Abstract: Primary coolant piping in nuclear engineering are welded in centrifugally cast austenitic stainless steel (CCASS). The welding quality is directly related to the safety of nuclear reactors. Narrow gap welding is mostly used in CCASS, while lack of fusion in side walls appeared easily. Detecting it in common testing methods is difficult. The paper combined the theoretical research, analysis calculation, numerical simulation, phased array ultrasonic testing and radiographic testing. The tesing technique of CCASS is underlined and the technology has been applied to the primary coolant piping weld in nuclear engineering.