Shohei Kawano

Simulation of helium bubble behavior in neutron-irradiated stainless steel during welding

A simulation model of helium bubble behavior and crack initiation at grain boundaries in the heat-affected zone (HAZ) during welding is proposed, and the effects of welding conditions and helium concentration on bubble evolution and cracking are evaluated. The model was based on the following assumptions; homogeneous bubble nucleation, bubble coalescence by random motion, bubble growth by vacancy absorption and ductile fracture. The result of calculations for different welding heat inputs reveal that the final bubble size increases with increasing the weld heat input and with decreasing bubble spacing at the grain boundary. The calculated critical heat input for cracking as a function of helium concentration is in good agreement with the results of welding experiments on neutron-irradiated stainless steels and helium-doped stainless steels.

Development of Underwater Laser Cladding and Underwater Laser Seal Welding Techniques for Reactor Components (II)

Stress corrosion cracking (SCC) is one of the major reasons to reduce the reliability of aged reactor components. Toshiba has been developing underwater laser welding onto surface of the aged components as maintenance and repair techniques. Because most of the reactor internal components to apply this underwater laser welding technique have 3-dimensional shape, effect of welding positions and welded shapes are examined and presented in this report.Copyright

Effect of weld thermal cycle, stress and helium content on helium bubble formation in stainless steels

Helium bubble structure was examined on a helium-implanted stainless steel after applying thermal and stress cycle using a weld thermal and stress cycle simulator. Helium ions were implanted on Type 304 stainless steels up to 200 appm uniformly to a depth of 3.5 μm. The specimens were heated at various temperatures between 1073 and 1473 K for 2 s in Ar gas atmosphere. Tensile stresses from 0.5 to 8 MPa were applied during the thermal cycle. TEM observations revealed that size of the bubbles at grain boundaries was larger for the specimens with a higher tensile stress and with a higher annealing temperature. Densities of bubbles increased with increasing helium content. A theoretical model calculation showed a good agreement with the experimental results.

Weldability of helium-containing stainless steels using a YAG laser

Bead-on-plate welding experiments using a 400 W YAG laser were conducted on SUS304 stainless steels implanted with helium ions of 0.5, 5 and 50 appm uniformly to a depth of 0.25 mm. High heat input welding at 20 kJ/cm caused surface grain boundary cracking in the heat-affected zone at 50 appm He. Cross-sectional observations after etching in oxalic acid solution revealed that bubble growth at grain boundaries in the heat-affected zone was enhanced at higher heat input and at higher helium concentrations. Bubble growth was negligible for the laser welding condition of 1 kJ/cm even at 50 appm He. The results suggest that YAG laser welding is a promising welding technique for stainless steels containing high amounts of helium.

Development of Ultrasonic Shot Peening Technique for Reactor Components to Improve Structural Integrity Against Stress Corrosion Cracking

The ultrasonic shot peening (USP) technique has been developed for boiling water reactor (BWR) components as a countermeasure against stress corrosion cracking. The effects on residual stress of USP for type 316L stainless steel and alloy 600 were evaluated. Compressive residual stress layer of 0.5 mm from the surface were formed on the specimens after USP using stainless steel ball with a diameter of 3 mm. Cross-sectional hardness measurement revealed that the increase of hardness due to USP is not significant compared with shot peening (SP). The FEM calculation showed the plastic strain induced by the impacts of 3 φ shot with 5 m/s is lower than those of 0.6 φ shot with 50 m/s. It suggests USP process suppresses the degree of work hardening in comparison with SP process. Dissimilar weld joint specimens which simulate the material and dimension of the shroud weld line H7 were examined to confirm the applicability of USP. The experimental result reveals that USP technique is applicable to reactor internal components as stress modification process.Copyright

Weld repair technique of helium‐containing reactor structural materials

Structures in the inside of reactors of commercial nuclear power plants are comprised of austenitic stainless steels and nickel base alloys which have excellent corrosion resistance in high temperature water. These materials accumulate nuclear transformation helium within themselves when they are irradiated with neutrons near the reactor core. In 1984, when the Savannah River reactor in the United States was welded during repair, cracking occurred at the toe of the weld of the stainless steel tank. The results of the investigation made it clear that the weld cracking was caused by the nuclear transformation helium in the material. Because 20 years have now passed since the power plants built in the 1970s started operating, a lot of projects have been carried out in recent years both in Japan and abroad to develop techniques for preventive maintenance and repair so that the long-term safe operation of these plants can be secured. What has been the subject of the attention in these studies is the weldability of the materials used for the structures in the reactors which contain helium. Therefore, this paper will outline the studies made on the weldability of stainless steels with helium as well as introduce future direction of work.

Laser de-sensitization treatment for inside surface of SUS304 stainless steel pipe welds in nuclear power plants

A technology to prevent the occurrence of Intergranular Stress Corrosion Cracking (IGSCC) by irradiating a high power Nd:YAG laser beam was developed. Laser Desensitization Treatment (LDT) process was realized by irradiating a laser beam onto the sensitized Heat Affected Zone (HAZ) surface of SUS304 stainless steel. LDT was formed by both a molten layer of approximately 0.2mm depth and a solution heat treated layer. The results of a Creviced Bent Beam (CBB) test showed that no cracks had appeared on the surface of LDT. After LDT was applied in the vicinity of welding joints on the inside surface of pipes, tensile residual stress was measured there. On the other hand, the tensile stress of outside surface of the pipes was decreased. From these results, LDT processing on the inside surface of a pipe can be expected to prevent the occurrence of IGSCC owing to the effect of both metallurgical improvement and decrease of the residual stress on the outside surface of the pipe. We developed the LDT processing system and successfully applied on the pipes of some actual nuclear power plants.A technology to prevent the occurrence of Intergranular Stress Corrosion Cracking (IGSCC) by irradiating a high power Nd:YAG laser beam was developed. Laser Desensitization Treatment (LDT) process was realized by irradiating a laser beam onto the sensitized Heat Affected Zone (HAZ) surface of SUS304 stainless steel. LDT was formed by both a molten layer of approximately 0.2mm depth and a solution heat treated layer. The results of a Creviced Bent Beam (CBB) test showed that no cracks had appeared on the surface of LDT. After LDT was applied in the vicinity of welding joints on the inside surface of pipes, tensile residual stress was measured there. On the other hand, the tensile stress of outside surface of the pipes was decreased. From these results, LDT processing on the inside surface of a pipe can be expected to prevent the occurrence of IGSCC owing to the effect of both metallurgical improvement and decrease of the residual stress on the outside surface of the pipe. We developed the LDT processing sy...