Method for welding of fuel rod

A resistance welding method for a fuel rod is presented in which the gas seal opening is closed with a sealing material of the same composition as the end plug material. The method enable to perform welding stably in a pressurized welding chamber, and the process is amenable to automation. The sealing material body is made of a wire, and is pressed against the conical shaped opening provided on the end plug. The welding method avoids contaminating the weld with undesirable impurities from the electrode material, and the welding operations is designed so that there is no need to reshape the end of the electrode or adjust the gap between the sealing material and the seal opening.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a technique of resistance welding of a 
seal opening provided on an end plug of a fuel rod containing fuel pellets 
and pressurized to a specific gaseous pressure with an inert gas filled 
through the seal opening. 
2. Technical Background 
In general, a fuel rod for use in a light water pressurized reactor is made 
by packing fuel pellets inside a fuel tube, and the ends of the fuel tube 
are fitted with end plugs and the end plugs are joined to the ends of the 
fuel tube by means of joining techniques such as the tungsten inert gas 
(TIG) welding. Further, there is a (gas) seal opening disposed on one of 
the two end plugs for filling the fuel tube interior with an inert gas 
under pressure. The gas seal opening is sealed off, by joining methods 
such as TIG welding, so as to maintain the gaseous pressure of the fuel 
tube at a specific value. 
TIG welding for general welding applications is performed in an open 
atmospheric pressure, and an inert gas is circulated so as to protect the 
tungsten electrode and the object to be welded from the chemical effects 
of the ambient air atmosphere, and to provide cooling. 
However, because the welding of the seal opening of a fuel rod must be 
carried out in a pressurized welding chamber, it is difficult to flow the 
inert gas between the welding object and the tungsten electrode. It is 
also difficult to generate the welding arc in the pressurized welding 
chamber, and it is often necessary to overheat the electrode in the 
initial stage of the welding operation. Accordingly, when performing TIG 
welding in a pressurized welding chamber, there is a rapid wear of the 
tungsten electrode, and depending on the welding conditions, there is a 
danger of inclusion of the tungsten in the metal of the weld. It is not 
desirable that tungsten becomes mixed in the weld, because when it 
happens, the weld quality becomes degraded, for example, by the loss of 
the corrosion resistance of the weld. Therefore, it was necessary that the 
welding operation be carried out with utmost care. The tungsten electrode 
suffered shape changes after each welding operation, due to such factors 
as melting and vaporization of part of the electrode, thus necessitating 
adjustments in the arc gap for every welding as well as frequent 
exchanging of the electrode for grinding of the electrode so as to reshape 
the tip to the required shape. 
SUMMARY OF THE INVENTION 
The present invention was made in view of the technical problems described 
above, by developing a method for resistance welding in a welding chamber 
of a gas seal opening provided on an end plug, comprising the steps of: 
(a) pressing a sealing material body against the gas seal opening; and (b) 
passing a current through contact electrodes disposed on the sealing 
material body and on the fuel tube so as to heat a joint between the 
sealing material body and the end plug to cause resistance welding of the 
joint. 
According to the welding method of the invention, welding is performed 
while Dressing a sealing material body, preferably made of the same 
material as the end plug, against the gas seal opening of the end plug, 
and passing a current between the end plug and the sealing material body, 
thereby heating the end plug and the sealing material body by specific 
resistance and contact resistance heating effects. The temperature rises 
until the joint is hot enough to form a metallurgical bond between the end 
plug and the sealing material body, and due to the pressing applied, 
resistance welding of the joint is achieved smoothly. 
The technique allows the welding to be performed, even under the condition 
of an inert gaseous pressure, stably in terms of the bead position and the 
melt-in shape, and enables to continuously weld a plurality of end plugs. 
The technique is further superior to the conventional TIG welding because 
it enables to eliminate shape adjustments of the tungsten electrode and 
gap setting between the tungsten electrode and the end plug before 
undertaking every welding operation. A further feature of the invented 
welding method is that it enables automation of the welding process of the 
fuel rod. 
A preferred method is to include the steps wherein prior to the steps (a) 
and (b), the fuel tube is readied for loading fuel pellets, and is loaded 
with a plurality of fuel pellets. 
The method of resistance welding is thus able to eliminate the danger of 
including the electrode material such as tungsten in the welded metal; to 
eliminate the necessity for operational adjustments and confirmation 
required before undertaking each welding operation, such as adjusting the 
arc gap and reshaping of the electrode which were required in the 
conventional TIG welding. 
In a further embodiment of the invention, the welding is conducted in a 
chamber filled with an inert gas to a specific gaseous pressure which is 
equalized through the gas seal opening with the inner gaseous pressure of 
the fuel tube. 
Through the embodiment of the invention, stable continuous welding can be 
obtained even under pressurization.

PREFERRED EMBODIMENTS 
A first embodiment of the present invention will be explained with 
reference to FIGS. 1 and 2. 
In the figures, the reference numeral 1 refers to a fuel tube for making a 
fuel rod which houses pellets and a coil spring. At each end of the fuel 
tube 1, an end plug 2 is inserted and joined to the tube by welding the 
peripheries. A (gas) seal opening 3 is formed on the upper end plug of the 
above two end plugs 2 for the fuel rod, and the exterior shape of the seal 
opening 3 is a conical opening 4 so as to enlarge its diameter as it 
extends outward away from the fuel rod. 
The fuel rod of the above configuration is welded in a welding chamber C as 
follows. First, an inert gas He is charged into the fuel tube 1 from the 
seal opening 3 of the one end plug 2. Then, as shown in FIG. 1, a sealing 
material body 5 of a wire or rod shape, made of zircalloy which is the 
same material as the one end plug 2, is pushed up against the conical 
opening 4. The diameter of the wire 5, such as 2 mm diameter, is chosen so 
as to permit the wire 5 to be in contact with the conical opening 4. To be 
able to flow a large amount of current (example, several thousand amperes) 
concentrated in the pressed region, the vicinities of the end plug 2 and 
the wire 5 are clamped with contact electrodes 6, 7 made of such materials 
as copper alloys, and a large current is passed through. Then, while 
pressing the contact electrodes 6, 7 towards each other, the end plug 2 
and the wire 5 are heated by the specific and contact resistance effects 
at the joint, and the temperature rises to induce welding in the joint 
region. Accordingly, due to the pressing force exerted on the joint 
region, the wire 5 is resistance welded with assistance from the force of 
pressing in the conical opening 4 so as to weld to the conical opening 4 
to seal the gas seal opening 3, thus sealing the fuel rod under a specific 
gaseous pressure of the inert gas He maintained in the welding chamber C. 
When the above resistance welding operation is completed, the excess wire 
5 is cut off to complete the welding operation as shown in FIG. 2. The 
wire 5 is forwarded by such method as pinch rollers to supply the wire for 
the next welding operation. Therefore, it becomes possible not only to 
perform every welding operation using the freshly cut surface of a wire 5, 
but also to use a long length of wire 5, thus enabling to weld a plurality 
of end plugs 2 continuously under stable welding conditions, thus opening 
a way to automating the welding process. 
In a variation of the first embodiment, it is also possible to shape the 
end of the wire 2 so as to provide a corresponding conical plane 8 for 
plane contact between the wire 5 and the conical opening 4 as illustrated 
in FIG. 3. By utilizing such a configurational arrangement, it provides a 
wider range of flexibility in the choice of joining conditions. 
The present application claims the priority of Japanese patent application 
filed in Japan, No. H04-312467, on Nov. 20, 1992, which is herein 
incorporated by reference.