Date of Award
Doctor of Philosophy (PhD)
Engineering Physics and Nuclear Engineering
A technique to measure void fraction in a two-phase flow system using the multiple scattering of neutrons from portable neutron sources was investigated. There exists evidence from a previous investigation with a neutron beam extracted from a nuclear research reactor, that the basic technique is attractive. The counting rate of multiply-scattered, thermalized neutrons was found to be nearly linear with void fraction and independent of flow regime. However, early investigation with a portable neutron source revealed that these advantages were not realized in all situations. In the present work, the objective, then was to determine the conditions under which the advantages of linearity and flow pattern independence could be achieved with portable neutron sources. Experiments were carried out to study the effect of various parameters like symmetry of neutron sources, test-section to detector distances, and detection energy bn did not affect the technique significantly. More significant parameters were flow patterns, test-section inside diameter, test-section wall thickness, presence of neutron reflector material around the test section, and incident neutron spectrum. From the trends observed in the parametric study, it was found that near linearity with void fraction and flow pattern independence could be achieved by tailoring the energy spectrum of the incident neutron beam, e.g. by moderating neutrons emitted by a given source. The amount of moderation required decreased with test-section inside diameter, test section wall thickness, and he addition of neutron reflector material surrounding the test-section. This finding was verified with experiments conducted for four different test sections. The reasons for these experimental observations were clarified by studying the effect of monoenergetic incident neutron groups with Monte Carlo simulation, the result of which was further interpreted with thermalization probability considerations. The Monte Carlo study revealed that each monoenergetic neutron group results in a different response in terms of thermalized neutron counting rate, which, in general, is neither linear with void fraction nor flow regime independent. However, linearity and flow regime independence could be achieved by superimposing the effect of different energy groups, i.e. by tailoring the neutron spectrum. In practice, this is achieved by appropriately moderating the neutron beam from a given source. Based on the understanding achieved, a conceptual design of a portable void fraction meter, and a design procedure are suggested.
Yuen, Peter S.L., "Development of an epithermal/fast neutron scattering technique for void fraction measurement in two-phase systems with portable neutron sources" (1985). Open Access Dissertations and Theses. Paper 3305.