Fast Neutron Method for Local Void Fraction Distribution Measurement in Two-Phase Flow

Abstract

<p>A new method is presented for measuring local void-fraction and its distribution in gas-liquid two-phase flow. The method is based on reconstructing the physical distribution of steam and water (phase distribution) from the energy spectrum of fast neutrons scattered in the test section. The unique relationship between neutron energy and angle of single scatterings in the liquid phase is utilized to reconstruct the phase distribution.</p> <p>Monte Carlo simulations of scattering in the test sections have been used in developing the method. The validity of the Monte Carlo methodology for this purpose is discussed within the logic of the MORSE computer code employed in this work. The usage of the code is verified by simulating actual experiments during the course of the work.</p> <p>The neutron scattering problem is formulated into non-linear mappings of the phase distribution to the measured neutron spectrum via matrices. The phase distribution is reconstructed by solving the inverse problem posed by the mappings. The non-linearity is overcome by applying a series of successive-approximations to the mappings. The mathematical foundations of the inverse problem are discussed and its implementation in the form of a numerical algorithm is shown.</p> <p>The energy spectra of the scattered neutrons are measured for a few test sections, and the results are shown to compare favourably with those obtained from Monte Carlo simulations. The techniques used in the measurements are also discussed.</p> <p>The conclusion of this study is that the proposed new method can be applied successfully for measuring phase distributions of gas-liquid flows. The method in contrast to other radiation methods, requires only a single exposure of the test section to radiation. The inverse problem of reconstructing the phase distribution is solvable and the required neutron energy spectrum can be measured using available techniques. The only restriction is that the test section diameter has to be of order of one mean-free-path of the source neutrons in the scattering medium (about 100 mm in water for 14 MeV neutrons). This is necessary to reduce the contributions from neutron rescattering that complicate the reconstruction problem.</p>