Thermal hydraulics design of nuclear reactors require extensive studies on two-phase flow pattern. In particular, for reactors using convection driven flow, such as advanced heavy water reactor being designed by the Bhabha Atomic Research Centre (BARC), Bombay, India, very little data exists on flow pattern behavior. Also such reactors using natural circulation face the problem of flow pattern instabilities due to various reasons and no data exists for such instabilities. We have used real-time neutron radiography methods in order to study flow pattern instability and develop thermal hydraulics model for such a reactor. A real time neutron imaging system has been developed for the visualization and measurement of void fraction inside metallic pipes. The system has been installed at the APSARA Reactor, BARC, Bombay, India. Experiments have been conducted on subsystems (SS) test sections of different diameters at pressure ranging from 1 to 40 bar and at various power levels. We report in this paper on the method and components used for this imaging system and some preliminary results of these experiments on void fraction measurement using this system.