rod to the sheath which is being brought into contact with the tie rod step via the tie rod step. Accordingly, even if a small error in the beam axial center position occurs to cause a slight deviation from the target position, the heat is transferred to the sheath after passing the contact portion of the step with the sheath without fail, thereby eliminating possibility of a welding failure which is caused by the melt-down of the sheath. Thus, the present invention prevents the melt-down of the sheath to secure a good weldability without high precision control of the beam axial center position which has been performed in the conventional method. Therefore, the present invention facilitates the laser welding control as well as the manufacture of the control rod, and achieves a reduction in manufacturing cost.

(2) According to another aspect of the present invention, steps of a tie rod are formed by a drawing process, each of sheaths is fixed to the tie rod by performing a laser welding using a laser beam with the sheath being fitted onto the tie rod to achieve a continuous weld of at least a part of the tie rod in a longitudinal direction thereof.

As described in the above item (1), by shifting the laser beam axial center position toward the tie rod axis center, the melt-down of the sheath can be prevented to thereby secure the good weldability even if the tie rod step is not machined into a precise rectangular shape and thereby an R-shape (curved) or the like remains on a corner of the step. Thus, it is possible to omit a machining step from a typical control rod manufacturing method comprising a process step of forming the drawn tie rod having a substantially cruciform cross section by a drawing process and a process step of machining of the steps to achieve the rectangular shapes, thereby making it possible to manufacture a multiple of the tie rods each provided with the steps at one time by the drawing process only. Therefore, the present invention facilitates the manufacture of the control rod by a process corresponding to the machining process omitted, to thereby achieve a reduction in manufacturing cost.

(3) According to further aspect of the present invention, a step for fixing sheaths is formed on a lower end of a handle attached to an axially upper part of a tie rod; an upper edge of each of the sheaths is fitted onto the step of the handle; and each of the sheaths is fixed to the handle by performing a laser welding using a laser beam with the sheath being fitted onto the handle to achieve a continuous weld of at least a part extending along the upper edge of the sheath. An axial center position of the beam is shifted from an end face position of the step of the handle to a side opposite to the sheath.

In this case, the laser beam is not irradiated directly on the sheath, but firstly on a surface of the handle so that heat is transferred from the surface of the handle to the sheath which is being brought into contact with the handle step via the handle. Accordingly, even if a small error in the beam axial center position occurs to cause a slight deviation from the target position, there is no possibility of a welding failure which is caused by the melt-down of the sheath. Therefore, the same effect as that in Item (1) is obtained.

(4) According to a still further aspect of the invention, a step for fixing sheaths is formed on an upper end of a lower part support member or a velocity limiter attached to an axially lower part of a tie rod; a lower edge of each of the sheaths is fitted onto the step of the lower part support member or the velocity limiter; and each of the sheaths is fixed to the lower part support member or the velocity limiter by performing a laser welding using a laser beam with the sheath being fitted onto the lower part support member or the velocity limiter to achieve a continuous weld of at least a part extending along the lower edge of the sheath. An axial center position of the beam is shifted from an end face position of the step of the lower part support member or the velocity limiter to a side opposite to the sheath.

In this case, the laser beam is not irradiated directly on the sheath, but firstly on a surface of the lower part support member so that heat is transferred from the surface of the lower part support member to the sheath which is being brought into contact with the step of the lower part support member via the lower part support member. Accordingly, even if a small error in the beam axial center position occurs to cause a slight deviation from the target position, there is no possibility of a welding failure which is caused by the melt-down of the sheath. Therefore, the same effect as that in Item (1) is obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a method for manufacturing a control rod for boiling water reactor and a control rod for boiling water reactor according to the present invention will hereinafter be described with reference to the accompanying drawings.

FIG. 1is a partly exploded perspective view showing a general structure of a control rod for boiling water reactor according to a first embodiment of the present invention.FIG. 2is a cross-sectional view taken along the section plane indicated by II—II inFIG. 1, in which fuel assemblies N are also shown.

