In repair welding for nuclear reactor vessel, low alloy steels are affected by heat input during welding process. The conventional repair welding for wall steel constructions requires post weld heat treatment (PWHT) to achieve the desired microstructure properties. However, post weld heat treatment is very difficult for some structures in operating plants. In such case, temper-bead welding technique is available as a repair welding method. Temper-bead welding employs a multi-pass deposition of welding metal. Each layer of beads provides heat for thermal treatment of the previous weld bead or layer, which lowers hardness of the heat affected zone (HAZ) and improves mechanical properties like the toughness. Toshiba has developed underwater laser cladding and laser seal welding techniques for reactor components repair welding. In this report, some experimental results of laser based underwater temper-bead welding are presented.
A remote maintenance technology to process cooling pipes internally for the ITER (International Thermonuclear Experimental Reactor) has been developed. The fundamental welding properties of austenite stainless steel obtained by using high power YAG laser indicates the tendency of porosity increase as the welding depth increases. It is also found out that the occurrence of porosity was prevented when N2 gas was used as a shielding gas and that the material strength of weld joint is kept unchanged. Furthermore, a processing equipment that can access the cooling pipes internally has been developed. The main body of this processing equipment can be inserted into a cooling pipe and consists of an focusing optics, clamping and driving mechanism, and an optical fiber. Operational experiments of this processing equipment is carried out and the basic verification experiments such as welding and cutting tests were conducted. As a result, a good quality of pipe welding joint was obtained without any defects.
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.
Patulous eustachian tubes are associated with symptoms such as aural fullness, and voice and breath autophony, which are very distressing symptoms for patients. Many patients with a patulous eustachian tube are also reported as being mentally unstable. We conducted a questionnaire survey to clarify the psychological profiles of patients with patulous eustachian tubes, and report the results with a review of the literature. A total of 103 patients who visited our outpatient clinic were assessed by the Self-rating Depression Scale (SDS) and the State-Trait Anxiety Inventory (STAI). The median scores at the initial diagnosis were PHI10 26 (20–32), SDS 45 (39–55), STAI state anxiety 50 (43–57), and trait anxiety 48 (44–56). The Spearman's rank correlation coefficient for PHI10 and SDS was 0.457 (p value 0.00161), that for PHI10 and STAI state anxiety was 0.457 (p value 0.00143), and that for PHI10 and characteristic anxiety was 0.487 (p value 0.000608), all representing significant correlations. The PHI10 improved in 73.1% of the 26 patients who required follow-up visits after the treatment. On the other hand, there was no statistically significant difference in the change of the PHI10 between the group of patients with very severe depression and those with STAI characteristic anxiety on SDS. The subjective symptoms were significantly improved with lower SDS values. There have been several reports on the relationship between a patulous eustachian tube and anxiety and depressive tendencies. The present study suggests that patients with high levels of depression and trait anxiety may be resistant to treatment and may have difficulty in receiving the usual treatments, suggesting the need for considering the psychological aspects of the disease.
Development of damaged fuel debris cutting technique in Fukushima Daiichi nuclear plant is one of the important issues for the defueling operations. Previous knowledge in the Three Mile Island Unit 2 (TMI-2) nuclear plant accident shows that fuel debris was made of uranium oxides, stainless steel, zircaloy and it was difficult to cut fuel debris by machine processing. Laser cutting is convenient to cut complex materials. However, it is difficult to know the thickness of fuel debris in advance, so it is hard to apply the conventional laser cutting method. In this study, we have been developed the cutting technique for fuel debris by using laser gouging method. Test pieces that simulate fuel debris were fabricated by embedding ceramic pellet on stainless steel. Laser gouging test was performed by cutting surface of the test pieces with high power fiber laser. According to the test results, laser gouging process has the ability to grind the simulated fuel debris under atmospheric condition and underwater.
Development of damaged fuel debris cutting technique in Fukushima Daiichi nuclear plant is one of the important issues for the defueling operations such as removing from the reactor, handling and storage, and so on. Previous knowledge in the Three Mile Island Unit 2 (TMI-2) nuclear plant accident is reported that fuel debris was made of uranium oxides, stainless steel, zircaloy and so on. Therefore, it is difficult to cut fuel debris by machine processing. Laser cutting is convenient to cut complex materials. However, it is difficult to know the thickness of fuel debris in advance, so it is hard to apply the conventional laser cutting method. In this study, we have been developed the cutting technique for fuel debris by using laser gouging method. Test pieces that simulate fuel debris ware fabricated by embedding ceramic pellet on stainless steel. Laser gouging test was performed by cutting surface of the test pieces with high power fiber laser, whose maximum irradiation power was 10kW. According to the test result, laser gouging process has the ability to grind the simulated fuel debris under atmospheric condition. In addition, same kind of cutting effect could be performed underwater.
Welding of structures such as pressure vessels is carried out for maintenance of nuclear power plants. For shortening the work process and reducing exposure radiation, development of a technique to weld is carried out without discharging the water from the pressure vessel. However, the welding must be done in an inert gas condition, and therefore it is necessary to remove the water around the welding point. We developed the air jet curtain technology which made possible the removal of water by feeding gas to keep the welding point in the gas phase.
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