Tungsten Cermet Reactors

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Tungsten Cermet Reactors

• John Darrell BessAugust 1, 2006

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Failings of Previous Fuel Types

• Brittleness of materials
• Engine vibrationscracked the fuel apart
• Thermal instability,cracking, and coefficient of
  thermal expansion
• Hydrogen erosion of carbide fuels at high
  temperatures
• Carbide coatings provided insufficient protection
• Loss of fission products in exhaust

INL CSNR Forum October 26, 2005
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Cladding Failure of Early NTR Designs
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Fuel Endurance
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Tungsten Cermet

• 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

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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)

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Future of Cermet Fuel

• Bi-modal design for power production
• Reusable nuclear rocket engines
• Orbital/Space Station refueling
• LANTR (LOX-Augmented NTR) concept
• Develops technology for high performance fission
  surface power
• Fuel and engine testing would enable Mars
  missions and beyond

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Fuel Additives

• Tungsten compatible materials
• Provide desirable mechanical properties
• Reduce brittleness, improve toughness
• Adjust ductile to brittle transition
• Stabilizers
• Decrease fission product migration
• Reduce UO2 fuel inventory
• Candidate materials
• Examples Re, Mo, ThO2, Gd2O3

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Cross Section (Probability)

• Various modifiers
• Particle energy
• System temperature
• Target atom
• Types of interactions
• Scattering
• Elastic
• Inelastic
• Capture
• Absorption
• Fission

http//www.ncnr.nist.gov/
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Fission Cross Section
http//www.uic.com.au/
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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

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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

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Thermal Poison Rhenium-187
Figures courtesy of Mike Houts, MSFC
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Fuel Element Design(Past Present)
19-HoleDesign(2 mm)
Dumbo Design
5-FinDesign(3 mm)
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Cooling the Reactor System
MSFC NTP One Year Review, June 20-21, 2006
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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.
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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)

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Specific Impulse Comparison
Multidisciplinary Analysis of Nuclear Thermal
Propulsion Design Options for Human Exploration
Missions. R. Joyner et al. AIAA 2006
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Reactor Controls

• Requires semiautonomous controls
• Requires knowledge of real-time status
• Neutron or gamma flux
• Power level
• Dose
• Temperature
• Propellant flow
• Strain/deformation

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Measuring Flux, Power, and Dose

• Direct detection
• Requires detector to differentiate between
  neutrons and gammas
• Gamma detectors
• Correlated to neutron flux and power level
• Indirect detection
• Neutron thermometer
• Interpolation from temperature gradient
  information
• Gamma thermometer
• Only viable candidate for in-core detection

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Temperature Measurements

• Require temperature profile of core, propellant,
  and fuel elements
• Thermocouples can function in high temperature,
  high radiation environments
• Fiber Bragg Gratings as developed by Luna
  Innovations deal with relatively high
  temperatures (1100C) and high dose (8.7 x 108
  Gy gamma)
• Higher temperatures use platinum-rhodium and
  tungsten-rhenium thermocouples (gt2700 K) but
  decalibrate with neutron exposure
• Johnson noise thermometry
• Mean kinetic energy of atomic ensemble
• Needs preamplifier electronic development

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Propellant Flow

• Flow from storage tanks
• Flow through turbomachinery
• Propellant flow necessary for rocket thrust
• Also necessary for cooling non-fuel reactor
  components to prevent damage
• Extensive modeling available through programs
  such as FLUENT

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Strain/Deformation

• Important for keff expansion yields negative
  reactivity and contraction yields positive
  reactivity
• In ground vacuum chamber testing, can use CCD
  camera to measure
• Gleeble testing of components
• Luna Innovations fiber optics can also measure
  strain only tested at low temperatures

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Nuclear Dosimetry using MCNP5

• Turbomachinery SS
• Neutron
• 1.2 0.3x109 cm-2s-1
• 2.1 0.4x104 remhr-1
• Photon
• 3.4 0.3x109 cm-2s-1
• 6.8 0.7x103 remhr-1
• Payload H2O
• Neutron
• 0
• Photon
• 0

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Summary

• Tungsten-Cermet fuels demonstrate potential for
  long-lived, high Isp, nuclear rockets with
  high-integrity containment of uranium and fission
  products
• Fuels development and testing necessary to
  confirm potential oftungsten-cermet fuelsin
  reactors for NTRs

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