Development of a Prototypic Tie-Tube for a Low Enriched Uranium (LEU) Nuclear Thermal Rocket (NTR) Team Members: Kelsa Benensky, Jacob Harry, and Jeffrey Clemens NASA Propulsion Academy Principal Investigator: Omar Mireles NASA Marshall Space Flight Center ER-24 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 Presentation Overview 1. Background and Project Overview 2. Tie-Tube Mechanical Design 3. Neutronic Studies 4. Moderator Production Experiments 5. Tie-Tube Test Rig (T3R) Setup 6. Conclusions and Recommendations 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 2 Nuclear thermal propulsion (NTP) systems use the energy from fission to provide high power levels for long periods of time Liquid hydrogen stored in the propellant tank first cools the reactor Thrust provided to the rocket via expansion of the propellant through the nozzle NTP is considered by many to be the preferred form of propulsion for manned missions to Mars 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 3 NTP is a proven technology with 20 reactors ground tested in the Rover/NERVA Programs Tie-tubes played an important role in the past to provide in-core cooling and neutronic moderation for fuel test reactors during the later Rover/NERVA tests 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 4 A LEU NTR has the potential to greatly decrease the cost of developing NTP One of the major costs associated with developing NTP is the specific facilities needed to produce, handle, and test highly enriched uranium for nuclear fuel forms 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 5 In the LEU Reactor, tie-tubes function as in-core reactor components to provide core cooling and house the neutron moderator System Specifications Moderator Element/Tie-Tube Reactor System Performance Core power (MW) 519.2 Core average fuel power density (MW/l) 17.21 1.8 cm Max Fuel Temp. (K) 2849.9 84 cm U-235 Enrichment (a%) 19.75 U-235 inventory (kg) 34.14 Engine System Interface Points Flow Rate Pressure Temp. Interface Point (lbm/s) (psia) (R) Tie Tube Inlet 14.6 1059 49.3 Tie Tube Outlet 14.6 698 900 There are expected to be 680 tie-tube elements within a 25,000 lbf thrust NTR 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 6 The team was tasked to develop a feasible mechanical tie-tube design by testing the material and thermo-mechanical response of the system Mechanical Design and Moderator Production / Tie-Tube Test Rig Computational Analysis Materials Fabrication Design & Assembly This task was split into three sub-tasks that were pursued in parallel by each team member 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 7 Tie-tubes are oriented axially along the length of the core and play an important role in satisfying the system power balance and providing neutron moderation 1 • Tie-tubes surround 12 21 17 8 the fuel elements 63 5 6 8 15 • Tie-tubes connect 25 3 2 1424 16 into the injectifold 15 10 to provide power 2616 9 11 for the turbo 14 13 pumps 5 4 7 4 • Tie-tubes provide 2717 22 18 1323 structural support 19 to the reactor 30 20 2112 31 32 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 8 The design of the tie-tube assembly requires the successful integration of many components to ensure safe reactor operation Moderator Radial Support Injectifold Support Piece Spacer Connector Piece Axial Compression Turning Vane Inner Tie-Tube Spring Neutron Outer Tie-Tube Inlet Hydrogen Moderator Flow A 13 inch tie-tube prototype was also modelled for sub-scale testing 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 9 The neutronics of the core was investigated to determine power profiles and the impact of mechanical design / material selection on reactivity Reactivity vs. Spacer Placement 80 0 0 2 4 6 -0.1 70 -0.2 60 Reactivity ($) Reactivity -0.3 50 -0.4 Spacer Number 40 Reactivity vs. Hydrogen Concentration 4 LENGTH (CM) LENGTH 2 30 0 1.5 1.6 1.7 1.8 1.9 2 20 -2 -4 Reactivity ($) Reactivity 10 -6 -8 Hydride fraction ZrHx 0 It was found that core reactivity is strongly affected by number of spacers and placement, as well as the hydride content of the moderator A zirconium hydride (ZrHx) was chosen to best moderate neutrons within the reactor core – this ensures that the fuel is used more efficiently Effect of hydrogen to uranium ratio on neutron multiplication factor for different moderators of interest Moderator elements surround the fuel to ensure the fuel is used efficiently Unfortunately at high temperatures ZrH can become very brittle, therefore it is desirable for ZrH to be in ε-phase which is the most malleable