Sensitivity Evaluation in Double Wall Radiography of Pipes and Multilayer Plates by Assessment the Monte Carlo Simulation

摘要

Image quality indicator (IQI) is selected based on the single and/or double wall thickness of pipe with or without considering the radiographic techniques or applied radiation source (x ray or gamma ray), in accordance with the sensitivity requirements of reference codes. Discrepancies in the codes and standards for sensitivity evaluation and IQI selection have been investigated in the present work and some experiments have been performed to measure the gamma ray or x-ray radiographic sensitivity for pipes and plates to compare the results with sensitivity requirements of the standards. Due to the considerable effect of scattering on the sensitivity, Monte Carlo method has also been used to simulate the scattering for gamma ray radiography of pipes and plates. Experiments revealed that 2 percent sensitivity based on the single wall thickness was attainable for double wall exposure and single wall viewing (DWSI) technique, but it could not be easily obtained for double wall exposure and double wall viewing (DWDI) gamma ray technique. Higher sensitivity was obtained for x-ray radiography and a considerable sensitivity reduction was found for in-service radiography of pipes. Achieved sensitivities were in good agreement with measured effective attenuation coefficients and simulation results which confirmed that scattering was, respectively, higher for DWDI radiography of pipe, radiography of single layer plate, and DWSI radiography of two layer plate with identical penetrated thickness. Therefore, IQI selection was proposed to be based on the single wall thickness of component except for DWDI Gamma ray radiography, which was proposed to be based on the double wall thickness.