Preliminary study on the neutron radiography of spent fuel sample from nuclear reactor

<bold>Background</bold>The non-destructive examination of internal structure of spent fuel rod is a very important way to evaluate the performance of spent fuel rods from nuclear reactor after irradiation. Compared with other examination methods, neutron radiography technology has more advantages on the irradiated nuclear fuel with high activity, such as strong penetration, high image sensitivity, and indirect radiography that can avoid radiation damage to radiography components.<bold>Purpose</bold>This study aims to develop and evaluate neutron radiography methods for examining internal structures of highly radioactive spent fuel rods, comparing the advantages and limitations of direct and indirect radiography approaches.<bold>Methods</bold>Firstly, a spent fuel sample transfer and neutron radiography device was designed to satisfy the radiation protection requirements and located at the neutron radiography facility of the China Advanced Research Reactor (CARR). Then, non-destructive testing experiments using both neutron imaging plate (NIP) for direct neutron radiography and neutron conversion screen plus imaging plate (IP) for indirect neutron radiography were carried out on fuel rod simulations with artificial defects and real spent fuel samples using indirect radiography and direct radiography methods respectively under the experimental conditions of neutron flux rate of 1×10⁸ n·cm^-2·s^-1 and neutron beam collimation ratio of 63. Finally, the advantages and disadvantages of the two radiography methods were compared by analyzing the internal structure images of radioactive spent fuel samples from a domestic commercial nuclear power plant.<bold>Results</bold>Experimental results show that both radiography methods obtain the internal structure images of simulated samples and small-sized spent fuel samples. Both methods detect gaps as small as 0.2 mm between pellets whilst direct radiography using NIP plate shows higher grayscale fluctuations (3107-8899) compared to indirect radiography (953-3879), indicating that direct radiography has higher image clarity. However, the coefficient of variation (COV) value of direct photography (0.032 7) is significantly higher than indirect radiography (0.007 4), indicating that indirect radiography has less noise.<bold>Conclusions</bold>Results of this study demonstrate that the direct radiography method using NIP plate has high test efficiency and high clarity, but the signal-to-noise ratio (SNR) is inferior to that of the indirect radiography method. The image of the indirect radiography method is not affected by gamma rays and has a good SNR, but the operation steps are complicated and requires the bonding of the metal conversion screen and the IP plate.