Tham khảo tài liệu 'volume 17 - nondestructive evaluation and quality control part 18', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Ị Fig. 7 Nuclear pressure vessel of 65 mm 2 in. thick carbon steel in which longitudinal girth and nozzle welds were radiographically inspected using 50-Ci cobalt-60 as the radiation source Eastman Kodak type AA radiographic film and the inspection setups shown. Dimensions given in inches For radiographic inspection cobalt-60 was selected as the radiation inspection because of the thickness of the material and Eastman Kodak type AA film was selected for its relative speed to give the shortest exposure time. The combination of Co-60 and type AA film can sometimes result in marginal sensitivity that occasionally requires the reshooting of a specific region. Each longitudinal seam was radiographically inspected with eight shots spaced 350 mm 14 in. apart using a 180 X 430 mm 7 X 17 in. film. The penetrameters were steel ASTM No. 45 with a mm I l in. thick shim. The Co-60 radiation source was placed 690 mm 27 in. from the film. The cassette was loaded with a mm in. thick lead screen two pieces of film a mm in. thick lead screen and a 3 mm C in. thick lead back shield. Section A-A in Fig. 7 shows the setup for radiographic inspection of the longitudinal seam. The girth weld was inspected using the panoramic technique in which the Co-60 50-Ci radiation source was placed at the center of the vessel and 28 film cassettes were placed around the outer surface section B-B Fig. 7 . The technique involves only one exposure of 25 min to radiograph the entire seam but its success depends on the placement of the radiation source exactly in the center of the vessel. The disadvantages of this technique include a lengthy setup time a fairly complicated setup and additional film costs if an error is made. An alternative technique is to place the radiation source outside the vessel and inspect the seam with 28 separate exposures. This method would result in higher-quality radiographs but a much longer total exposure time would be required. _ L Three equally .
