Preliminary Study on the Neutron Radiography Examination of Spent Fuel Sample from Nuclear Reactor

The non-destructive examination of internal structure is an 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.[Purpose]: 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.[Methods]: In this study, a spent fuel sample transfer and neutron radiography device that meets the radiation protection requirements was designed at the neutron radiography facility of the China Advanced Research Reactor (CARR). Then, non-destructive testing experiments were carried out on fuel rod simulations with artificial defects and real spent fuel samples using indirect radiography and direct radiography methods, respectively, and the advantages and disadvantages of the two radiography methods were compared. The internal structure images of radioactive spent fuel samples from a domestic commercial nuclear power plant were obtained.[Results]:Both radiography methods can obtain the internal structure images of simulated samples and small-sized spent fuel samples under the experimental conditions of neutron injection rate of 1×108n/cm2/s and neutron beam collimation ratio of 63. Observing the gaps between artificial defective fuel rods simulated by direct photography and indirect photography, both methods could detect gaps as small as 0.2 mm between pellets. Further comparison of the grayscale fluctuations of the gaps between pellets by the two methods, direct radiography using NIP plate showed higher grayscale fluctuations (3107-8899) compared to indirect radiography (953-3879), indicating that direct radiography has higher image clarity. Compared with direct radiography, the contours of the simulated pellets and the gaps between the pellets and the cladding are clearer in indirect radiography. By comparing the COV values, the COV value of direct photography (0.0327) was significantly higher than indirect radiography (0.0074), indicating that indirect radiography has less noise. The neutron radiography results of the small-sized spent fuel samples by the two radiography methods are consistent with those of the simulated samples. Analysis of the reasons shows that the NIP plate of direct radiography is interfered by the background of gamma rays, resulting in uneven background noise. The poor clarity of the indirect radiography image may be due to the fact that the metal screen and the IP plate are not tightly fitted during radiography, and a slight relative displacement between the two causes a decrease in resolution.[Conclusions]: Current research shows that the direct radiography method using NIP plate has high test efficiency and high clarity, but the data signal-to-noise ratio is not as good as the indirect radiography method. The image of the indirect radiography method is not affected by gamma rays and has a good signal-to-noise ratio, but the operation steps are more complicated and the requirements for the bonding of the metal conversion screen and the IP plate are high.