document: Uranium carbide was traditionally used as fuel kernel for the US version of pebble bed reactors as opposed to the German version based on uranium dioxide. For the Generation IV nuclear systems, mixed uranium–plutonium carbides (U, Pu) C constitute the primary option for the gas fast reactors (GFR) and UCO is the first candidate for the very high temperature reactor (VHTR). In the former case the fuel high actinide density and thermal conductivity are exploited in view of high burnup performance. In the latter, UCO is a good compromise between oxides and carbides both in terms of thermal conductivity and fissile density. However, in the American VHTR design, the fuel is a 3:1 ratio of UO2:UC2 for one essential reason, well explained by Olander [2] in a recent publication. During burnup, pure UO2 fuel tends to oxidize to UO2+x. UO2+x reacts with the pyrocarbon coating layer according to the equilibrium:(1)UO2+x + xC → UO2 + xCO