InFIGS. 1 and 2, a control rod1for boiling water reactor is provided with a control rod supporting structural body2and four blades3each of which extends from an axis center2A of the control rod supporting structural body2(or an axis center4A of a tie rod4to be described later in this specification) toward four directions. The control rod1as a whole has a cruciform cross section.

The control rod supporting structural body2is provided with a tie rod4having the cruciform cross section, a handle5fixed to an upper end of the tie rod4and a velocity limiter6fixed to a lower end of the tie rod4.

Each of the blades3comprises hafnium flat tubes7serving as a neutron absorbing member and sheaths8covering the hafnium flat tubes7. Each of the blades3is provided with four hafnium flat tubes7, two of which being provided in a direction of the axis center2A of the control rod supporting structural body2, other two of which being provided in a direction extending outward from the axis center2A of the control rod supporting structural body2. Here, upper ends of the hafnium flat tubes7which are provided at an upper part are fixed to the handle5with pins (not shown), lower ends of the hafnium flat tubes7which are provided at a lower part are fixed to a base member6aof the velocity limiter6with pins (not shown), and the sheaths8press the hafnium flat tubes7to fix the hafnium flat tubes7to the control rod supporting structural body2.

Each of the sheaths8is prepared by bending a stainless steel plate, for example, to form a U-shape, and each of tips of the sheath8is provided with projections8aand recesses8b. The projections8aare welded to each of tips4aof the arms of the tie rod4, an upper edge8cof the sheath8is welded to a lower end5aof the handle5, and a lower edge8dof the sheath8is welded to an upper end6a1of the velocity limiter base member6a, to thereby fix the sheath8to the control rod supporting structural body2. Each of the sheaths8is provided with a plurality of cooling holes9which serve as paths for a coolant.

The method for manufacturing each of main parts of the thus-structured control rod for boiling water reactor according to the first embodiment of the present invention will be described with reference to FIG.3.

A plate10is prepared by rolling a material in Step10of FIG.3. Next, in Step20, the plate10is cut in such a manner as to form the projections8aand the recesses8b, and then punched to form the cooling holes9, to thereby obtain a flat sheath11. In Step30, the sheath8is obtained by bending the flat sheath11in such a manner as to form a U-shape using a press machine.

A drawn tie rod12is formed by drawing a material in Step40ofFIG. 3, and then the drawn tie rod12is cut in Step50to give a tie rod4(hereinafter, for the distinction from the drawn tie rod12, the tie rod4will be referred to as cut tie rod4when so required). Details of shapes of the drawn tie rod12and the cut tie rod4will be described below with reference toFIGS. 4A and 4B.

FIG. 4Ais a top view or a bottom view of the drawn tie rod12, andFIG. 4Bis a top view or a bottom view of the cut tie rod4. In these drawings, the drawn tie rod12is the tie rod formed by drawing a material in Step40, which is provided with steps12bformed at both sides of a tip12aof each of arms. A corner12b1of each of the steps12bis slightly R-shaped (curved).

In turn, the cut tie rod4is provided with steps4bformed at both sides of a tip4aof each of arms like the drawn tie rod12. It is formed by cutting each of the steps12bof the drawn tie rod12in Step50to eliminate the R-shape of the corner12b1. Thus, each of the steps4bhas a precise rectangular shape. The steps4bare provided for the purpose of fitting the projections8aof the tips of the sheath8thereonto at the time of welding the sheath8to the tie rod4in Step110which will be described later.

Referring back toFIG. 3, the handle5, velocity limiter6and other members constituting the control rod1for boiling water reactor are manufactured by subjecting materials to machining, assembling, welding and so forth in Step60.

(4) Control Rod Supporting Structural Body2

In Step70ofFIG. 3, the handle5manufactured in Step60is fixed to an upper end of the cut tie rod4by an assembly welding; the velocity limiter6manufactured in Step60is fixed to a lower end of the cut tie rod4in the same manner; and other members are properly assembled and welded, so that the control rod supporting structural body2is completed.

