Uranium-molybdenum alloys have been shown to transform from the stable to the metastable phase due to neutron bombardment. This phenomenon has been explained on the basis of the smoothing out of concentration gradients due to the action of “displacement spikes” or “thermal pulses” generated within the sample. A kinetic study of this reaction in U-9 w/o Mo alloy specimens was performed in which the phase reversal was followed by electrical resistivity measurements on the samples while they were being irradiated and held at low temperatures in-pile. The special facility which was constructed to perform this work, as well as the results of the first in-pile experiment, are described. The results of this test are felt to verify the displacement spike model of radiation damage.