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Fused fluoride mixtures containing UF4 have been developed as fuel solutions for high temperature nuclear reactors. The purpose of the present investigation was to evaluate the corrosion properties of alloys composed basically of Ni and Mo for containment of these mixtures. These evaluations utilized thermal convection loops which circulated salt mixtures between a hot-zone temperature of 815°c 0 and a cold-zone temperature of 650°C. Initial corrosion studies were made with Ni-base alloys containing 17 to 20 per cent Mo and various percentages of Cr, Al, Ti, V, Fe, Nb, and W. Loops of these alloys were exposed to the salt mixture NaF-LiF-KF-UF4 (ll.2-4S.3- 4le0=2.S mole per cent) for periods of 500 and 1000 hours. Measurements of the concentrations of corrosion products in after-test salt samples indicated the corrosion susceptibility of alloying additions to increase in this order: Fe, Nb, V, Cr, W, Ti, and Al. However, metallographic examinations of loop surfaces showed relatively light attack for all alloys except those containing combined additions of Al and Ti or Al and Cr. A Ni-base alloy containing 17 per cent Mo, 7 per cent Cr.11 and 5 per cent Fe., designated IN0R-8.11 was found to afford the best combination of strength and corrosion resistance A Ni-base alloy containing 17 per cent Mo, 7 per cent Cr.11 and 5 per cent Fe., design~ted IN0R-8.11 was found to afford the best combination of strength and corrosion resistance among the alloy compositions tested and was selected for fur­ ther corrosion studies. Corrosion of this alloy was found to occur by the selective oxidation of Cr and, accordingly, was controlled by the rate at which Cr diffused to surfaces undergoing attack. To determine these rates, the diffusion coefficients for Cr in INOR-8 were measured over the temperature range 870°c to 680°c by exposing the alloy to a fused fluoride mixture containing radioactive Cr in the form of CrF2• The diffusion coefficients were computed from measurements of the intake of radioactive Cr by the alloy and from an analysis of the concentration profile of radioactive Cr below the exposed surfaces of the nBtal. The magnitudes of the diffusion coefficients ranged from 10-12 to 10=14 cm2/sec and compared closely to the coefficients determined for Cr in the Ni-base alloys.