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

• 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

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. Gas purge of the system 2. Pull vacuum on the system 3. Hydrogen gas testing • Furnace temperatures from 200° C – 500° C by increments of 50° C • Flow rates from 0.25 SCFM – 4.0 SCFM by increments of 0.25 SCFM

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 24 Procedures

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 25 Procedures

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 26 Testing

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 27

Preliminary Results – Temperature Data

200oC

300oC

250oC 350oC 400oC 450oC 500oC

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 28 Discussion of Preliminary Results – Temperature Data

Recommendations for future work: • Use Hydrogen gas as the working fluid • Allow longer times for transients to settle out before moving on to next mass flow rate • Add redundancies for inlet and outlet

thermocouples

200oC

300oC

250oC 350oC 400oC 450oC 500oC

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 29 Preliminary Results – Pressure Data

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 30 Discussion of Preliminary Results – Pressure Data

Recommendations for future work: • Use Hydrogen gas as the working fluid • Implement digital pressure transducers • Move pressure transducers closer to inlet/outlet • Implement digital flow controller

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 31 Suggestions for Future Work

• Expand capability of T3R to test full-scale tie-tube • Move from analog to digital data collection to improve accuracy • Hydrogen gas as the working fluid • Argon purge gas to replace Nitrogen because of concerns with nitriding of zirconium • Replace surrogate tie-tube materials with selected design materials to test the response to the operating environment

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 32 Acknowledgements

A big thank you is owed to Omar Mireles and Dan Cavander, our mentors this summer!

Another thank you is given to NASA’s Propulsion academy for such an outstanding research opportunity!

Thank you to the Alabama, Pennsylvania, Iowa, and Texas State Space Grants for their support of our internship!

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 33 Thank You!

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 34 BACKUP SLIDES

2015 Nuclear and Emerging Technologies for Space Conference Albuquerque, NM February 23 - 25 35 CFD analysis was used for a design basis and benchmarked by testing in the Tie-Tube Test Rig

Inlet pressure vs. Volumetric Flow Rate Mass Flow Rate vs. Inlet Pressure 7 6 40 35 5 CFD Flow 30 4 Rate 25

3 Experimenta 20 (SCCM) 2 l Flow Rates 15 10

1 5 Volumetric Volumetric FLow Rate 0 MassFlow Rate (g/s) 0 0 1 2 3 4 5 700 800 900 1000 1100 1200 1300 Pressure (psig) Inlet Pressure (psig) Zirconium Hydride Phase Diagram Zirconium Hydride XRD Diagram Zirconium Hydride Mass/Length Change Results

Initial Sample Initial Sample Run Sample Sample ∆W (%) Sample ∆L (%) Weight (g) Length (mm) 1 1 18.69 0.00 22.65 0.01 1 2 15.16 0.00 24.69 0.28 2 1 18.90 0.05 22.83 0.00 2 2 13.97 0.07 22.89 0.00 3 1 20.31 0.09 24.49 0.12 3 2 14.55 0.27 23.67 0.04 4 1 12.05 0.08 14.71 0.40 4 2 7.82 0.00 13.02 0.15