Lunar Applications for Nuclear Thermal Propulsion

1
Lunar Applications for Nuclear Thermal Propulsion

• Steven D. Howe
• 6/16/09

2
Agenda

• CSNR perspective
• Logical path
• Current projects

3
CSNR Perspective

• Goal of space exploration is understanding our
  neighborhood, i.e. the solar system
• Unmanned scientific missions for science and as a
  precursor to human missions
• Ultimate goal is the expansion of human
  civilization throughout the solar system

4
CSNR Perspective

• Current propulsion technologies are insufficient
  for human expansion past the Moon
• we need the steamship equivalent to the sailing
  ships of the past
• what is the next propulsion technology - fission,
  fusion, electric propulsion, sails, beams?
• According to the Independent Review Panel
  convened in 1999 to review the propulsion
  technologies examined in the NASA Advanced Space
  Transportation Program
• The Review Team categorized fission as the only
  technology of those presented 45 concepts were
  presented which is applicable to human
  exploration of the near planets in the near to
  mid-term time frame

• The CSNR proposes that the Nuclear Thermal Rocket
  (NTR) is the only near term option for improved
  transportation of humans to other planets

5
Recent Assessments

• NASAs Mars Architecture Study (Dec 2007)
  concluded that the NTR was preferred for human
  missions to Mars
• National Research Council committee (S. Howe
  served as one of 23 members) that reviewed the
  NASA Exploration Technology Development Program
  (ETDP) reported (8/21/08) that the one technical
  gap in program was no funding for the NTR.

6
Benefits of the NTR have been shown for several
missions

• Moon - Reduce costs of implementing a Lunar
  Outpost
• Mars - Faster missions for humans reduced
  radiation exposure lower costs for cargo
  adaptability to hazards
• Good Asteroid - rendezvous
• Bad Asteroid/comet - rapid interception
• Outer solar system time to first science
  within a decade for orbitor missions to outer
  planets and to Kuiper Belt fly-through

In short, the NTR opens up access to the entire
solar system for humans and robotic probes
7
Therefore, if we eventually need a NTR for human
Mars missions, how do we develop a system that is
reliable, safe, and known operational performance?

• Use the NTR to support lunar outpost development
  and cargo supply
• Get more mass to the Moon per Ares V launch
• cost savings
• Fewer launches
• Higher mission success probability
• Get operational experience
• Reliability data
• Find weak links for space ops
• Develop man-rating for a Mars mission

8
Lunar Trajectory Objectives

• Minimum ?V trajectory
• Time-of-Flight (TOF) is not a significant
  concern.
• Insertion into either equatorial or polar LLO

Lunar Orbit Capture (LOC)
Low Earth Orbit (LEO)
Trans-Lunar Orbit (TLO)
Earth
Moon
Low Lunar Orbit (LLO)
Trans Lunar Injection (TLI)
9
NTR-Based ESAS Architecture
EDS
NTR
10
Enhanced mission performance(2006 CSNR Summer
Fellows study)
11
Can we develop and test a NTR in the current world

• According to the Independent Review Panel
  convened in 1999 to review the propulsion
  technologies examined in the NASA Advanced Space
  Transportation Program
• Previous studies during the Space Exploration
  Initiative prioritized the critical issues for
  developing a nuclear propulsion system as 1)
  ground testing, 2) fuels development, and 3)
  enhanced performance

12
Ground testing - Sub-surface Active Filtering of
Exhaust (SAFE)

• Nuclear furnace proved abiltiy to scrub exhaust
• Scaling to full power engines implies a costly
  faciltiy
• SAFE offers one cheaper option if proven feasible
• If fuel doesnt leak, then cheaper scrubber is
  possible

13
Testing in the Current Environment

• INL/CSNR completed NTP testing assessment for
  NASA Prometheus program office (2007)
• Desert Research Institute sub-contract completed
• Validated previous SAFE evaluation by Howe et al
  in 1998
• Produced design of a sub-scale proof-of-concept
  experiment for 1M

