In a light water nuclear reactor test assembly for emulating the performance of a nuclear boiling water reactor fuel bundle, a system of the measurement of subchannel void fraction in the two-phase region of the emulated fuel bundle is disclosed. The emulated fuel bundle has individual emulated nuclear fuel rods typically heated by individual electrical currents instead of nuclear reaction. The interior of the emulated fuel rods are thus cylindrically hollow. A gamma-emitting source, such as americium 241, is placed on a probe and mounted for vertical excursion interior to a selected emulated hollow fuel rods. A detector, typically a Geiger-Müller counter, is placed for corresponding vertical excursion interior to another and preferably adjacent fuel rod. Gamma radiation from the source to the detector through the walls of the emulated fuel rods is measured. Attenuation of the gamma rays between source and detector is used to measure the void fraction within the two-phase region of the emulated fuel bundle. Preferably, both detector and source are collimated so that the detector does not receive gamma ray scattering from the interior of the test assembly. Preferably the detector is both collimated as to the gamma rays emitted by the source and cooled by an overlying heavy metal shield, such as gold, this shield conducting heat from the vicinity of the detector to a water-cooled line passing through the metal shield. By the expedient of utilizing a relatively low energy neutron source -- such as americium 241 with its 60 KeV gamma ray emission -- gamma signal at the detector can be directly attributed to gamma absorption of the liquid moderator fraction present between the emulated fuel rod containing the detector and the emulated fuel rod containing the source. Utilizing the disclosed test assembly an accurate measurement of the presence of moderator within a fuel bundle can be determined.