Evaluating the microstructure and origin of nonmetallic inclusions in as-cast U-10Mo fuel

https://doi.org/10.1016/j.jnucmat.2021.152949Get rights and content

Highlights

  • Inclusions characterized on sub-nanometer to millimeter scale

  • Inclusions are hypostoichiometric oxides (UO2−x) and carbides (UC1−x)

  • Inclusions are 0.42 ± 0.25 % by area with average diameter of 4.6 ± 0.8 µm

  • ~20 at. % 235U is measured for all carbide inclusions and the surrounding matrix, and higher 235U enrichment of ~25 - 27 at. % was measured in some oxide inclusions

Abstract

Low enriched uranium alloyed with 10 wt. % molybdenum (U-10Mo) has been identified as a promising alternative to highly enriched uranium oxide dispersion fuels for use in high performance research and test reactors. Manufacturing U-10Mo alloy fuel involves several complex thermomechanical processing steps and understanding of the microstructure and its evolution throughout the various fabrication steps is critical to enable the deployment of a reliable fuel production capability. Nonmetallic inclusions are often found in U castings and may affect subsequent fuel processing steps and microstructure evolution. Yet, the origin of these inclusions is not well established. To elucidate their origin and formation mechanisms, nonmetallic inclusions in U-10Mo castings were characterized on sub-nanometer to millimeter scale. Inclusion distribution, morphology, size, and composition were determined for the metallic fuel samples. Inclusions were estimated to comprise ~ 0.4 % of the fuel (by area), were identified at both grain boundaries and grain interiors, and were found to have varying morphologies (e.g., core-shell, elongated, blocky). All inclusions were either uranium carbides or oxides, or a combination of the two (i.e., dual-phase inclusions). Analysis of inclusions via atom probe tomography revealed that carbides and oxides were hypostoichiometric, with minor amounts of additional impurity elements present (e.g., Si, H). All analyzed inclusions were found to be enriched to ~20 at. % 235U, consistent with the surrounding γ-UMo matrix and target enrichment for the low enriched U fuel, indicating that the inclusions formed during the downblending of highly enriched U metal with depleted U via the melting and casting processes.

Introduction

Metallic nuclear fuels have been studied extensively since the 1940s for use in commercial and research reactors due to their more favorable machinability, higher uranium (U) density and improved thermal conductivity as compared to the more traditional ceramic or dispersion systems [1], [2], [3]. Metallic fuels are typically U alloys that maintain a high fissile density of 235U, with better chemical and mechanical properties than unalloyed U [4,5]. High assay low-enriched U (LEU) nuclear fuels (i.e., 5 wt. % < 235U < 20 wt. %) are currently being developed to replace highly enriched uranium (HEU) oxide dispersion fuels, to reduce the significant proliferation risks associated with continued HEU operations [6]. LEU alloyed with 10 wt. % molybdenum (U-10Mo) in a plate-type (i.e., monolithic) design is a leading LEU fuel candidate, particularly for the United States high-performance research reactors, because a high 235U density is possible [6]. Alloying U with 10 wt. % Mo stabilizes the high-temperature γ-UMo phase with body-centered cubic structure, which provides isotropic properties or behavior (e.g., swelling in an irradiation environment) unlike other U allotropes [7], [8], [9]. In addition, U-10Mo provides reasonable corrosion resistance, ductility, and fracture toughness for application in an operating reactor environment [4,10,11]. In order to deploy a reliable production capability, the evolution of U-10Mo microstructure throughout the fabrication process must be well understood. Inhomogeneities in the as-cast microstructure (e.g., Mo segregation, and morphology and elemental/isotopic composition of nonmetallic inclusions) can directly affect microstructure evolution in subsequent processing steps and fuel performance in an irradiation environment.

