Applications of neutron computed tomography to thermal-hydraulics research

The characterization and visualization of multiphase flows are significant for advancing thermal-hydraulics and nuclear engineering research, such as the safety and efficacy of nuclear reactor operation, etc. Neutron imaging technique has been established as a non-invasive measurement technique with high accuracy and reliability for multiphase flow study to provide a visual knowledge of the distribution of characteristics within thermal-hydraulics systems under determination. Various information can be extracted and quantified from neutron images for further analysis, such as flow patterns, void fraction distribution, bubble coalescence and breakup, bubble velocity, pressure loss, gas volume fraction, and effects of rising bubbles. Two-dimensional Neutron Radiography (NR) is produced by measuring an amount of transmitted neutron intensity after passing through a test section. This modality of imaging has been successfully utilized for visualization and quantification purposes in many applications and multiphase flow studies such as geological and porous media research, thermal-hydraulics phenomena, etc. However, certain shortcomings of two-dimensional NR are prominent, including limited temporal and spatial resolutions, which result in challenges in evaluating materials with complex geometries and capturing the high-speed fluid flow and dynamic phenomena. Three-dimensional Neutron Computed Tomography (NCT) provides a clearer insight into an object and a capability to observe dynamic phenomena. The CT technique is based on a measurement of radiation attenuation in a projection through an object from different orientations. Two main types of reconstruction algorithms are analytical reconstruction algorithms and iterative reconstruction algorithms. The analytical reconstruction algorithm provides accurate reconstruction without requiring long computational time and a high level of complexity. However, it requires a fully sampled projection data set with an angle coverage of 360° around the object, which is not always feasible to obtain under extreme experimental environments such as high heat, physical stress, and intense radiation. An investigation of multiphase flow studies in neutron imaging facilities encounters the mentioned environments, limited access, and high cost of implementation. Therefore, a research direction in NCT reconstruction methods lies in a trade-off between many projections with a low number of counts against a reduction in projections but increased acquisition time per orientation angle. Iterative reconstruction techniques produce improved image quality with a smaller amount of projection data. Nevertheless, the techniques exhibit increased computing complexity and longer computational time. This study reviews recent studies of NR and NCT in multiphase flow applications, such as an investigation of heavy oil flow behaviors for upgrading a packed bed of metallic reactors and determination of flow regime and void fraction distribution, etc. In addition, this study presents an in-depth explanation of iterative reconstruction algorithms and discusses the direction in which the NCT study is gradually shifting.