The role of carbon diffusion in densification of uranium dicarbide

Because uranium dicarbide fuel is being considered for space reactor applications, we conducted an investigation for the Department of Energy of process parameters and densification mechanisms for this fuel. Uranium dicarbide feed powder was prepared by carbothermic reduction of uranium dicarbide and graphite at 2050 K in vacuum. Uranium dicarbide pellets were prepared by cold pressing at 110 MPa and sintering at 2370 K in contact with niobium, tantalum, or tungsten substrates in an argon atmosphere. A high density front was observed to move through the pellets, starting from the pellet-metal substrate interface. The primary factor controlling this phenomenon is the diffusion of carbon from the uranium dicarbide pellet into the metal substrate. When the carbon level reduces so that no free carbon is present, rapid densification of the uranium dicarbide occurs. This technique can be used to control densification of uranium dicarbide and to control carbon level and grain size in sintered compacts.