InFIG. 3, a hafnium plate13is formed by rolling a material in Step80. Both ends of each of two hafnium plates13are bent, and then, in Step90, the hafnium plates are assembled in such a manner as to face each other, followed by welding seams thereof, so that a hafnium flat tube7is completed.

(6) Control Rod1for Boiling Water Reactor

InFIG. 3, the hafnium flat tube7, which has been manufactured in Step90in the manner described in the item (5), is fixed to the control rod supporting structural body2which has been manufactured in Step70in the manner described in the item (4). Here, the upper and lower ends of each of the hafnium flat tubes7to be provided in the upper and lower parts are fixed to the handle5and the base member6aof the velocity limiter6with pins as described above, respectively.

The hafnium tubes7fixed to the four positions as described above are then covered with the sheaths8, respectively, in such a manner that the sheaths8respectively incorporate the hafnium tubes7from a tip of the U-shape, and the projections8aof each of the sheaths8are fitted onto the steps4bof each of the arms of the tie rod4. Here, the lower end5aof the handle5and the upper end6a1of the base member6aof the velocity limiter6are provided with a step5b(seeFIG. 9) and a step6ab(seeFIG. 11) similar to the steps4b, and an upper edge8cand a lower edge8dof each of the sheaths8are fitted onto the steps5band6ab, respectively.

In Step110, fitting portions of the projections8ain the steps4b, the upper edges8cin the step5b, and the lower edges8din the step6abare subjected to a laser welding. Thus, the sheaths8are fixed to the control rod supporting structural body2, so that the control rod1for boiling water reactor is completed.

In the method for manufacturing the control rod1for boiling water reactor through the above-described process steps, the greatest characteristic is that, in performing the laser welding with the projections8a, the upper edge8cand the lower edge8dof each of the sheaths8being fitted onto the steps4b, the step5band the step6a1, a beam axial center position of the laser beam is shifted from an end face position of the sheath8to a side opposite to the sheath8to weld them. Hereinafter, details of the laser welding will be described taking an example when welding the sheath8on the tie rod4.

FIG. 5is a conceptual block diagram showing a general construction of a YAG laser welding machine used in the laser welding. InFIG. 5, a YAG laser welding machine14is provided with a machining table15on which the tie rod4and the sheath8are placed, a holding fixture16for holding the sheath8, a laser beam machine17, a laser oscillator18for emitting a YAG laser beam23which will be described later and a control device19. The laser welding machine17is provided with rails17a, a frame17bcapable of moving in directions indicated by the arrow A, a support member17chaving a substantially L shape which is mounted on the frame17b, a slider17dcapable of moving in directions indicated by the arrow C, a support rod17eextending downward from the slider17dand a machining head17fcapable of moving in directions indicated by the arrow B along the support rod17e. Owing to this structure, the machining head17fcan move in three axial directions of A, B and C with respect to the machining table15.

The control device19is connected with the frame17bof the laser beam machine17and with the laser oscillator18respectively by a signal line20and a signal line21, while the laser oscillator18is connected with the machining head17fby an optical fiber22. Further, an operation panel (not shown) is connected with the control device19, so that an operator uses the operation panel to control a position of the machining head, laser output and so forth. Here, the projections8aconstitute the tips of the U-shape of each of the sheaths which are recited in the appended claims.

Next, details of the first embodiment of the method for manufacturing a control rod for boiling water reactor using the cut tie rod4of the above-described structure will be described with reference toFIGS. 6 and 7.FIG. 6is a perspective view showing an enlarged part of welded portion of the sheath8and the tie rod4which are welded using the YAG laser welding machine14, andFIG. 7is a cross-sectional view taken along the section plane indicated by VII—VII in FIG.6.

