The extent of reaction with water of fuel materials having uranium metal as the major constituent is reported. This chemical reaction occurs as a result of the fission heating and meltdown of these materials during a nuclear transient. Observations of the physical changes that occur are also noted. The technique involved the transient irradiation of fuel samples immersed in water in stainlesssteel capsules in the Transient REActor Test facility (TREAT). Uranium wires, 64 mil diam by 1 in. long; uranium, 5 wt% zirconium, 1.5 wt% niobium alloy pins, 0.2 in. diam by 0.5 in. long (both Zircaloy-2 clad and bare); and clusters of uranium pins, 0.2 in. diam by 5-5/8 in. long were heated under water by the transient neutron pulse of the TREAT reactor. Subsequent measurement of the hydrogen evolved was used to determine the extent of reaction. The extent of reaction was correlated as a function of the fission energy developed in the fuel samples. Unbonded strain-gauge transducers were used to measure the pressure history in the autoclaves that contained the fuel samples. The extent of reaction was determined for energies ranging from 37 to 394 cal/g and reactor periods from 63 to 515 msec. With reactor periods of the order of 100 msec, melting of fuel samples began at a fission energy input of 40 cal/g, and complete melting occurred at 95 cal/g. The extent of reaction varied from a few tenths of a percent for those samples which had not melted appreciably to 4% for those which were completely melted in the experiments conducted in 25°C water (20 psia helium overpressure). Above 95 cal/g, the samples fragmented, and the extent of reaction increased with increasing fission energy to 50% at the highest energy of 394 cal/g. Three experiments were performed in 285°C water (1000 psi steam overpressure). The three samples in 285°C water reacted from two to three times as much as those in the 25°C water at the same energies. The effect of longer reactor periods was to decrease the temperature reached by the samples and thus the extent to which it reacted. Although the samples were of various sizes initially, the average particle size of the samples after the transient was reduced to the order of 15 mil when the fission energy input was above 200 cal/g A few of the samples were initially irradiated to a burnup of 0.3% of the total uranium atoms prior to the TREAT experiments; however, no effect on the extent of reaction or degree of fragmentation was observed. The effect of original sample size and the presence or absence of cladding were also minor. The results are directly applicable to the analysis of reactor excursion accidents.