Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Having particular end closure or seal
Patent
1991-10-15
1993-10-05
Walsh, Donald P.
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Having particular end closure or seal
376261, G21C 310
Patent
active
052512479
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a method for sealing a fuel rod by welding after the fuel rod has been pressurized with helium.
BACKGROUND ART
In the manufacture of nuclear fuel for light water reactors, tubes of usually a zirconium alloy, such as Zircaloy-2 or Zircaloy-4, are provided with an end closure, whereupon the tube is filled with fuel pellets to the desired length. After that, the rod is pressurized with helium before it is completely sealed. By filling the fuel rod with helium, the heat transfer from the pellet out towards the coolant is improved.
During the sealing of the fuel rod, the rod is usually provided with plugs of a zirconium alloy which are designed such that part of the plug may make contact with the tube end of the rod, and by welding around the joint between tube and plug, the rod is sealed. A number of different welding methods, such as TIG welding, EB welding or resistance welding, are used in this connection. When using welding methods which do not require vacuum, for example TIG and resistance welding, pressurization of the rod with helium can be carried out in connection with the plug being welded on. For welding methods which require vacuum, for example EB welding, pressurization of the rod must be carried out separately. This is done by providing the plug with a fill hole through which helium may be supplied to the rod and which is thereafter sealed by means of TIG welding.
The disadvantages of the prior art methods of sealing a fuel rod after pressurization are that TIG welding functions badly in helium environments. High voltages are required for igniting the welding arc, and welding electrodes are consumed very rapidly and have to be replaced. Since this type of manufacturing usually takes place in continuous lines, frequent replacements of welding electrodes entail considerable drawbacks.
Conventional resistance welding, of the joint between the plug and the tube end of the rod entails problems in that the type of defects which may occur at the weld joint after resistance welding, such as, e.g., oxide streaks, cannot be controlled by some form of non-destructive testing. Since nuclear fuel rods operate at high pressures and contain substances, the spreading of which outside the rod is not desirable, it is of the utmost importance that sealing welds do not contain defects which may lead to continuous cracks in the weld.
SUMMARY OF THE INVENTION
To solve the above problems, according to the invention the circular weld between the plug and the rod may be carried out with a method such as, e.g., EB welding or TIG welding, which permit inspection by means of non-destructive testing and, after that, pressure reduction with helium may be carried out via the plug through a fill hole provided for the purpose. For sealing the fill hole, a body of a zirconium alloy of suitably the same type as that of which the plug consists is placed over the fill hole and formed such that the body has a volume which is larger than the volume of the upper part of the fill hole and the body makes contact with the edge of the fill hole along a border so that the contact surface is as small as possible. This can be accomplished by giving at least part of the body a tapering diameter, for example conical or spherical. When the rod is to be sealed, this can be done by resistance projection welding. The rod is thereby placed between two electrodes, one of which is placed over the body. A force is applied to one electrode and an electric current is passed through the body and the plug. Since the contact surface between body and plug is small, a high current concentration is obtained and the heating of the parts of the workpiece, which are in contact with each other, is good. When the body is pressed against the fill hole, material from the edge of the fill hole is welded together with material from the body and the body is pressed into the fill hole. The upper surface of the body is also deformed and may be pressed so that the upper surface of the body gets on a level with the surface of
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patent: 3902038 (1975-08-01), Heichel
patent: 4273983 (1981-06-01), Ogawa et al.
patent: 4478787 (1984-10-01), Nadkarni et al.
Sahlin Thorbjorn
Wisen Katarina
ABB Atom A.B.
Chelliah Meena
Walsh Donald P.
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