Test fabrication of JOYO fuel, (5)

The cause of blow holes that nucleate in the welding of an end plug and a cladding has been investigated. The rejection of a fuel pin with a welded upper plug gives rise to heavy loss, because the rejected pin must be broken into pieces in a glove box and mixed oxide pellets must be removed from it. It will give a great advantage if the cause of welding defects could be removed as completely as possible. TIG method was selected for this work. The welding conditions were as follows. (1) Welding current 8.5 A (2) Rotation of fuel pin 20 rpm (3) Arc-gap 0.5 mm (4) Torch angle 600 to fuel pin axis (5) Atmosphere He for upper plug welding, He + 20% Ar for lower plug welding (6) Torch gas He 8 l/min. It was found that blow holes were not nucleated by H2 and N2 released due to the different solubility in the liquid and the solid phases of austenitic stainless steel. It is considered that blow holes are due to the space of mismatch between an end plug and a cladding tube. (Iwase, T.)