Research On Local Dry Underwater Robotic Welding Power Supply For Nuclear Fuel Pool

In consideration of the effect of the nuclear radiation to human body health, instead of welders’ welding, using robot has become an inevitable trend of underwater welding when the nuclear power plant need repair. If we make a comprehensive consideration of these factors of spent fuel pool, such as water depth, water quality, convenience of robot welding and to fill the requirements of the welding quality, local dry process is the optimizing way of robot welding repairs the nuclear spent fuel pool under the water. But with thin wall of spent fuel tank and the water cooling effect over the weld peripheral, it’s easily appears difficulties such as arc starting, arc breaking, the uniformity of melt dripping transition issue, and difficult to get high quality welding effect. Therefore, how to improve the stability of arc welding process so as to improve the quality of underwater welding is the key to study of nuclear spent fuel pool underwater local dry process with the robot welding power source.Firstly introduce the topic background in this paper, and make an analysis of the present situation and development trend of local dry underwater welding in domestic and foreign research, combined with the technological requirements of local dry underwater welding, high frequency inverter technology and digital control technology, designed a kind of under local dry robot welding power supply for nuclear spent fuel pool. It contains three parts in the welding power source designing, main circuit design, hardware circuit design and control software design. Main circuit design including the design of three-phase EMI filter, primary and secondary inverter circuit design and other key parts design, inverter device IGBT as the power switch tube of the inverter module. Hardware circuit design mainly choose STM32F405RGT6 as the core to build the peripheral circuit, including the design of control circuit, protection and fault diagnosis circuit and communication circuit design. Control software design with FreeRTOS real-time kernel environment for writing welding power source software, including control system overall program design, PID control strategy and communication system software design. Finally, choose the local dry underwater robot welding power supply prototype as test platform, the whole machine performance test is carried out under the simulated load, through a large number of local dry underwater welding process test by changing the parameters of the drain cover, gas pressure, welding current, duty cycle, the welding frequency and the welding speed, using oscilloscope and digital clamp table test instruments collecting experimental waveforms and data, and analyzing the formation of weld under different parameters.Experimental results shows that: using the underwater local dry drain cover proposed in this paper for underwater welding is better than directly using wet welding in arc stability and weld quality. Experiments under different gas pressure, when gas pressure is small, it’s not enough to form stable gas phase zone, arc stability get worse, easy to broken arc, the formation of weld is poorer. When gas pressure is big, the arc will be compressed due to gas pressure, and arc unstable, the formation weld will be narrow and collapse. It can be concluded in the welding current and duty cycle test that, appropriate increase of average welding current, could improve the welding heat input, improve the formation of weld. Make a comparison of different welding frequency test, we found when the pulse frequency is low, the stiffness of the arc is low, and the arc is soft, and will be easily influenced by protection gas and drainage gas pressure. By the welding speed test, increase the welding line energy input can help improve the formation of weld, reducing undercut, hump and so on.