Visualization and measurements of two-phase flows in metallic ducts using neutrons as microscopic probes (3rd report, Quantitative measurement of neutron radiography image)

The quantitative measurement of an image obtained by neutron radiography technique was proposed in order to accurately measure the void fraction of a two-phase flow in a metallic duct. It was shown experimentally that the spatial distribution of the dark-current component was homogeneous and the temporal variation could not be ignored. Since the scattered neutrons falling on the image converter could be homogenized by setting the test section at a distance from the converter, it was clarified that the corrections for dark-current and scattered neutrons could be represented by an offset value. It was proposed that the offset value could be determined by using the total macroscopic cross section of the material (∑-scaling method). By comparing the calculated void fractions with the measured ones obtained by simulating the known void profile using a standard test section, the void fraction could be measured by this method within 2% error. The measurement error was estimated to be up to 10% when no corrections for scattered neutrons were made or no arbitrary offset value was used.