Improvement of Validation Technique for Long Term Soundness of In-core Component Materials of Light Water Reactor Plant after Repair and Maintenance

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The objective of this study is to investigate damage microstructure development including He bubble formation in the austenitic stainless steel for in-core components including transmutant He atoms due to heat treatment during re-welding and neutron irradiation after rewelding by means of the simulation experiments using light ion implantation/irradiation by using an accelerator. The formation of He atoms was simulated using He ion implantation. The heat treatment during re-welding was simulated using a static heat treatment in a furnace.Damage microstructure development including helium (He) bubble formation in solution annealed type 316L austenitic stainless steel due to heat treatment (850 and 1050_oC for 5-500 min) after He ion pre-implantation (10-1000 appm) at room temperature and hydrogen (H) ion irradiation (0-0.5 dpa) at the temperature of 300_oC after the heat treatment was investigated in order to evaluate the microstructural degradation in a HAZ (heat affected zone) region … More of in-core components of fission reactors during re-welding and post welding neutron irradiation. The growth of He bubbles was observed due to the increment of He concentration, annealing time and annealing temperature of heat treatment, and displacement damage by H irradiation. He bubbles formed during the heat treatment and after H ion irradiation were distributed both in the grain boundary and in the matrix. The significant growth of He bubbles in the grain boundary after H irradiation up to 0.5 dpa was observed.It was clarified by this study that the growth of He bubbles significantly occurred due to the increment of He-concentration, annealing time and temperature of heat treatment, and displacement damage followed by He-ion implantation and heat treatment. He bubble formation in the grain boundary is well known to cause an embrittlement, weld metal cracking and degradation of weldability. Thus, in order to keep the highly reliable operation of fission reactor plants, the identifying the He concentration before re-welding repair, re-welding under the precisely controlled conditions, and the operation term and inspection periods after re-welding repair would be significantly important. Moreover, thermal stress and its cycling occurring due to the re-welding are also considered to cause the growth of He bubble. Therefore, the effect of thermal stress level and cycling on the He bubble formation and growth behavior must be investigated in future work. Less