Chemical and physical data on twenty binary uranium compounds that may prove suitable for refractory nuclear fuels were assembled. The compounds were those with aluminum, boron, carbon, iron, nickel, nitrogen, siliqon, or sulfur.
Diffusion rates of uranium through graphite were determined in the temperature range of 3,000' to 4,350"F. The diffusion couples consisted of sintered UC, disks in contact with graphite rods. The observations indicated two distinct types of uranium transport which could be associated with volume diffusion and with migration along pores respectively.Volume diffusion was characterized by steep concentration gradients and shallow penetration. The m u s i o n coefficient D,, in sq. cm./ sec. between 3,300" and 4,250°F., is given by an equation. Above 4,250"F. incipient melting of the UC, was evident and the diffusion coe5cients were much higher than those given by the equation.As an example of the penetration resulting from volume diffusion, calculations show that, after 1,000 hr. at 4,200"F. the uranium concentration at 0.1 cm. from the interface will be 1,000 mg./cc., compared with 10,000 mg./cc. for pure UC,.Pore migration resulted in uranium penetration far beyond that arising from volume diffusion at equivalent temperatures and diffusion times. However, uranium concentrations were very small compared with those corresponding to volume diffusion. Pore migration is strongly temperature dependent.To estimate the practical importance of pore migration, the uranium flow through a graphite wall at 3,000"F. was measured. With a wall thickness of 0.32 cm., the average flow per unit area was 0.015 mg.!(sq. cm.),'(hr.) for a 40-hr. test.Graphite WIS used in the first nuclear reactor because it is a good moderator and because large amounts of pure graphite were obtainable. The early reactors were designed to operittc! at relatively low kmper:itures. However, graphite also is very well suited as a moderator arid structural material for power-producing reactors. Ksrept for its poor oxidation resistance, it has generally attrartive properties for Iiigh-temperature serviw-high thernial wnductivity , good strength, and escellent resistance to thermal shock arid to creep.There are several important areas in which more data are needed to extend the usc of graphite in power reactors.Among these is the nced for inform :t t' ion on its rompatability with niirlenr fuels at high temperatures.Cranium dioxide and uranium tlicarhide are two high-melting fuel compounds which have been considered for use in contact with graphik. AY the oxide reacts with graphite at high temperatures to form the carbide, uranium dicarbide appears t o be the more suitable compound to use with graphite.Uranium dicarbitie, UC,, is known to be relatively unreactive with graphite up t o its melting point, but few data on the diffusion of uranium from UC? into graphite have been reported. I n fact, prior to this investigation, the only systematic study of the problem appears to be that made by Loftness (1).Loftness measured the diffusioli of uranium from UC,-impregnated graphite into pure graphite at temperatures between 3,900" and 4,iW"E'. The impregnated graphite had a uranium roncentration of approximately 2.50 mg./cv. Loftness roncluded that volume diffusion ...
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