14
Tungsten Cermet Fuel

• Hot hydrogen compatibility
• Better thermal conductivity
• Potential for long life reactors
• High melting point (3700 K)
• Resistance to creep at high temperatures
• Smaller reactor core then carbide fuels
• Good radiation migration properties
• Cladding from same metallic material
• Contains fission products and uranium oxide in
  fuel
• More radiation resistant than carbon

W
15
Tungsten Loss Rate
e.g. Loss of 20 microns in 2 hours implies 3200 K
16
Accident Scenarios for Homogenous Core Design
k is normalized to critical configuration
sk 0.003
Only scenarios resulting in submersion in
seawater and wet sand are required for
criticality accidents.
17
Tungsten NTR Fuel elements
Tungsten fuel elements loaded with CeO2 (40
Vol.) as UO2 simulant
18
Fuels Development

• The requirements of the NTR place rigorous
  constraints on the fuel
• While normal power reactor fuel cant work in
  the NTR, NTR fuel could work in a power reactor
• Development of one fuel form to serve both power
  and propulsion could ultimately be a cost savings
  for the program
• The 2009 CSNR Summer Fellows are examining
  concepts for high temperature power conversion to
  utilize the NTR fuel in a lunar reactor

19
Conclusions

• The benefits of using a NTR for many types of
  missions have been shown for many years
• The NTR opens the solar system to rapid
  exploration
• Testing and fuel development are major issues
• A single solution to these issues is the fuel
  form
• Most questions about the candidate fuel forms can
  be addressed for modest expense using
  electrically heated testing
• Development of one fuel form for power and
  propulsion could provide significant program
  savings

20
backups
21
Why arent nuclear rockets in use today?

• Concept proven during Rover/NERVA
• Performance demonstrated for high-thrust,
  restarts, lifetime
• TRL-5 or 6 demonstrated by 1969
• 37 years after the proof, we are still using
  chemical rockets with 50 of the performance

22
Tech summary

• Rover/NERVA demonstrated that a nuclear core at
  full power (keep the hot parts at 2550K and the
  cool parts cool) could operate for the require
  duration, have multiple restarts, produce high
  thrust, have high Isp, and operate safely
• Through the CY2000, some expertise remained in
  human resources and some parts remained in
  physical resources. While blueprints and
  documents remain regarding design, the rest is
  essentially gone. Thus, there is little carry
  over
• The major issues with the NERVA system were 1)
  mid-band corrosion (lifetime) and 2) radioactive
  effluent (impacts testing and space operations)
• Any new program will start with knowledge but no
  hardware and should be targeted to address the
  major issues

23
Issues - Emissions

• NERVA tests showed significant emission of
  radioactive gases and particulate during
  operation
• NTR performance benefit is enhanced if operations
  begin in LEO
• Emission of radioactive species into LEO may be
  precluded in public viewpoint
• Arguing relative amounts compared to galactic
  cosmic ray background does not erase the mental
  image of radioactivity raining down onto the
  Earth
• Radiation emitted by the operating NTR can impact
  big observatories indicating that a hot reactor
  may not be allowed to orbit but must be ejected
  on the first burn
• No periapsis pumping

24
Fractional release rate
25
Issues - proliferation

• Launch aborts must be considered
• Fast reactors offer less chance for criticality
  on submersion than epi-thermal systems but
  contain more fissile material
• Dispersion upon reentry is not attractive from an
  environmental impact perspective
• Even though the engine has no fission product
  inventory and is cold
• Engine should stay intact upon reentry
• Dropping a few hundred kilograms of fissile
  material into foreign states could be considered
  a high risk
• Could constrain launch profile
• Could dictate fuel form

26
Cladding Failure of Early NTR Designs
27
Lifetime of Cermet Fuels

• Not limited by erosion of tungsten-cermet fuels
• Actual limitation
• Quantity of nuclear material
• Integrity of non-nuclear rocket components
• Poison buildup
• Possible space-cold effects(ductile to brittle
  transition)
• Operation temperature(max Isp of 950 s)