InFIGS. 6 and 7, according to the present embodiment, the operator uses the operation panel, while moving the machining head17fin a longitudinal direction (in a direction of the arrow D) of the tie rod4, to perform a continuous laser welding of the projection8aof the sheath8on the step4bof the tie rod4. In this laser welding, a shield gas24is fed from the machining head17fat the same time with the irradiation of the YAG laser beam23to prevent oxidization of the welded portion. Further, since a welding bead (not shown) immediately after the welding is susceptible to the oxidization, a trailer gas26is blown to the welding bead from a trailer nozzle25to prevent the oxidization. It is the greatest characteristic of the present embodiment that, the axial center position23A (seeFIG. 7) of the YAG laser beam23is shifted from an end face4b1of the tie rod step4btoward the tie rod4(to the side opposite to the sheath8) to irradiate a surface of the tie rod4directly with the YAG laser beam23for laser welding.

In the conventional technique, wherein the axial center position of the beam is shifted toward the sheath8(in a side opposite to the tie rod4) from the end face4b1of the tie rod step4b, it is necessary to control an irradiation position of the YAG laser beam23to be located in a very narrow overlap L1(seeFIG. 7) as mentioned above. If an error in the axial center position23A of the YAG laser beam23occurs to irradiate a portion which is shifted from the overlap L1toward the sheath8with the YAG laser beam23, the sheath8is heated too much since heat generated by the irradiation of the YAG laser beam23is difficult to be transferred to the tie rod4. Thus, in the conventional technique, the projection8aof the sheath8has been melted down, resulting in a welding failure in some cases.

By contrast, according to the present embodiment, the surface of the tie rod4is firstly irradiated with the YAG laser beam23, and then heat generated by the irradiation is transferred from the surface of the tie rod4to the sheath8via the tie rod step4b. Accordingly, even if a small error in the axial center position23A of the YAG laser beam occurs and the YAG laser beam slightly deviates from the target position, the heat is transferred to the sheath8after passing the contact portion of the tie rod step4bwith the sheath8without fail, to thereby prevent the welding failure which otherwise would be caused by the melt-down of the projection8aof the sheath8. Therefore, as compared with the conventional technique, the present embodiment prevents the melt-down of the sheath8to secure a good weldability without controlling the axial center position23A of the YAG laser beam23with high precision. Thus, the present embodiment facilitates the laser welding control and the manufacture of the control rod1for boiling water reactor, and achieves a reduction in manufacturing cost.

Description has been made on an example in the welding of the sheath8on the tie rod4, while the following describes an example in the welding of the sheath8on the handle5.FIG. 8is a perspective view showing an enlarged part of a welding portion in the welding of the sheath8on the handle5using the YAG laser welding machine14.FIG. 9is a cross-sectional view taken along the section plane indicated by IX—IX in FIG.8. Among the elements shown inFIGS. 8 and 9, those also shown inFIGS. 6 and 7are denoted by the same reference numerals, and explanations therefor will be omitted in the following description.

In the welding of the sheath8to the handle5, the machining head17fis moved in a direction along the upper edge8cof the sheath8(in a direction indicated by the arrow E) to perform a continuous laser welding of the upper edge8cof the sheath8on the step5b(seeFIG. 9) of the lower end5aof the handle5as shown inFIGS. 8 and 9. Here, in the same manner as in the welding of the sheath8on the tie rod4described above, the axial center position23A of the YAG laser beam23is shifted from an end face5b1(seeFIG. 9) of the handle step5btoward the handle5(to the side opposite to the sheath8) to directly irradiate a surface of the handle5with the YAG laser beam23for laser welding. In this case, too, heat generated by the irradiation of the YAG laser beam23is transferred from the surface of the handle5to the sheath8via the handle step5b. Therefore, similarly to the above described welding of the sheath8on the tie rod4, the present embodiment prevents the melt-down of the sheath8to secure the good weldability without controlling the axial center position23A of the YAG laser beam23with high precision. Thus, the present embodiment facilitates the laser welding control and the manufacture of the control rod1for boiling water reactor, and achieves the reduction in manufacturing cost.

