Patent Number: 060524243
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method of welding to fabricate double-wall structures composed of inner and outer wall and reinforcing ribs provided between the outer and inner walls. The invention also relates to a double-wall structure produced by the method. Double-wall structures that can be produced by the method include ship, airplane and car bodies, as well as bridge structures, various containers for use in industrial plants, and vacuum vessels. The present invention relates particularly to a method of welding a vacuum vessel composed of a double-wall structure reinforced with ribs between the outer and inner walls. The invention also relates to a vacuum vessel produced by this welding method. The invention further relates to a method of welding a double-walled vacuum vessel for a nuclear fusion device which consists of an inner wall to be exposed to a plasma and an outer wall that surrounds it, with the walls being reinforced with ribs and welded to each other. The invention also relates to a vacuum vessel for a nuclear fusion device that is produced by this welding method. An electron beam welding method is conventionally applied to fabricate double-wall structures such as ship, airplane and car bodies, as well as bridge structures and vacuum vesslels by welding the inner and outer walls together with reinforcing ribs. In the class of nuclear fusion devices called "tokamak", a plasma created within a vacuum vessel is heated and sufficiently maintained with a strong external magnetic field to initiate a fusion reaction. The plasma-confining vacuum vessel is required to be capable of sustaining a high vacuum and function as a primary barrier against radiations. It is also required to withstand the large electromagnetic force produced by plasma disruption. In addition, in order to secure plasma control characteristics and reduce the loss of magnetic flux that occurs when a plasma is brought to energy break-even conditions, the electrical resistance of the vacuum vessel as measured in the toroidal direction of the torus must be held higher than a certain value. These requirements can be met by a vacuum vessel of double-wall structure that consists of an inner wall to be exposed to a plasma and an outer wall that surrounds it and in which the inner and outer walls are joined with ribs to segment the vessel wall in compartments. If the reinforcing ribs are arranged in a poloidal direction, only a small effect is caused on the electrical resistance of the vacuum vessel in the toroidal direction. An example of this type of vacuum vessel for use in fusion devices is described in Japanese Patent Public Disclosure No. 731919/1990. The double-wall structure adopted by such vacuum vessels has the additional advantage of securing mechanical strength and rigidity. On the other hand, extensive use of welding is required to fabricate this double-wall structure. In addition, the welding of the outer wall to ribs, which is performed after inserting a shield between adjacent ribs, can only be effected from outside. Conventionally, the outer wall is externally joined to ribs by plug welding which is illustrated in FIG. 13. However, it is difficult to secure the desired weld strength by this method. If slot welding is substituted with a view to increasing the throughput, the distortion in welding is also increased and, what is more, the man-hour required is exorbitant. SUMMARY OF THE INVENTION The present invention has been accomplished under these circumstances and has as an object providing a method of welding a double-wall structure such that even if its size is increased, the fabrication efficiency is not compromised and the distortion in welding is sufficiently reduced to enable precise assembling of a structure that can reasonably withstand the great electromagnetic force caused by plasma disruption. Another object of the invention is to provide a double-wall structure that is produced by the method. A particular object of the invention is to provide a double-walled vacuum vessel by the method that is suitable for use in a fusion device. The first object of the invention can be attained by a piercing welding method for fabricating a double-wall structure consisting of an inner wall, an outer wall surrounding said inner wall and reinforcing ribs that connect said outer and inner walls, in which an electron beam is externally applied at right angles to the outer wall such that the applied electron beam penetrates the outer wall to reach the abutting rib, whereupon the outer wall is welded to the rib, characterized in that the welded structure of the outer wall and the rib is composed of at least two piercing weld beads that are spaced apart by an unwelded area, with the sum of the widths of the weld beads being at least 25% of the rib width and the length of the unwelded area exterior to the root of each bead being no more than 20% of plate thickness. The second object of the invention can be attained by a double-wall structure consisting of an inner wall, an outer wall surrounding said inner wall and reinforcing ribs that connect said outer and inner walls, said structure being produced by a piercing welding method in which an electron beam is externally applied at right angles to the outer wall such that the applied electron beam penetrates the outer wall to reach the abutting rib, whereupon the outer wall is welded to the rib, characterized in that the welded structure of the outer wall and the rib is composed of at least two piercing weld beads that are spaced apart by an unwelded area, with the sum of the widths of the weld beads being at least 25% of the rib width and the length of the unwelded area exterior to the root of each bead being no more than 20% of plate thickness. The particular object of the invention is attained by a double-walled vacuum vessel for a fusion device which is a torus-shaped plasma container consisting of an inner wall to be exposed to a plasma, an outer wall surrounding said inner wall and reinforcing ribs that connect said outer and inner walls, with shields being inserted between the two walls and the outer wall being combined with each rib to form a T shape, further characterized by being fabricated by a piercing welding method in which an electron beam is externally applied at right angles to the outer wall such that the applied electron beam penetrates the outer wall to reach the abutting rib, whereupon the outer wall is welded to the rib and that the welded structure of the outer wall and the rib is composed of at least two piercing weld beads that are spaced apart by an unwelded area, with the sum of the widths of the weld beads being at least 25% of the rib width and the length of the unwelded area exterior to the root of each bead being no more than 20% of plate thickness. In a preferred embodiment, wraparound welding is applied to the piercing welded end of each rib such that the distortion in welding is sufficiently reduced to enable precise assembling of a structure that can reasonably withstand the large electromagnetic force caused by plasma disruption.