Void fraction measurements of acrylic discs via neutron radiography

摘要

Simulation experiments have been initiated to verify that neutron radiography methods can accurately measure void fractions under various operating conditions in a steam-water flow channel. Recent neutron radiography experiments measured void fractions in an air-water channel at atmospheric pressure and room temperature conditions. Because steam-water densities at atmospheric pressure and low temperature vary significantly from those at high pressure and high temperature, questions have been raised as to the ability of the neutron radiographic technique to deduce vapor fractions under various steam-water operating conditions. In response to this concern, the macroscopic neutron cross sections presented by two extreme steam-water conditions were simulated using acrylic discs containing various size holes. Acrylic, which contains hydrogen as a major constituent closely resembles the properties of water as seen by a thermal neutron beam. Through holes appear to the neutrons as steam voids at atmospheric pressure. The effect of increased vapor density, (and neutron macroscopic cross section), which would occur at high pressure, was simulated by not drilling the holes completely through the discs. The shallow hole plus the remaining material was made to simulate a through hole filled with a vapor density on the order of 14% of water. The measured void fractions deduced from digitally imaged neutron radiographs are in good agreement with the expected values for the two cases studied.