Postirradiation examination using neutron tomography

Reconstructive tomography using multiple neutron radiographs, or neutron tomography, is being developed at the Idaho National Engineering Laboratory (INEL) for the examination of nuclear fuel bundles which have been irradiated in test situations. A study has been in progress for about a year to define suitable data collection and processing requirements and to estimate achievable image quality. An iterative maximum entropy reconstruction algorithm, MENT, was judged appropriate for this application and used to generate all reconstructions for the study. Cross-sectional images of a 91-pin fast breeder reactor fuel bundle were obtained from 36 penetrating neutron radiographs of the bundle. The fuel pins and most of the spacer wires can be distinguished in these reconstructed images. Computer simulations were also conducted. Results show that reconstructions of nearly symmetrical arrays of fuel elements can be misleading. A reconstruction which appears robust and plausible may inaccurately show only the symmetrical features. Important irregularities such as mispositioned spacer wires may not appear if an insufficient number of radiographic views is used. Simulations were also performed to investigate the spatial resolution requirements for the 36 neutron radiographs used for tomographic analysis of the 91-pin fuel bundle. For the number of views considered, it is shown that little image improvement can be realized by increasing the radiograph resolution beyond a certain point. Adequate and sufficient resolution corresponds to 150 to 200 equally spaced samples across the diameter of the fuel bundle