Design, preparation and diffusion research of the W coating used in W-based cermet for nuclear thermal propulsion

Abstract

The fissile fuel agglomeration and loss in the tungsten (W) based ceramic-metallic (cermet) fuel element are the major challenges for application in nuclear thermal propulsion (NTP). The previous research indicated that a W coating deposited on the surface of uranium dioxide (UO2) by the fluidized bed chemical vapor deposition (CVD) technique is an efficient way to achieve the uniform distribution of fuel particles and reduce fuel loss simultaneously. However, the W layers' resulting thicknesses differ, which is not conducive to achieving high fuel loading. Therefore, in this study, the appropriate thickness of the W coating is first investigated by COMSOL Multiphysics modeling. Through contrastive analyses, the W layer with a thickness of about 5–10 μm is considered to be the optimal choice, where the maximum loading capacity of the fissile fuel can be achieved with a volume fraction of about 60%. Based on the above calculations, the CVD technique is employed to produce the optimized thickness of the W layer by precisely adjusting the deposition time at 900 °C. This approach is taken after determining the critical influencing factor that governs the performance of the W layer. Experimental results show that the highly dense W layer presents a uniform thickness and fine-grained size. As the only structural and protective layer, the application of this W coating provides a barrier for blocking the inward diffusion of hydrogen (H2) to stop the detrimental chemical reactions between UO2 and H2, thus lowering fuel loss. The spark plasma sintering (SPS) demonstrates that zirconium dioxide (ZrO2) particles as the surrogate for UO2 exhibit a homogeneous distribution without any direct contact in the W matrix because of the presence of the W coating. Despite the fact that the interface between the W coating and the W matrix exhibits remarkable bonding, making it difficult to clearly discern the boundaries due to surface diffusion and atomic binding, the existence and integrity of the W coating after high-temperature sintering is unequivocally confirmed through interface TEM observations and the uniform dispersion of coated particles within the W-Y2O3 matrix. Furthermore, the fine-grained W coating has been demonstrated to effectively hinder the diffusion of H2 isotopes through permeation and diffusion tests. This comprehensive study not only aids in establishing the optimal geometrical thickness for the W coating but also underscores its pivotal role in W-based cermet fuels for NTP system.