The difficult problem of directly measuring the void fraction of rapidly boiling water in rod-bundle channels, such as those simulating a large loss-of-coolant accident (LOCA) in a nuclear reactor, is overcome by using a fast-neutron scattering device (scatterometer). The neutron scatterometer measures the number of neutrons scattered from a test section exposed to fast neutrons, and relates these measurements to the channel void fraction. The concept of the device and its design features are discussed. Such a scatterometer was constructed and installed on a test section (channel) of the RD-14M Thermalhydraulic Test Facility of Atomic Energy of Canada Limited. Experiments (30-mm diameter break in the inlet header) were conducted to evaluate the scatterometer's performance under transient conditions. Results of these tests show that the scatterometer can continuously measure the channel void fraction to within +/- 10% void (95% confidence interval) when corrections for non-linearity and time response are applied. (c) 2005 Elsevier Ltd. All rights reserved.