Yasumi Nagura

Optical fiber transmission of 2 kW cw YAG laser and its practical application to welding

The YAG laser has an excellent characteristic that makes beam transmission with optical fibers. In addition, the oscillator of the 2 kW class has recently been developed, and so its applications are changing largely from conventional fine material processing. The authors have adopted the YAG laser welding that uses optical fibers favorable to application to narrow, complicated areas. First, the authors have analyzed the beam intensity distribution near the focusing point using ray tracing method, and manufactured the compact condensing optical lens systems, and metal mirrors which have high reflectivity and durability that can be inserted in the tubes of about 16 mm I.D. to weld them. And also have developed the beam transmission system that can transmit beam for a long distance of about 200 m using small diameter optical fibers. In addition, with these systems the welding conditions to get about 2 mm penetration for stable welding has been obtained and applied YAG laser welding of steam generator tubes in practical nuclear power plants.

Development of advanced CRDM without canopy seal welds

Leakage of primary coolant from the canopy seal welds of control rod drive mechanism (CRDM) for PWR has been reported not only in Japan but also all over the world since the 1970s. The canopy seal weld is a circular weld designed to retain the pressure of reactor coolant while the strength of joints is maintained by thread. Since the coolant inside the canopy seal welds makes it easier for dissolved oxygen density to increase when a plant starts up, stress corrosion cracking (SCC) becomes a potential issue. Consequently, an advanced CRDM housing that eliminates all canopy seal welds has been studied, verified, and now applied to Vessel Head Replacement (VHR) plants in Japan.

Intelligent Monitoring of Laser Welding by Neural Network Model and Multivariate Analysis.

An intelligent monitoring device for laser welding in small-size pipes is described. It monitors the reflected laser light and the welding speed, etc, learns the normal criteria and the anomalous criteria of welding, and discriminates between normal and anomalous welding. Using the learned criteria, it categorizes the anomaly as one of several types. As a result of evaluation tests, the degree of correspondence between this device and nn expert is about 90%. This paper describes the new methods, the multivariate analysis for discriminating between normal and anomalous welding, and a neural network model for distinguishing the types of anomalies.