The major goal of the work is to develop effective additives for U-10Zr (wt.%) metallic fuel to mitigate the fuel-cladding chemical interactions (FCCIs) due to fission product lanthanides and to optimize the fuel phase mainly by lowering the gamma-onset temperature. The additives Sb, Mo, Nb, and Ti have been investigated. Metallic fuels with one or two of the additives and with or without lanthanide fission products were fabricated. In this study, Ce was selected as the representative lanthanide fission product. A series of tests and characterizations were carried out on the additive-bearing fuels, including annealing, diffusion coupling, scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and differential scanning calorimetry (DSC).
Sb was investigated to mitigate FCCIs because available studies show its potential as a lanthanide immobilizer. This work extends the knowledge of Sb in U-10Zr, including its effect in the Zr-free region. Sb forms precipitates with fuel constituents, either U or Zr. However, it combines with the lanthanide fission product Ce when Ce is present. Those Sb-precipitates are found to be stable upon annealing, and are compatible with the cladding. The additive does not change the phase transition of U-10Zr.
Mo, Nb, and Ti have been investigated for phase optimization based on the known characteristics shown in the binary phase diagrams. The quaternary alloys, i.e., two Mo-bearing alloys and two Nb-bearing alloys, were investigated. Compared to U-10Zr, a few weight percentages of Zr are replaced by those additives in the quarternary alloys. The solid-state phase transitions were determined (alpha and U2Ti transfer into gamma). The transition temperature varies depending on the compositions. The Mo-bearing alloys have lower -onset temperatures than the Nb-bearing alloys. All of them have lower gamma-onset temperatures than that of U-10Zr. Since low gamma-onset temperature is favorable, the results indicate that the fuel phase can be optimized by the replacement of a few weight percentages of Zr into those additives.
All the experiments were out-of-pile tests. Therefore, in-pile experiments will be necessary to fully evaluate the performance of the additives in the future. / Doctor of Philosophy / Fuel is the "heart" of a nuclear reactor, and fuel development is a key to improving the performance and reliability of a nuclear reactor. This study investigated the effects of metallic fuel additives in a sodium-cooled fast reactor (SFR). SFRs are an advanced reactor design. Metallic fuel, e.g., U-10Zr (wt.%), is one of the common candidates for SFR fuel. The aim of this study is to develop effective additives for U-10Zr metallic fuel to improve fuel performance. The study has two main objectives. The first one is to mitigate the fuel-cladding chemical interactions (FCCIs), while the second one is to optimize the fuel phase.
Four additives, i.e., Sb, Mo, Nb, and Ti have been investigated. The study is a pioneer for the application, thus, the experiments were performed without considering the irradiation effect. Metallic fuels with one or two additives were fabricated, with a series of tests being performed at a laboratory scale.
The additive, Sb, was used to mitigate the FCCIs, since FCCIs are a limitation of fuel utilization (i.e., burnup). Lanthanides are produced during fuel operation and attack cladding, being one of the reasons for FCCIs. It is known that the additive Sb has the potential to bind lanthanides into stable precipitates. This work brings the investigation a step further, providing more evidence to demonstrate the stability of the precipitates and the compatibility with cladding. The results are favorable as they demonstrate that the lanthanides will not attack the cladding if they can be caught by the additive Sb in the fuel.
The additives Mo, Nb, and Ti were investigated to optimize the phase. One of the favorable phase properties is the gamma-onset temperature - the lower the better. For example, the gamma-onset temperature is 776°C in pure U, while it is 680°C in U-10Zr (meaning that 10 wt.% Zr lowers the gamma-onset temperature by 96°C). In this work, the exploration moves forward by replacing a few percentages of Zr with Mo+Ti, or Nb+Ti. After the change, the gamma-onset temperatures are further decreased, with the temperatures decreasing more in the Mo-bearing fuels than in the Nb-bearing fuels.
The significance of this work is twofold. Firstly, it extends the knowledge of Sb as an additive for mitigating FCCIs; secondly, it shows that Mo, Nb, and Ti can optimize the fuel to achieve a favorable phase property. The results provide strong reasons for additional irradiation tests in the future.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/111207 |
| Date | 11 July 2022 |
| Creators | Zhuo, Weiqian |
| Contributors | Mechanical Engineering, Zhang, Jinsuo, Bai, Xianming, Liu, Yang, Benson, Michael T., Pierson, Mark Alan |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/vnd.openxmlformats-officedocument.wordprocessingml.document |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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