Next, the welding of the sheath8on the base member6aof the velocity limiter6will be described.FIG. 10is a perspective view showing an enlarged part of the welding portion of the sheath8on the base member6aof the velocity limiter6which are welded by using the YAG laser welding machine14.FIG. 11is a cross-sectional view taken along the section plane indicated by XI—XI in FIG.10. Among the elements shown inFIGS. 10 and 11, those also shown inFIGS. 6 and 7are denoted by the same reference numerals, and explanations therefor will be omitted in the following description.

In the welding of the sheath8on the velocity limiter base member6a, the machining head17fis moved in a direction along the lower edge8dof the sheath8(in a direction indicated by the arrow F inFIG. 10) to perform a continuous laser welding of the lower edge8dof the sheath8on the step6ab(seeFIG. 11) of the upper end6a1of the velocity limiter base member6aas shown inFIGS. 10 and 11. Here, in the same manner as in the welding of the sheath8on the tie rod4described above, the axial center position23A of the YAG laser beam23is shifted from an end face6ab1(seeFIG. 11) of the step6abof the velocity limiter base member6atoward the velocity limiter base member6a(to the side opposite to the sheath8) to directly irradiate a surface of the velocity limiter base member6awith the YAG laser beam23for laser welding.

In this case, too, heat generated by the irradiation of the YAG laser beam23is transferred from the surface of the velocity limiter base member6ato the sheath8via the velocity limiter base member step6ab. Therefore, similarly to the above described welding of the sheath8on the tie rod4, the present embodiment prevents the melt-down of the sheath8to secure the good weldability without controlling the axial center position23A of the YAG laser beam23with high precision. Thus, the present embodiment facilitates the laser welding control and the manufacture of the control rod1for boiling water reactor, and achieves the reduction in manufacturing cost.

In addition, although the tie rod4, the handle5and the velocity limiter base member6aas members to be welded are directly irradiated with the YAG laser beam23in the first embodiment of the present invention, a welding rod may be used for promotion of fusion (see FIG.23). In this case, since the welding rod is irradiated with the YAG laser beam23, heat generated by the irradiation is transferred from the welding rod (more precisely, a melted welding rod) to the sheath8to prevent the melt-down of the sheath8, thereby achieving the good weldability.

Next, a method for manufacturing a control rod for boiling water reactor and the control rod for boiling water reactor according to a second embodiment of the present invention will be described with reference toFIGS. 12to15. In the present embodiment, the control rod for boiling water reactor is manufactured by using the above-described drawn tie rod12which is formed only by drawing, or, without the cutting process.

FIG. 12is a partly exploded perspective view showing a general structure of the control rod for boiling water reactor according to the present embodiment. Among the elements shown inFIG. 12, those also shown inFIG. 1are denoted by the same reference numerals, and explanations therefor will be omitted in the following description.

InFIG. 12, a control rod1′ for boiling water reactor is provided with a control rod supporting structural body2′ comprising the drawing tie rod12, a handle5which is fitted onto an upper end of the drawing tie rod12and a velocity limiter6which is fixed to a lower end of the drawn tie rod12.

Process steps for manufacturing the control rod for boiling water reactor according to the second embodiment of the present invention will be described with reference to FIG.13. Among the elements shown inFIG. 13, those also shown inFIG. 3are denoted by the same reference numerals, and explanations therefor will be omitted in the following description.

Steps10to30for manufacturing sheaths8, Step40for manufacturing the drawn tie rod12and Step60for manufacturing the handle5, velocity limiter6and other members are the same as those of the first embodiment.

Although the cutting process is performed in Step50succeeding to Step40in the first embodiment as shown inFIG. 3, the cutting process is not performed in the present embodiment. The handle5and the velocity limiter6are fixed to the upper end and the lower end of the drawn tie rod12, respectively by an assembly welding, and the other members are assembled and welded as required in Step70, so that the control rod supporting structural body2′ is completed.

