Some Considerations on Internal Welding of Small-size Tubes for Nuclear Plants

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For nuclear plants, thermal power generation, etc. it has become necessary to design a steam generator and a high-pressure heater with closelyspaced lattice and tube arrangement ; those tubes become small in diameter relative to wall thickness, such as 15.9 mm in outside diameter and 3.2 mm in wall thickness. Conventional tube-to-tubesheet joint configuration which is fillet-welded defies reliable non-destructive inspection technique. In worst case, therefore, trouble occurs at these welds under commercial operation. Accordingly above-mentioned joint configuration doesn't meet the requirement at all from viewpoint of safety operation for nuclear plants where service conditions are severe. From this consideration, crevice-free type of tube-to-tubesheet joint is most desirable that can be fully inspected by means of radiography. Herein, stubs fit for tubes are formed from tubesheet by machining and thereafter internal tube-to-stub welding is employed. It is, however, very difficult satisfactorily to weld tube-to-stub joint with high reproducibility by inserting a tungsten electrode through tubesheet hole. In this paper, authors describe the effect of pulse current, pressure difference between internal and external shield gas and composition of shield gas itself on internal welds, that is, weld reproducibility and quality of internal welds and they make it clear that tube-to-stub welding can be internally conducted with satisfaction and high reproducibility under stringent welding conditions.
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Some Considerations on Internal Welding of Small-size Tubes for Nuclear Plants

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