Thermodynamic studies of the solid solutions of refractory compounds with uranium monocarbide were started at the RIPRA “Luch” when no data on the thermodynamic properties in the homogeneity region at high temperatures were available in the liter­ature. Only basic approaches for estimating thermodynamic properties were known. Structural and fuel materials of the reactor core are based on refractory zirconium and niobium carbides, their solid solutions with uranium, carbide compositions with carbon inclusions [4, 5, 7], and zirconium hydrides for a moderator unit (Table4.1). These materials belong to the class of so-called interstitial phases [8]. The majority of refractory compounds possess highly symmetric cubic lattice. The majority of monocarbides, mononitrides crystallizes in FCC lattice of NaCl type, nonmetallic atoms located in octahedral positions. Melting point, elastic properties and factors of thermal expansion are structurally tolerant characteristics and, in essence, they depend on bonding energy, energy of a crystal lattice necessary for division into separate ions.

High brittleness of compounds is caused by low dislocation mobility owing to an orientation of bonding, high value of forces Peierls-Nabarro and low rate mul­tiplication of dislocations. Dislocations submit to the same laws in compounds, as in metals. However in polyatomic compounds with ionic-covalent bonding, more complex geometry of dislocations’ structure is observed. Starting stresses of disloca­tions’ movement in crystals with high Peierls barrier are on one to two orders above, than in metals. The stresses demanded for the yield beginning, in turn, exceed start stresses on two orders at the moderate temperatures 0.3 Tml.

The data on the physical-mechanical properties of many refractory compounds and their compositions (some of them were used in devices for the first time) revealed the possibilities of these materials and influenced the determination of their operating conditions and estimates of their prospects [9-11]. Unlike uranium mono compounds, fuel based on solid solutions of UC-ZrC and UC-NbC carbides with a composi­tion close to the stoichiometric composition provide the heating of hydrogen up to 3,000K.