Steps80and90for manufacturing hafnium flat tubes7, Steps100and110for manufacturing the control rod1′ for boiling water reactor are the same as those of the first embodiment.

Here, the drawn tie rod12constitutes the tie rod for fixing the sheaths which is prepared by drawing and provided with steps at each of the tips of the arms of the cruciform as recited in the appended claims.

Next, details of the method for manufacturing the control rod for boiling water reactor of the present embodiment will be described. As shown inFIGS. 14 and 15, a machining head17fis moved in a longitudinal direction of the drawn tie rod12(in a direction indicated by the arrow G) to perform a continuous laser welding of the projection8aof the sheath8on the step12bof the drawn tie rod12in the present embodiment. Here, in the same manner as in the first embodiment, an axial center position23A (seeFIG. 15) of a YAG laser beam23is shifted toward the drawn tie rod12(to the side opposite to the sheath8) from an end face12b2of the drawn tie rod step12bto directly irradiate a surface of the drawn tie rod12with the YAG laser beam23for laser welding.

Since each of corners12b1(seeFIG. 15) of the step12bof the drawn tie rod12is in a slightly R-shape due to the omission of the cutting process as described above, the projection8aof the sheath8is not completely fitted onto the end face12b2of the step12b. More specifically, an overlap L2(seeFIG. 15) in the present embodiment is narrower than the overlap L1of the sheath8with the cut tie rod4of the first embodiment.

In the conventional technique, wherein the laser welding is performed with the axial center position23A being shifted toward the sheath8(in a direction opposite to the drawn tie rod12) from the end face12b2of the step12b, it is necessary to control the irradiation position of the YAG laser beam23to be located inside the very narrow overlap L2which is yet narrower than the overlap L1in the first embodiment, to thereby increase the possibility of the melt-down of the sheath8due to an error in controlling the laser irradiation position. Also, since a thermal transfer from the sheath8to the drawn tie rod12is smaller due to the narrowed overlap L2, the possibility of the melt-down of the sheath8, which results in a welding failure, is further increased in the conventional technique.

By contrast, in the present embodiment, since the surface of the drawn tie rod12is irradiated with the YAG laser beam23, heat generated by the irradiation is transferred from the surface of the drawn tie rod12to the sheath8via the drawn tie rod step12. Therefore, the present embodiment prevents the melt-down of the projection8aof the sheath8and the welding failure even if a small error in the irradiation position of the YAG laser beam23occurs.

More specifically, according to the present embodiment, by shifting of the irradiation position of the YAG laser beam23toward the drawn tie rod12(in a direction opposite to the sheath8), it is possible to prevent the melt-down of the sheath8and to secure the good weldability even if the drawn tie rod12is not machined into the precise rectangular shape and remains the R-shape on the corner12b1. Thus, it is possible to omit the machining step from a typical tie rod manufacturing process consisting of the process steps of formation of the drawn tie rod12from a material by drawing and machining of the steps12bto achieve the rectangular shape, thereby enabling the control rod1′ for boiling water reactor to be manufactured by using the drawn tie rod12prepared only by the drawing process. Therefore, the present embodiment facilitates the manufacture of the control rod by a step corresponding to the omitted machining process, which leads to a reduction in manufacturing cost.

In addition, the welding rod described in the first embodiment may be used also in the present embodiment for promotion of fusion (see FIG.23). In this case, too, it is possible to achieve the above-described effects of the present embodiment.

Next, a third embodiment of the method for manufacturing a control rod for boiling water reactor of the present invention will be described with reference toFIGS. 16to23. In the present embodiment, weldings of sheath8on the tie rod4, the sheath8on the handle5, the sheath8on the velocity limiter base member6aare automated.

