## Diffusion Characteristics

The statistical thermodynamic theory of three-component interstitial phases developed at the institute and the established properties of diffusion-controlled processes [12] such as sintering, nitration, carbonizing treatment, and oxidation facilitated the optimization of technologies used in investigations. Diffusion processes have a significant impact on the HGA materials’ interaction with hydrogen, and on consolidation of carbide powders [3]. Experimental diffusion data were used to estimate the load-bearing capacity of fuel rods at high temperatures and the healing of surface defects. Kinetics of diffusion phenomena in the majority of cases is controlled by a number of elementary processes, whose individual contribution is rather difficult to differentiate.

Thus, densification rate during sintering and creep rate are controlled by volume, boundary, and surface diffusion [12], and by dislocation sliding. Two-component or multicomponent nature of interstitial phases generally complicates the analysis of diffusion-controlled processes, as the contribution of metallic and nonmetallic atoms in various temperature ranges may vary. Lastly, the comparison of the experimental and the theoretical data on diffusion-controlled processes requires clarification of the concept of an effective self-diffusion factor Def which is usually introduced for the multicomponent bodies [2].

For a single-component body D is a factor of a self-diffusion; for two — or multicomponent bodies one should use substitute Def into the respective equations:

Def = (D*aD*b/CbD*a + CaD*b) ■ g,

where g is the thermodynamic factor, Ci is concentration of a specific component, Di is partial (chemical) self-diffusion factors that are typically defined through experiments with radioactive isotopes. If the diffusion mobility of one component in the alloy or compound is non-negligible (for example, diffusion mobility of interstitial elements), and D*b > D*aD, then the equation for the Def may be simplified as follows:

Def — (1/Ca) ■ D*Ag

In this case the rate of diffusion-controlled processes is defined by diffusion rate of the slowest diffusing element [2]. The developed statistic and thermodynamic theory of three-component interstitial phases, and our research of diffusion-controlled processes (creep, sintering, nitriding, carburization, and oxidation) make it possible to formulate evaluation recommendations concerning composition, processing, and operating conditions of NRE fuel elements (Table4.3).