Eutectoid decomposition of γ-phase (cI2) into α-phase (oC4) and γ'-phase (tI6) during the hot isostatic pressing (HIP) of the U-10 wt.% Mo (U10Mo) alloy was investigated using monolithic fuel plate samples consisting of U10Mo fuel alloy, Zr diffusion barrier and AA6061 cladding. The decomposition of the γ-phase was observed because the HIP process is carried out near the eutectoid temperature, 555°C. Initially, a cellular structure, consisting of γ'-phase surrounded by α-phase, developed from the destabilization of the γ-phase. The cellular structure further developed into an alternating lamellar structure of α-and γ'-phases. Using scanning electron microscopy and transmission electron microscopy, qualitative and quantitative microstructural analyses were carried out to identify the phase constituents, and elucidate the microstructural development based on time-temperature-transformation diagram of the U10Mo alloy. The destabilization of γ-phase into α-and γ'-phases would be minimized when HIP process was carried out with rapid ramping/cooling rate and dwell temperature higher than 560°C.

Microstructural anomalies in the co-rolled-and-HIP'ed U-10 wt.% Mo (U10Mo) metallic fuel plate with Zr diffusion barrier assembly were examined as a function of HIP temperature (from 520 to 580°C) and duration (45, 60, 90, 180 and 345 min) by scanning and transmission electron microscopy. The anomalies observed in this study are carbide/oxide inclusions within the U10Mo fuel alloy, and regions of limited interaction between the U10Mo alloy and Zr barrier, frequently associated with carbide/oxide inclusions. In the U10Mo alloy, the cF8, Fm3m (225) UC phase (a = 4.955 Å) and cF12, Fm3m (225) UO 2 phase (a = 5.467 Å) were observed throughout the U10Mo alloy with an approximate volume percent of 0.5 to 1.8. The volume percent of the UC-UO 2 inclusions within the U10Mo alloy did not change as functions of HIP temperature and time. These inclusion phases, located near the surface of the U10Mo alloy, were frequently observed to impede the development of interdiffusion and reaction between the U10Mo alloy and Zr diffusion barrier. The regions of limited interaction between the U10Mo and Zr barrier decreased with an increase in HIP temperature, however no noticeable trend was observed with an increase in HIP duration at constant temperature of 560°C.

The Mo 2 Zr phase has been identified as a major interaction product at the interface of U-10Mo and Zr. Transmission electron microscopy in-situ irradiation with Kr ions at 200°C with doses up to 2.0E+16 ions/cm 2 was carried out to investigate the radiation stability of the Mo 2 Zr. The Mo 2 Zr undergoes a radiation-induced structural change, from a large cubic (cF24) to a small cubic (cI2), along with an estimated 11.2% volume contraction without changing its composition. The structural change begins at irradiation dose below 1.0E+14 ions/cm 2 . The transformed Mo 2 Zr phase demonstrates exceptional radiation tolerance with the development of dislocations without bubble formation.The high performance research reactor fuel development program (HPRR-FD) aims to develop low enrichment (b 20% 235 U) U-Mo fuels to ensure a safe and secure use of research and test reactors worldwide. It is well known that the microstructural development under irradiation has a strong effect on the performance of fuels and materials in the reactor. Significant efforts have been put into developing a comprehensive understanding of the U-Mo fuel microstructural evolution under irradiation. The most popular fuel format for these reactors is the plate type of fuels, either in dispersion or monolithic configuration, sandwiched with aluminum alloy Al 6061cladding on both sides. The advantage for a monolithic fuel plate is its higher uranium loading capacity compared to a dispersion fuel. More information on these types of plate fuels can be found in the literature [1][2][3]. To mitigate the undesired strong interaction between the U-Mo fuel and the Al 6061 cladding, a Zr thin foil of 25 μm is added as a diffusion barrier between the U-Mo and Al 6061. It is found that the Zr foil also develops reaction products at both interfaces of U-Mo/Zr and Zr/Al 6061 from the fuel plate fabrication process [4]. A comprehensive microstructural characterization is required to investigate the radiation stabilities of the relevant interaction product phases at the fuel/cladding interface.While neutron irradiation experiments are an essential part of a fuel development program, ion irradiation experiments with a focus on a certain aspect of a fuel material property are very useful and complementary due to their cost effectiveness, quick turn-around cycle, wellcontrolled irradiation conditions, and negligible radioactivity of the irradiated samples. The previous ion irradiation studies on the stability of interaction product phases at the U-Mo/Al-Si interface provided useful information [5,6]. The Mo 2 Zr phase has been identified as a major interaction product developed at the U-Mo/Zr interface [4,7] or in the U-10Mo-χZr ternary alloy (χ = 1 to 6) [8]. In the open literature, the common crystalline structure for Mo 2 Zr is MgCu 2 type (cF24, a 0 = 0.7588 nm) as listed in U-Mo-Zr ternary or Mo-Zr binary system phase diagrams [9]. There is also a less common structure for Mo 2 Zr phase that is bcc type (cI2, with a 0 = 0.3185 nm) found in a twophase Mo 2 Zr co...