[en] The characterizatoin of two-phase flow in a nuclear reactor is of significance for both operational and safety reasons. This study reports on the exploration of flow-regime characterization by using measurements of the neutron-flux fluctuations induced by the two-phase flow passing through a nuclear-reactor core. It is shown that many significant properties of the flow can be determined by analysis of the induced neutronic fluctuations. These characteristics have been categorized as propagation, structure, and dynamics of the flow. In order to carry out this investigation, it was necessary to develop models for the response of the neutron field to flow perturbations and to relate measured flow quantities to phenomenological models of the flow. These modelling activities are summarized in this report. The experimental program of two-phase-flow characterization was carried out using an air/water loop in the University of Washington Nuclear Reactor (UWNR) and a parallel loop for visual studies. Details of the construction of the apparatus are summarized