An investigation report on the defective occurrence of the lower end plug welded section for JOYO MK-III.; An investigation result about the inclusion occurrence

Kajiyama, Tadashi; Numata, Kazuaki; Otani, Seiji; Goto, Tatsuro*; Takahashi, Hideki*

Fabrication of the cladding tube with the lower end plug for the core fuel assembly of the experimental fast reactor JOYO and the proto-type fast breeder reactor MONJU has been performed by Tamatsukuri inspection branch of quality assurance section, Technical administration division, Plutonium fuel center since 1989. The fabrication process of the cladding tube with the lower end plug consists of numbering process of fuel element number to the lower end plug, welding process of the lower end plug and the cladding tube, and inspection process after welding (visual inspection, dimensional inspection and X-ray radiography). The processing of cladding tube with the lower end plug which is used for the initial loading core fuel assembly of JOYO MK-III has been carried out from August to May in 1996. The many tungsten inclusions were observed by X-ray radiography in the welds of the cladding tube with the lower end plug that were fabricated in the fourth fabrication campaign performed from July to August in 1996. The investigation of the causes was carried out about the welding process of the cladding tube and the lower end plug based on the fabrication record. The following three items were confirmed as a result. (1)As for the ingredients of inclusions observed by the X-ray radiography, it was proved by EPMA(Electron Probe X-ray Micro-analyzer) analysis that it was the same as the tungsten electrode. (2)Cracks, chips and consumption were observed in the tip of the electrodes used for welding of the lower end plug. And, cracks and chips were observed in the electrodes as well which hadn't been used. (3)It was proved that the tip of the electrode was exhausted remarkably when the distance between the tip of the electrode and the welding section became less than 0.08mm. (here in after to as the distance between the electrodes) Based on the above result, examination of reappearance for the inclusion generation was performed simulating the shape of cracks and chips ...