Developments Towards Hybrid Liquid Metal Plasma-Facing Components for Future Long-Pulse Fusion Energy Reactors
Open Access
- Author:
- Marchhart, Trevor
- Graduate Program:
- Nuclear Engineering (MS)
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 20, 2023
- Committee Members:
- Arthur Motta, Professor in Charge/Director of Graduate Studies
Jean Paul Allain, Thesis Advisor/Co-Advisor
Mr. Martin De Jesus Nieto-Perez, Committee Member
Xing Wang, Committee Member - Keywords:
- Nuclear fusion
Plasma-facing components
Liquid metals
Spark plasma sintering
Porous tungsten
UHV instrumentation - Abstract:
- Nuclear fusion reactors have the potential to play a significant role in the transition to a carbon-free energy grid that is plentiful, equitable, and secure. This stems from its high energy density, nearly limitless fuel, and scalability. However, one of the greatest challenges in the advancement of long-pulse future fusion reactors is the development of materials that can withstand the harsh plasma environment present in the reactor, throughout the course of its lifetime. The materials of these plasma facing components (PFC) are exposed to combined loadings of extreme heat fluxes, ion bombardment, and neutron irradiations. These can cause large thermal gradients and stresses, microstructural changes such as nanostructure formation, and material property degradation such as embrittlement. A possible PFC configuration that aims to address some of these challenges utilizes liquid metal as the plasma-facing media. A potential concept in this category is a hybrid liquid metal PFC, which incorporates a tungsten-based capillary porous system (CPS) filled with molten lithium. The CPS is used to supply the molten lithium to the surface, as well as stabilize the liquid against magnetohydrodynamic (MHD) forces that are present in the device. Research towards the advancement of this concept PFC is presented in this thesis. Spark plasma sintering (SPS) has been identified as a unique and versatile method for fabricating tungsten-based CPS structures. SPS is an advanced powder metallurgy technique that utilizes combined uniaxial pressure and pulsed DC current to rapidly sinter a packed bed of powders into fully-dense substrates. However, under reduced pressures and temperatures, it can be used to create porous media. 50-70\% dense porous tungsten samples with micron-sized pores have been fabricated using this method. However, initial investigations have found that these samples are quite mechanically weak, and contain a significant concentration of impurities. Additionally, there is little control over pore parameters, as the porosity is coupled to sintering temperature. The combination of these results suggests that samples fabricated this way would not be suitable for a fusion reactor environment. An alternative SPS method for manufacturing porous media utilizes a secondary constituent powder that acts as a placeholder for the tungsten. Both the tungsten and the placeholder powders are sintered in the SPS press using higher temperatures and pressures, and then the placeholder is dissolved out of the sample. What is left is a porous tungsten structure. The benefit of this method is that it allows for much more control over the resulting pore structure. Pore size, ligament size, and porosity can all be tuned by varying the powder size and volumetric ratios. A series of partially porous tungsten structures have been fabricated with this method using zirconium placeholders, which have been dissolved with mixtures of concentrated nitric and sulfuric acids. An important of aspect of the hybrid liquid metal PFC concept is the replenishment of the liquid metal to the surface of the CPS. This is an important parameter that is relevant during off-normal high heat flux events in a reactor, which have the capability of evaporating large volumes of liquid out of the CPS. In order to elucidate how various CPS factors effect the replenishment time, two analytical models were developed. The first utilizes a two-phase formulation of Darcy’s law, while the second analyzes a simplified porous domain and relates the forces acting on a fluid using the Navier-Stokes equations. Both models present replenishment times as a function of pore size and back-pressure applied to the liquid metal. Results generally suggest that larger pore sizes and back-pressures will cause faster replenishment of the liquid metal, except in the cases where MHD-drag effects are included, which suggest slower replenishment with larger pores. These models provide a useful parameter space for the design of CPS structures. In order to support the development of CPS structures, the Liquid Metal In-Vacuo Injection (LIVIn) system has been designed and developed. This ultra-high vacuum instrument is capable of testing the hydrodynamic performance of liquid metals in various architected porous media. It is integrated with the IGNIS-2 facility at Penn State University, which is an advanced surface science facility being developed to research a wide range of fusion-relevant materials. A thorough outline of the design process for LIVIn system is presented, including extensive computational validation of many novel subsystems.