For the purpose of automating the laser welding, the inventors of the present invention have conducted welding experiments using the sheath8and the tie rod4under various welding conditions to find out a welding condition under which a prevention of the melt-down of the sheath8as well as a perfect weld penetration are achieved.FIG. 16shows a range of the welding conditions. As a result of the welding experiments, states after the weldings are broadly classified into three states of a state wherein a penetration bead is not formed and the perfect weld penetration is not achieved, a state wherein the perfect weld penetration is achieved and a state wherein the sheath8is melted down.

Then, the inventors have converted the three states onto numerical values by using a heat input parameter Po which relates to an amount of heat input. More specifically, the state wherein the perfect weld penetration is not achieved due to an insufficient heat input is represented by Po=−1, the state wherein the perfect weld penetration is achieved is represented by Po=0, and the state wherein the sheath8is melted down is represented by Po=1.

Further, the inventors have conducted multiple regression analyses of welding conditions associated with the above three states to obtain an analysis parameter P represented by the following equation:
P=0.184+1.11×G+0.964×A+1.07×H−1.17×D−0.11×W−0.807×L
where G represents a gap (mm) between an inner surface8a5(seeFIG. 21) of a sheath projection8aand a base4b3of a tie rod step4bin a state where the sheath projection8ais fitted onto the tie rod step4b; A represents a distance (mm) (hereinafter referred to as “laser irradiation position A” when so required) from an axial center position23A of a YAG laser beam23to an edge8a1(seeFIG. 20) of the sheath projection8aon the premise that a direction toward the sheath8is a positive direction and a direction toward the tie rod4is a negative direction; H represents heat input (kj/cm) by the YAG laser beam23; D represents a beam converging diameter (mm) of the YAG laser beam23; W represents a supply (g/m) of a welding rod30for one meter of welding length; and L represents an overlap (mm) (seeFIG. 20) of the inner surface8a5of the sheath projection8awith the base4b3of the tie rod step4bin the state where the sheath projection8ais fitted onto the tie rod step4b.

FIG. 17shows a relationship between the heat input parameter Po and the analysis parameter P. According toFIG. 17, the heat input parameter Po becomes 0 when the analysis parameter P is in the range of −0.5 to 0.5 to achieve the perfect welding. More specifically, as can be seen fromFIG. 17, if values for the gap G, laser irradiation position A, heat input H, beam converging diameter D, control rod supply W and overlap L are given, it is possible to find out the state after welding by the heat input parameter Po since the heat input parameter Po is dependent on the analysis parameter P. According to the inventors' research, the most satisfactory conditions of the above values are in the following range: 0 to 0.3 mm of the gap G; 0 to −0.5 mm of the laser irradiation position A; 0.89±0.2 kj/cm of the heat input H; 0.57 to 0.6 mm of the beam converging diameter D; 3.16 to 4.06 g/m of the welding rod supply W; and 0.3 to 0.6 of the overlap L.

FIG. 18is a conceptual block diagram showing a general construction of an automatic YAG laser welding machine which performs the automatic welding using the above analysis parameter P. Among the elements shown inFIG. 18, those also shown inFIG. 5of the first embodiment are denoted by the same reference numerals, and explanations therefor will be omitted in the following description. InFIG. 18, the automatic YAG laser welding machine27is provided with a laser scanning two-dimensional displacement sensor (not shown) attached to the machining head17f, an welding rod supply device (not shown) for performing an automatic supply of a welding rod30, which is to be described later in this specification, as being attached to the machining head17f, a processor29which is connected with the control device19via a signal line28, and a servo motor (not shown) for moving the machining head17fto a welding start position and a welding completion position which are instructed by the control device19.

The processor29calculates the welding start position, welding completion position, gap G and overlap L from values detected by the laser scanning two dimensional displacement sensor, and further calculates the laser irradiation position A, heat input H, beam converging diameter D and welding rod supply W from the gap G and overlap L using the analysis parameter P.

