| dc.description.abstract | The concept of Integrated Computational Materials Science and Engineering (ICMSE) has emerged due to the need to accelerate the process of materials research and development at a fraction of the cost. In this work, we propose to apply ICMSE to the research and development of Gen-IV metallic fuels. As an illustration, the roles of thermodynamics and kinetics on the origin of uranium-niobium’s discontinuous precipitation (DP) was investigated. For this, the used integrated computational framework included first-principles calculations, CALPHAD, and phase-field modeling.
Particularly, first-principles calculations were coupled with CALPHAD to consistently assess the thermodynamic properties of uranium-niobium. The assessment results were in good agreement with experiments. The consistent thermodynamic description was then used to estimate atomic mobility and diffusivity of bcc uranium niobium. In turn, phase-field simulations were carried out to investigate the roles of thermodynamics and kinetics on the occurrence of DP via three possible hypotheses: the two local equilibria between α and γ, the kinetics of reaction front, and the ordering tendency of γ:
The two-local-equilibrium hypothesis was inferred from X-ray experiments. In its original form, the hypothesis assumed that α forms two common tangents with γ between which the first common tangent explains for the occurrence of DP while the second common tangent is responsible for discontinuous coarsening (DC), which is another interesting discontinuous reaction that follows after DP in the uranium-niobium system. In the current work, this hypothesis was re-examined using ICMSE’s quantitative advantage. Study showed that this hypothesis is a possible explanation for DP when further taking into account the thermodynamic effect of strain due to lattice/volume misfit as well as fast grain-boundary diffusion at the reaction front.
The kinetic hypothesis was proposed during our phase-field investigations of the kinetic effects on the occurrence of DP under the two-local-equilibrium hypothesis. It was found that when the kinetics of the reaction front is fast enough it can actively sustain the characteristic metastable phase of DP and ultimately leads to the stable growth of the reaction’s lamellar microstructure. This however requires that the strain energy is needed in order to shift the first inflection point of γ’s miscibility gap to a higher composition than that of the metastable phase so that γ 1−2 falls within the metastable region of the gap.
The ordering hypothesis stems from the previous interesting finding which showed a pronounced tendency to short-range ordering in the equiatomic uranium-niobium alloy. This tendency, in principle, could lower the free energy of the system and allow an intermediate state during the decomposition of the γ phase which tends to legitimate the occurrence of the discontinuous reaction. Even though we can not directly verify the existence of such tendency in this work, we find through our first-principles calculations that this tendency is not likely to happen, at least for the investigated equiatomic bcc B2 and B32 uranium-niobium alloys.
The knowledge achieved in the current work contributes to a better understanding of the fundamental thermodynamics and kinetics that govern uranium-niobium and its discontinuous precipitation. Such fundamental understanding together with the integrated computational framework can serve as an infrastructure for future research and development of the fuel or for prognosis of its failure during nuclear operation, via which demonstrates the advantages of ICMSE in the research and development of nuclear fuels and Gen-IV Integrated Fast Reactor (IFR). | en |
