AbstractA negative Doppler effect has been observed in measurements on a metallic uranium sample (enriched to 93% in U235) placed in a typical fast-reactor spectrum which has a median fission energy of 195 keV. The plausibility of the negative sign is supported on theoretical grounds, although with the use of standard analytical techniques and the limited number of resonance parameters at present available, it cannot be calculated for this spectrum. The value forwas found to be -5.65×10-6/°C. New measurements on U238have been made, and the data on Th232, previously published, have been extended to 930 C. The agreement between analytical and experimental values for the latter two materials is good.