## 1 Introduction
Given its high melting point, good corrosion resistance, good conductivity, and continuous BCC region at high temperatures, U-Nb is considered to be a promising metallic fuel candidate for Gen-IV fast breeder reactors [20]. However, the fuel exhibits various metastable phase transformations, resulting  
Figure 1: The equilibrium U-Nb phase diagram [15; 16]. The \(\gamma\) phase is the body-centered-cubic solid solution phase, whereas \(\alpha\) and \(\beta\) are the orthorhombic and tetragonal uranium allotropes that have very limited solubility of Nb. Below the 647 \({}^{\circ}\)C monotectoid isotherm, the equilibrium state of U-6 Nb consists of -80 mole % \(\alpha\) (-0 at.% Nb) + -20 mole % \(\gamma_{2}\) (70–75 at.% Nb), indicated by the purple tie line. Decomposition of alloys with 4-8 wt.% Nb (9.65-18.22 at.%) in the 350 \({}^{\circ}\)C–650 \({}^{\circ}\)C temperature range takes place by cellular precipitation, initially to \(\alpha\) + metastable \(\gamma^{\prime}\) (blue dotted tie line) before further evolution to \(\alpha\) + equilibrium \(\gamma_{2}\). The decomposition paths and mechanisms of these same alloys below 350 \({}^{\circ}\)C are less clear, but will involve the formation of the metastable supersaturated \(\gamma^{0}\) or \(\alpha^{\prime}\) martensitic phase (see Figure 3) and its subsequent decomposition (tempering) as it evolves toward the \(\alpha+\gamma_{2}\) final equilibrium state [15]. in microstructures that strongly and negatively affect performance (see, for instance, [3; 4; 20; 21; 22]). [PERSON] et al. [20] studied discontinuous precipitation (DP) to a lamellar microstructure that is known to degrade U-Nb's corrosion resistance and ductility [23]. [PERSON] et al. [24] reported experimental observations of general precipitation, followed by discontinuous precipitation. In the U-Nb system, DP occurs as part of the monotectoid decomposition:  
\[\gamma\rightarrow{}^{(\text{DP})}\alpha+\gamma^{\prime}\rightarrow{}^{( \text{DC})}\alpha+\gamma_{2} \tag{1}\]  
in which, \(\gamma\) is a quenched BCC matrix, \(\alpha\) is orthorhombic precipitate, and \(\gamma^{\prime}\) is metastable BCC precipitate with an intermediate composition differing from that of stable \(\gamma_{2}\) (see Figure 2a). DC corresponds to discontinuous coarsening. [PERSON] [25] examined the decomposition of the \(\gamma\) phase in the U-21.2 Nb (at.%) alloy. Based on post-XRD analyses of samples that were quenched from isothermal heat treatment, [PERSON] [25] hypothesized that \(\alpha\) and \(\gamma\) form two local equilibria (LE) with each other, one at an intermediate composition, \(\gamma^{\prime}\), and the other at the global equilibrium composition, \(\gamma_{2}\). Due to the former LE, \(\gamma\) would decompose partially into \(\alpha\) and metastable \(\gamma^{\prime}\), if its initial composition \(\gamma\) was greater than \(\gamma^{\prime}\). This explains the occurrence of DP. After a suitably long incubation time, the \(\gamma_{2}\) phase will nucleate in the system and, since it corresponds to the _stable_ phase in equilibrium with \(\alpha\), it would evolve spontaneously, resulting in DC. As the isothermal holding temperature increased, [PERSON] [25] observed that the composition of the \(\gamma^{\prime}\) precipitate gradually shifted towards Nb-lean compositions. [PERSON] schematically described energy profiles reproduced in Figure 2b to demonstrate their hypothesis [20; 25]. In this Figure the common tangent to the \(\alpha\) and \(\gamma^{\prime}\) (\(\gamma_{1-2}\) on Figure 2b) phase curves is also given (dashed line). During the decomposition of the \(\gamma^{\prime}\) solid solution below the eutectoid temperature, the \(\alpha\) + \(\gamma^{\prime}\) phase mixture, corresponding to this tangent, is formed. Of course, being metastable, it continues to transform to the equilibrium \(\alpha+\gamma_{2}\) (the common tangent solid line).