Two in-pile capsule experiments have been performed to study the behavior during irradiation of a “paste” of fissionable particles settled at maximum density in a liquid-metal medium. The paste consisted of 150-µ nominal diameter, spherical particles of U-10 wt % Mo alloy in NaK and was irradiated to burnups of 0.0055 and 0.061 total at % in the Battelle Research Reactor. The irradiation capsule consisted essentially of a tube divided into two compartments by an orifice plate. The inside diameter of twin orifice tubes, projecting in either direction from the orifice plate, was such that the paste could flow by gravity from one compartment to the other. An underwater device to measure the gamma-ray emission from the irradiated fuel was used to check the mobility of the paste between increments of irradiation. The paste in the first experiment failed to flow after the first increment of irradiation, and examination of the capsule and particles failed to provide an explanation. The second experiment, performed after some refinements in procedure, was more successful. The mobility of the paste was demonstrated up to a burnup of 0.031 total at %, after which the flow became very sluggish even when assisted by vibration. Examination of the paste indicated that a buildup of oxide contamination in the capsule probably caused the sluggishness of the paste. There was no evidence in either experiment of particle agglomeration by a sintering or fission welding mechanism.