28
Design Benefits of a Fast Reactor

• Greater power density
• Lighter core design thanthermal reactors
• Burn-up of transuranics generated in the reactor
• Reflectors instead of moderating material
• Fast reactors can be controlled using the
  reflector systems with control drums

29
GE-710 HTGR PROGRAM

• 1962-1968
• Accomplished a flexible, basic fuel rod design,
  assessed a fabrication process and evaluated
  performance objectives through both non-nuclear
  and in-pile testing
• Four different program objectives
• Gas cooled reactors (fast spectrum open and
  closed loop operation)
• Gas cooled reactor for closed loop operation only
• Brayton cycle space power
• Fuel element technology development program

30
SINTERING STUDIES

• Consistent fuel loadings of 46wt UO2
• 1-2 mm diameter W particles
• Crucible design to achieve desired density
• Sintering temperature to minimize fuel
  dissociation
• Minimization of CTE difference between fuel and
  cladding

31
Maintaining Thermal Subcriticality

• Boron-carbide control drumsabsorb excess
  neutrons
• Melting of the core wouldput it in a
  non-critical state
• Loss of the beryllium reflectorensures the
  reactor cannot go critical
• Addition of tungsten and rhenium absorb neutrons
  at the thermal energies 4 to 5 orders of
  magnitude greater than carbon

32
Thermal Poison Rhenium-187
Figures courtesy of Mike Houts, MSFC
33
NTR Design
34
NTR-Based ESAS Architecture
MCNP Model 2D Renderings
Outer Pressure Vessel
Model parameters (densities and physical
dimensions) were used to determine engine and
shield masses.
LH2 Fuel Tank
Inner Pressure Vessel
Turbopumps
Radiation Shield
Core
Engine
Control Drums
Nozzle
35
NTR-Based ESAS Architecture
MCNP Model 3D Renderings
36
NTR-Based ESAS Architecture
MCNP Model 3D Renderings
37
Rocket Operation Parameters

• Single Reactor
• Specific Impulse 850 s
• Thrust 150 kN (34 klbf)
• Temperature 2300 2500 K
• Hydrogen Flow Rate 18.0 kg/s
• Thermal Power 650 MW
• Cermet W-Re(6.5 w/o)-UO2 (60 v/o, 93 HEU)

38
Fabrication of Frozen Pellet Bed samples using
the SPS furnace
39
2009 Summer Fellowship Topics

• Advanced Heat Exchanger Concepts NASA is
  pursuing technology development of Fission
  Surface Power (FSP) systems for the lunar and
  Mars surface. A potential FSP concept uses a
  pumped liquid metal reactor cooling loop coupled
  to either Stirling or Brayton power conversion.
  System performance is very sensitive to this heat
  transfer interface. The participants will
  develop heat exchanger concepts that are
  efficient, lightweight, reliable, compatible with
  the working fluids, and feasible to build.
• FSP Shield Options Reducing mass and complexity
  are important aspects of space system design.
  The use of water as a radiation shield has the
  potential to reduce the mass and complexity of
  fission surface power (FSP) systems. Landed mass
  can be further reduced if water for the shield
  can be obtained in-situ. Participants will
  investigate water shield design from both a
  radiation attenuation and thermal management
  standpoint. Potential shield canister materials
  that have adequate long-term compatibility with
  water in a moderate radiation environment will be
  identified. Detailed radiation transport and
  thermal management calculations will be
  performed. Variable and fixed-orientation
  shields will be investigated. Methods for
  effectively using potential in-situ sources of
  water will be devised.
• NTR Intercept of Short Period Comet Evaluate
  the potential performance of a NTR for
  interception of a massive low-period comet
  inbound to Earth. The participants will design
  the NTR for various thrust, specific impulse, and
  lifetime modes. Innovative NTR designs will also
  be investigated.
• Advanced High Temperature Power Reactor design
  assess feasibility of using the NTR core as a
  source of high temperature fluid for power
  conversion. Ultra-high temperature systems such
  as Brayton, Rankine, and MHD will be evaluated.
  Specific components benefiting from high
  temperature refractory alloys will be identified.