Ablation Study of Tungsten-Based Nuclear Thermal Rocket Fuel by Tabitha Elizabeth Rose Smith B.A. in Physics, May 2010, West Virginia University B.A. in Sociology & Anthropology, May 2010, West Virginia University M.A. in International Science and Technology Policy, May 2012, The George Washington University A Dissertation submitted to The Faculty of School and Engineering and Applied Sciences Of The George Washington University In partial fulfillment of the requirements For the degree of Doctor Philosophy May 18, 2014 Dissertation directed by Michael Keidar Professor of Mechanical and Aerospace Engineering The School of Engineering and Applied Sciences of The George Washington University certifies that Tabitha Elizabeth Rose Smith has passed the Final Examination for the degree of Doctor of Philosophy or Doctor of Science as of May 5, 2014. This is the final and approved form of the dissertation. Ablation Study of Tungsten-Based Nuclear Thermal Rocket Fuel Tabitha Elizabeth Rose Smith Dissertation Research Committee: Michael Keidar, Professor of Mechanical and Aerospace Engineering, Dissertation Director Alexey Shashurin, Research Scientist, The George Washington University, Committee Member Chunlei Liang, Assistant Professor of Engineering and Applied Sciences, Committee Member William Emrich, Senior Engineer, National Aeronautics and Space Administration Marshall Spaceflight Center, Committee Member Ronald Litchford, Principal Investigator, National Aeronautics and Space Administration Langley Research Center, Committee Member ii Dedication The author wishes to dedicate the work conducted for this Doctoral dissertation to her husband John and future children, including our baby boy who will be born this summer. iii Acknowledgements The author wishes to firstly acknowledge Dr Michael Keidar, her advisor for the three and a half years she spent conducting the Doctoral research. Dr Keidar was supportive, kind, and always there when needed. He offered guidance and insight into matters of all subjects, maintained interest in the various research ventures of the author, and was consistently focused, providing a guiding light which was needed in order to finish the research. The author has changed as a person after working with Dr Keidar, and will forever be grateful that he was her advisor. Secondly, the author wishes to acknowledge her husband John and his mother, Leona. Since they were 15 years old, John has been a pillar of strength and best friend, regardless of the circumstances for the author. He, his mother, and his family have been constant variables in a continuously changing environment that was necessary in order for the author to maintain a stable reference point to focus on the future. The author will always remember the many situations in which her husband John demonstrated his dedication, friendship, and love through trying times. Thirdly, the author wishes to acknowledge NASA, NASA Marshall Spaceflight Center, and the NTR community. There are dozens of unique, passionate, and special people that the author has met within this community, and she was greatly honored to be included as one of them after receiving funding from NASA to do research on the NTR. Specifically, the author would like to thank Ronald Litchford, William Emrich, Harold Gerish, Steve Howe, Jeramie Broadway, Omar Mireles, Joe Sholtis, Steven Bowen, Mike Houts, Gwyn Rosaire, Brad Appel, and last but not least, Lou Qualls. It has been a iv tremendous honor to simply know all of these special people and many more, and the author hopes to remain in the NTR community in some way, for many years. Fourthly, the author would like to express thanks to the GWU machine shop and the students at the mpnl laboratory. Those at the machine shop, William Rutkowski, Nicholas Batista, and Tom Punte, were instrumental to assisting in fabricating vital pieces of metalwork that were needed in order for the experiments to be conducted with the arc jet. Their friendship and company was also invaluable, as was their instruction in the skills of machining. Within the mpnl lab, George Teel and Alexey Shashurin provided vital help with their expert knowledge on conducting experiments with the arc jet. It is with sincere gratitude that the author expresses her thanks for their guidance and patience as she learned what was needed in order to complete the dissertation research. v Abstract of Dissertation Ablation Study of Tungsten-Based Nuclear Thermal Rocket Fuel The research described in this thesis has been performed in order to support the materials research and development efforts of NASA Marshall Space Flight Center (MSFC), of Tungsten-based Nuclear Thermal Rocket (NTR) fuel. The NTR was developed to a point of flight readiness nearly six decades ago and has been undergoing gradual modification and upgrading since then. Due to the simplicity in design of the NTR, and also in the modernization of the materials fabrication processes of nuclear fuel since the 1960’s, the fuel of the NTR has been upgraded continuously. Tungsten-based fuel is of great interest to the NTR community, seeking to determine its advantages over the Carbide-based fuel of the previous NTR programs. The materials development and fabrication process contains failure testing, which is currently being conducted at MSFC in the form of heating the material externally and internally to replicate operation within the nuclear reactor of the NTR, such as with hot gas and RF coils. In order to expand on these efforts, experiments and computational studies of Tungsten and a Tungsten Zirconium Oxide sample provided by NASA have been conducted for this dissertation within a plasma arc-jet, meant to induce ablation on the material. Mathematical analysis was also conducted, for purposes of verifying experiments and making predictions. The computational method utilizes Anisimov’s kinetic method of plasma ablation, including a thermal conduction parameter from the Chapman Enskog vi expansion of the Maxwell Boltzmann equations, and has been modified to include a tangential velocity component. Experimental data matches that of the computational data, in which plasma ablation at an angle shows nearly half the ablation of plasma ablation at no angle. Fuel failure analysis of two NASA samples post-testing was conducted, and suggestions have been made for future materials fabrication processes. These studies, including the computational kinetic model at an angle and the ablation of the NASA sample, could be applied to an atmospheric reentry body, reentering at a ballistic trajectory at hypersonic velocities. vii Table of Contents Dedication ......................................................................................................................... iii Acknowledgments ............................................................................................................ iv Abstract of Dissertation ................................................................................................... vi List of Figures .....................................................................................................................x List of Tables .................................................................................................................. xiii List of Symbols/Nomenclature ...................................................................................... xiv Chapter 1: Introduction ....................................................................................................1 1.1 Historical Development of Nuclear Propulsion and Fuel ..............................................1 1.2 Plasma Ablation as a Method of Materials Failure Testing ...........................................5 1.3 Objectives and Summary of Dissertation ......................................................................8 Chapter 2: Literature Review and Development of the Kinetic Model ......................10 2.1 Materials Development and Failure Testing ................................................................10 2.1.1 Literature Review ..................................................................................................11 2.1.1.1 Thermal Cycling and Oxides ..........................................................................15 2.1.1.2 Stoichiometry .................................................................................................15 2.1.2 Understanding Failure Mechanisms .....................................................................17 2.1.2.1 Mechanical Deformation, Defects, and Fractures .........................................17 2.1.2.2 Imperfections and Fatigue .............................................................................19 vii 2.1.1.3 Developing upon the Current Fuel Failure Studies .......................................23 2.2 Plasma Ablation as a Method of Failure Testing ........................................................26 2.2.1 The Kinetic Model ................................................................................................26 2.2.2 Chapman Enskog Expansion and the Thermal Conduction Parameter ............29 Chapter 3: Computational and Experimental Set Up .................................................34 3.1 Including Tangential Velocity ....................................................................................34 3.1.1 Modification of the Kinetic Model Equations ......................................................34 3.1.1.1 Calculations for Mass Conservation ..............................................................35