phase at high temperatures and has the lowest Young’s Modulus 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 11 Three processes were investigated through a literature search as potential methods to produce ZrH1.8 1. Sintering of ZrH2 2. Direct Hydride of 3. Direct Hydride of Zirconium Powder Zirconium Sponge Tube / Round Bar Ultimately, direct hydride of zirconium tube was chosen because of its capability to provide a better grain structure and a hydride closest to theoretical density (advantageous for neutronic analysis) 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 12 Zirconium hydride was produced using the Enclosed Hydrogen Tube Furnace (EHTF) in the Materials Building (4602) 1. Zirconium Samples were machined, measured, and cleaned pre-hydride 2. EHTF operation consists of 7 steps Sample Loading Argon Purge This system was designed by a previous intern but built and tested by the NTP core components team Zirconium hydride was produced using the Enclosed Hydrogen Tube Furnace (EHTF) in the Materials Building (4602) 1. Zirconium Samples were machined, measured, and cleaned pre-hydride 2. EHTF operation consists of 7 steps Sample Loading Argon Purge Zirconium hydride was produced using the Enclosed Hydrogen Tube Furnace (EHTF) in the Materials Building (4602) 1. Zirconium Samples were machined, measured, and cleaned pre-hydride 2. EHTF operation consists of 7 steps Sample Loading Argon Purge Hydrogen Initiation Furnace Operation Zirconium hydride was produced using the Enclosed Hydrogen Tube Furnace (EHTF) in the Materials Building (4602) 1. Zirconium samples were machined, measured, and cleaned pre-hydride 2. EHTF operation consists of 7 steps Sample Loading Argon Purge Hydrogen Initiation Furnace Operation System Cool Down System Shutdown Sample Removal 3. Post Hydride Samples were qualified using X-Ray Diffraction (XRD) Zirconium hydride was produced using the Enclosed Hydrogen Tube Furnace (EHTF) in the Materials Building (4602) 1. Zirconium samples were machined, measured, and cleaned pre-hydride 2. EHTF operation consists of 7 steps Sample Loading Argon Purge Hydrogen Initiation Furnace Operation System Cool Down System Shutdown Sample Removal 3. Post Hydride Samples were measured and qualified using X-Ray Diffraction (XRD) Four samples were hydrided in order to find the best method to produce the neutron moderator Sample Test Matrix to Create Zirconium Hydride Samples Time Temperature Pressure Flow Rate Run Gas Mixture (min) (⁰C) (psi) (SCCM) 1 45 600 10 5 H2 2 120 600 10 5 H2 3 45 600 10 50 5% H2/ 95% Ar 4 120 600 10 50 5% H2/ 95% Ar Experimental parameters for interest were duration of test and partial hydrogen pressure (controlled by gas mixture and flow rate) 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 18 Zirconium hydride production was proven qualitatively through XRD Sample ∆W Sample ∆L Run Sample (g) (mm) 1 1 0.00 0.01 1 2 0.00 0.07 2 1 0.01 0.00 2 2 0.01 0.00 3 1 0.02 0.03 3 2 0.04 0.01 4 1 0.01 0.06 4 2 0.00 0.02 Change in axial length and mass was observed and attributed to hydrogen pick up – since hydrogen has such a low atomic mass it is recommended more precise measuring instruments should be used 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 19 XRD results show evidence of hydrogen pickup within the zirconium metal The blue plot corresponds to Zr-702 while the black plot corresponds to the hydride sample 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 20 XRD results show evidence of hydrogen pickup within the zirconium metal The major peak shown in the sample XRD corresponds to a hydrogen peak at 2θ = 28⁰ 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 21 The Tie-tube Test Rig (T3R) was constructed to flow test a subscale tie-tube prototype Design Construction Preliminary Testing Over a ten week period, three major milestones were achieved to create a working system 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 22 The team was able to design both the mechanical and electrical system of the T3R Full-Scale T3R Parameters Parameters • The subscale tie-tube can Maximum 2850 1300 meet similar conditions to Temperature (K) the full length tie-tube Minimum 27 77 • The system can also be used Temperature (K) as a materials testing furnace • Moderator and spacer Maximum Pressure 1059 100 assemblies can be quickly (psi) tested with this system Mass Flow Rate 0.021 0.021 • The system is limited by the (lbm/s) capabilities of the Length (cm) 84 33 mechanical components it is comprised of 2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 23 Procedures 1.