Abstract: On RT testing of centrifugal casting pipe of CPR1000 PWR coolant system, an image on the film with a large area of black and white, and with unclear edge boundary was found. The image was also characteristic of irregular patch or defect image. Through a variety of physical and chemical methods and non-destructive examination methods for exclusion and verification and by comparing it with the defect image of typical casting defect map, it was verified that the image was the non-defect image caused by the crystal diffraction, and following on its formation mechanism was analyzed.