This thesis presents a technique for measuring the void fraction of boiling water flow in a small diameter pipe using a portable neutron source. The method relies on measuring the thermalization, by the liquid phase, of fast neutrons incident on the pipe. This is achieved by employing a thermal neutron detector positioned such that it measures neutrons scattered from the pipe volume.

The design of the device was optimized by selecting a source energy range most suitable for producing a linear and flow regime independent relationship between the void fraction and the detector count rate. A simple computer model, based on the collision probability theory, was employed in this optimization process. Discrete ordinates particle transport analysis showed that the desired energy range can be obtained by proper moderation of ²⁵²Cf or ²⁴²Am/Be neutron sources. Further Monte Carlo analysis indicated the adequacy of this arrangement.

A mockup of the device was subsequently designed for testing in the laboratory. The water phase was simulated in these experiments by lucite rods. Neutron scattering by the surrounding shielding material resulted in a large background signal. This difficulty was effectively overcome by housing the detector and the pipe together in a cadmium chamber.

The device was tested on an adiabatic air-water flow at the Thermo-Hydraulics Test Facility of the Ecole Polytechnique de Montreal. The results of these experiments were shown to favorably compare with the actual void fractions independently measured using the quick closing valves method. Field problems were identified and methods to overcome them were proposed.

A conceptual design of the device suitable for field use is finally presented. This design, not only optimizes the device performance, but also incorporates the experience gained in this work and provides adequate biological shielding for operators.