MICROSTRUCTURE AND PROPERTIES ANALYSIS OF INNER CLADDING TUBE AND END PLUG GIRTH WELD OF ANNULAR FUEL ELEMENT

摘要

Annular fuel element is a kind of new double-sided fuel element, and the welding between the inner cladding tube and end plug of it belongs to a new kind of welding. Actually the conception was first proposed by Massachusetts Institute of Technology and the China Institute of Atomic Energy of Reactor Engineering makes the physical design of annular fuel element product. Through this research, we now basically master the method of welding annular fuel element. In this research, we designed a girth-welding fixture for welding the inner cladding tube and end plug of annular fuel element. The influences of the power input on the weld penetration and morphology have been obtained. The metallurgical performance of welded joints was analyzed through optical microscope, hardness testing and scanning electron microscope (SEM). The mechanical tests results indicate that the tensile properties of the welded joints are closely related to the microstructure. And the welding joints are also tested in the autoclave. The research shows that the micro hardness along the longitudinal section of the inner cladding tube appears to be the trend: firstly gradually reduced, and then stayed. The highest hardness is in the welding zone. And the heat-affected zone has great impacts on the micro hardness: the far the area is away from the welding line, the lower the micro hardness becomes. The grains in both of the weld zone and heat-affected zone are obviously grown up, but the grain growth is more obvious in the weld zone. The tensile fracture of the welding joint all occurs in the welding zone, the tensile strength is larger than that of the bar, which is used for processing the end plug. And the maximum force belongs to the shear stress fracture. In microstructure picture, the weld fracture appears to be dimple-shaped. And some of the dimples showed equiaxial and some of the dimples showed the elongated dimples. And the surface-welding zone is coated with the uniform and compact black oxide film without the white and brown corrosion. All the results and study in the paper will be of guidance for the further processing of the annular fuel element used in the pressurized water reactor (PWR). Key words:Annular fuel element, Welding morphology, Micro-hardness, Tensile strength