The fast-neutron hodoscope detects fuel motion within test samples inserted in the center of the Transient Reactor Test Facility (TREAT) reactor core. The hodoscope system has been built to support the U.S. Energy Research and Development Administration nuclear reactor safety testing program in which fuel motion is induced under simulated hypothetical conditions. Optical detection methods cannot be used due to opacity of fuel capsules and sodium coolant required in the tests for the sodium-cooled fast breeder reactor program. The hodoscope system includes components necessary to acquire, store, decode, and process the collected data. An area at the test fuel plane 5.7 cm (horizontal) × 52 cm (vertical) is viewed through 334 slots in a steel collimator by an array of 334 Hornyak button fast-neutron detectors. Collimator and detectors are external to the reactor. Horizontal and vertical spatial resolution of fuel pin motion as small as 0.25 and 8 mm, respectively, with a signal-to-background ratio of 7, can be achieved. Test samples can be single- or multipin assemblies enclosed in autoclaves or flowing sodium loops. When a 7-pin cluster is inserted in a flowing sodium loop, horizontal spatial resolution of fuel motion deteriorates to ∼6 mm and the signal-to-background ratio for any one of the pins is reduced to ∼2 (as much as 5 cm of material radially surrounds the fuel pins in this case). Transients at TREAT typically range from 20 to 20 000 MW when the hodoscope is used. To satisfy test objectives, data must be recorded from each detector at count rates up to 2 million/ sec each, time-resolved down to millisecond intervals. This is accomplished in a relatively reliable and inexpensive manner by displaying counts from each detector sequentially in binary code on a lamp panel, which is photographed by a high-speed framing camera, producing a film record of the transient test.