Next, details of the method for manufacturing a control rod for boiling water reactor of the present embodiment using the above-described automatic YAG laser welding machine27will be described.FIG. 19shows a scanning method of the laser scanning two dimensional displacement sensor;FIG. 20is a cross-sectional view taken along the section plane indicated by XX—XX inFIG. 19; andFIG. 21is a longitudinal sectional view taken along the section plane indicated by XXI—XXI in FIG.19.

Referring toFIGS. 19to21, the automatic YAG laser welding machine27detects coordinates of an edge8b1(seeFIG. 20) of a recess8bof the sheath8and an outer corner4b2(seeFIG. 20) of the step4bnear a tip of the tie rod4by automatically scanning in a direction indicated by the arrow H in FIG.19. Also, the automatic YAG laser welding machine27detects coordinates of both edges8a2and8a3(seeFIG. 21) of the projection8aof the sheath, a height of the base4ba(seeFIG. 21) of the step4bof the tie rod4and a height of an outer surface8a4of the sheath projection8aby scanning in a direction indicated by the arrow I in FIG.19.

The processor29calculates the gap G between the sheath8and the tie rod4and the overlap L of the sheath8with the tie rod4from the data which are obtained by the two scannings of the laser scanning two dimensional displacement sensor as well as a length of the sheath projection8a(a distance between the edge8a1of the sheath projection8aand the edge8b1of the sheath recess8b) and a thickness of the sheath8which are inputted by, for example, the operator. The coordinates of the both edges8a2and8a3of the projection8aof the sheath8which are obtained by the above scanning are used as the welding start position and the welding completion position as they are.

Also, the processor29calculates the laser irradiation position A, heat input H, beam converging diameter D and welding rod supply W to achieve the analysis parameter P of not less than −0.5 to not more than 0.5 by using the thus obtained gap G and overlap L. At this point, a value of the laser irradiation position A is set to a negative value (i.e. to be shifted in a direction toward the tie rod4) in advance of the calculation by, for example, the operator in view of the prevention of the melt-down of the sheath8similarly to the first and the second embodiment.

The control device19, which has obtained from the processor29the welding start and completion positions, gap G, laser irradiation position A, heat input H, beam converging diameter D, welding rod supply W and overlap L, controls the laser welding machine17and the laser oscillator18, and performs the automatic laser welding of the sheath8on the tie rod4, so that the welding rod30is irradiated with the YAG laser beam23as being placed at a position corresponding to the axial center position23A of the YAG laser beam23which is shifted toward the tie rod4(in a direction opposite to the sheath8) from the end surface4b1of the tie rod step4bas shown inFIGS. 22 and 23, thereby automatically achieving the perfect weld penetration.

According to the present embodiment described above, since the YAG laser beam23irradiates the welding rod30, heat generated by the irradiation is transferred from the welding rod30to the sheath8. In particular, since the welding rod30is irradiated with the YAG laser beam23which is shifted toward the tie rod4in the same manner as in the first and second embodiments, a surface of the tie rod4is irradiated with the YAG laser beam23if the irradiation position is erroneously deviated from the welding rod30. More specifically, heat generated by the irradiation transfers from the surface of the tie rod4to the sheath8via the tie rod step4bin the same manner as in the first embodiment. Therefore, according to the present embodiment, the melt-down of the sheath8is prevented without fail to achieve the good weldability.

Further, according to the present embodiment, since the automatic YAG laser welding machine27performs the laser welding to automatically achieve the perfect weld penetration by calculating the laser irradiation position A, heat input H, beam converging diameter D and welding rod supply W, it is possible to prevent the melt-down of the sheath8more securely to achieve the good weldability. Moreover, owing to the automatic laser welding, effects such as a reduction in workload of welding operators and improvements in productivity of control rods are achieved.

Although the present embodiment is described in connection with the welding of the sheath8on the tie rod4, it is possible to perform the automatic weldings of the sheath8on the handle5, and the sheath8on the velocity limiter base member6aby the same process steps to achieve the same effects.