Nonmetallic inclusions and impurity elements have previously been identified in manufactured U-10Mo fuels, including UC and U2MoSi2C [12,13], yet their enrichment and origin have only recently been investigated [14,15]. Inclusions and impurity elements are present in the feedstock material, but may also be picked up from the environment during casting (i.e., melt interaction with the crucible and/or casting atmosphere). Intrinsic impurity phases in depleted uranium (DU) or HEU feedstock materials that do not dissolve in the liquid phase field at 1673 K (~1400 ⁰C) would retain their original U isotopic signature after casting, which would indicate that the inclusions originated from feedstock materials. On the other hand, if feedstock impurity phases dissolve completely, then carbides, oxides, or variants (e.g., oxycarbides) could precipitate during the casting process [16,17]. Isotopic enrichment of the nonmetallic inclusions that is very different from the LEU specification can directly affect irradiation performance by leading to nonuniform fission density (and thus nonuniform temperature and fission product formation) in the fuel [18]. While the effects of homogenization and thermomechanical processing on the kinetics of grain growth and phase transformations have been studied [19], [20], [21], the starting (i.e., as-cast) microstructure, and in particular the role and origin of nonmetallic inclusions, is not well understood. The presence of nonmetallic inclusions may influence grain growth and the local phase transformation kinetics during downstream fuel processing [13,22]. In addition, second-phase particles dispersed in a polycrystalline material can affect microstructure evolution during subsequent annealing and hot/cold rolling [23], [24], [25]. Hence, detailed understanding of inclusion characteristics (e.g., distribution, size, morphology) can help to advance the computational models that detail phenomena such as particle-assisted abnormal grain growth and particle pinning [22,26], and allow for better predictions of microstructure-processing relationships.

In this work, a multimodal, multi-length-scale approach was applied to investigate the as-cast U-10Mo microstructure. Grain size, morphology, and distribution of nonmetallic inclusions were characterized in two fuel plates via optical and scanning electron microscopy. To further detail the elemental distribution across inclusions at different locations in U-10Mo castings, scanning electron microscope energy dispersive x-ray spectroscopy (SEM-EDS) mapping was employed. For a subset of fine-scale inclusions, atom probe tomography (APT) was performed to investigate nanoscale morphology of inclusions and to identify their elemental and isotopic compositions in comparison with the surrounding matrix. The combination of these techniques provides insight into fuel plate processing history and the origins of nonmetallic inclusions.

Section snippets

Materials and Methods

DU, HEU, and Mo were mixed in proportions to meet the target U enrichment (~ 20 wt. % 235U) and the 10 wt. % Mo alloying specification. Raw materials were vacuum induction melted and cast into ingots at the Y12 National Security Complex. The melt was held at 1673 K (~1400°C) for 30 minutes, then poured into a preheated Er2O3-coated graphite crucible. The melt was then furnace-cooled to ~600°C and air-cooled to room temperature. Each vertical melt pour yielded three parallel cast plates which

Microstructure of as-cast U-10Mo

Microstructures of plates P1 and P2 were analyzed via OM, with representative images of top, middle, and bottom sections given in Figure 2. As seen in Figure 2(a) – (c), P1 has a dendritic microstructure [33]. The dendritic structure is significantly reduced during the subsequent prolonged homogenization treatment performed at temperatures above 833 K (560°C), which is within the γ-phase field [19,34,35]. Prior work has demonstrated that after subjecting dendritic, as-cast microstructures with

Cast fuel plate microstructures

Inclusions in U-10Mo castings were characterized across length scales using a multimodal approach to gain a detailed understanding of fuel microstructure prior to subsequent thermomechanical processing. Inclusion morphology, size, volume fraction and the elemental and isotopic compositions were studied via OM, SEM, SEM-EDS, and APT. Optical and electron micrographs indicate that significantly different solidification and heat transfer conditions exist in two neighboring plate molds, producing

Conclusions

The microstructure of as-cast U-10Mo fuel plates, including grain size and morphology, inclusion area fraction, number density, size, and elemental and isotopic compositions, was investigated via microscopy methods across length scales (sub-nanometer to millimeter). Findings and conclusions from the work presented herein are summarized as follows:

  • i

    Cast U-10Mo fuel plate microstructures contained only small percentages of inclusions (< 1 % by area). An average area percent of inclusions was

Author Contributions

E.J.K. and S.S. led manuscript writing. E.J.K. prepared all samples for APT, and performed APT data collection. E.J.K, M.F., and A.D. analyzed APT data. S.S. collected and analyzed all OM and SEM images. M.A. assisted with manuscript preparation and discussion of results. A.S.K. collected SEM images and EDS maps/line scans. A.S., V.J., and C.L. led fuel fabrication characterization efforts, and contributed to discussion of results and significance to the fuel fabrication process. All authors

Funding

This work was supported by the U.S. Department of Energy (DOE) National Nuclear Security Administration. A portion of this research was performed using facilities at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE's Biological and Environmental Research program and located at PNNL. PNNL is operated for the U.S. DOE by Battelle Memorial Institute under Contract No. DE-AC05-76RL01830.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank Mark Rhodes of Pacific Northwest National Laboratory (PNNL) for performing bulk metallographic sample preparation, Mr. Chad Painter for reviewing the manuscript, and Ms. Maura Zimmerschied for providing a technical review of the manuscript.

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