Verification of neutron radioscopic measurement of void fraction by Monte Carlo simulation

Neutron radioscopic images have been simulated using the Monte Carlo method to verify the Σ-scaling method used in void fraction measurement of two-phase flow. Continuous energy Monte Carlo code MCNP was used for the accurate treatment of the test section with complex geometry. Radioscopic images of water-layer thickness distribution in a concentric triple-tube test section were successfully reproduced by the Monte Carlo simulation. The simulated images at various distances between the test section and the converter were obtained using an ideal monodirectional (parallel) beam and realistic beam with unparallelness defined by the LD ratio, thus enabling the separate treatment of the unsharpness of the image caused by the scattered neutrons and the unparallelness of the incident neutron beam. The treatment of the scattered component and the unparallelness of incident beam employed in the Σ-scaling method was verified to be appropriate through the simulation, which shows that the Σ-scaling method could be successfully adopted for the quantitative measurement of void fraction of two-phase flow.