FUEL PERFORMANCE

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FUEL PERFORMANCE 5 Fuel Temperatures Oxide
fuel

• The fuel-performance code FRAPCON predicts
  fuel-element behavior during irradiation
• The objective is to determine if any
  fuel-related design margins are exceeded
• - maximum fuel temperature
• - fission-gas release/fuel-rod internal pressure
  
• - strain of the cladding due to pellet-cladding
• interaction (PCI)
• - waterside corrosion of the cladding
• - cladding embrittlement due to hydrogen pickup

FRAPCON-3 Thermal-mechanical behavior of
oxide fuel rods G. Berna, C. Beyer, K. Davis, D.
Lanning, NUREG/CR-6534 (1997)
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Input Information

• Cladding OD
• Cladding thickness
• Pellet-cladding gap (radial, as fabricated)
• Fuel stack height
• Plenum length and spring characteristics
• Fuel enrichment (not needed)
• Porosity of fuel (open and closed)
• Roughness of fuel cladding surfaces (height of
  asperities)
• Densification rate

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Cladding information - input

• Type (Zry-2, Zry-4, ZIRLO, M5)
• Fabrication cold-worked or stress-relieved-anneal
  ed
• Surface roughness
• Texture factor (fraction of grains of hcp Zr with
  basal planes parallel to the tube axis usually
  small)
• Fill-gas type and pressure (usually He at 10
  atm) liquid-metal fill accepted

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Reactor conditions

• Type (BWR or PWR)
• Thermal-hydraulic conditions (not needed if axial
  distribution of cladding OD temperatures is
  specified)
• - coolant inlet temperature
• - system pressure (7 MPa for BWR)
• - mass flux per fuel rod
• - rod pitch
• Average linear heat rate (LHR), q(z) qav ?(z)
• Relative axial LHR shape - ?(z) user input or
  chopped cosine as a function of elevation and
  time
• Cladding OD temperature profile
• - calculated based in LHR and coolant conditions
• - user-specified (from T-H team)

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Fuel radial temperature distribution

• Given
• - geometry (cladding OD
• thickness, initial gap size)
• - LHR and TCO at elevation z
• Calculate
• - conductance of fuel-cladding gap
• - temperature distribution in fuel

q
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Fuel cracking relocation

• If the fuel pellet remained intact, the gap
  thickness at the start of irradiation (hot) would
  be

radial displacements due to thermal expansion at
startup
average fuel temperature (parabolic
distribution)
But, the parabolic temp. distribution generates
tensile hoop stress
? thermal exp. coef.10-5 K-1
E fuel Youngs modulus
E 168 Gpa ?fract 0.13 GPa
r/R
? Poissons ratio 0.31
?(T0-TS)fract 105oC
occurs on startup
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• cracks predominantly radial
• do not interfere with heat conduction

Hourglassing of cracked pellet
hard PCMI
PCMI starts
PCMI pellet-cladding mechanical interaction
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Gap conductance

• Initial cracking/relocation 40-50 reduction of
  initial hot gap size
• When remainder of the gap is closed by fuel
  swelling, hard contact (PCI) begins
• cracks are closed due to high compressive
  stresses in the fuel due to PCI

q 2?Rhgap(TS TCi)
hgap gap conductance, W/m2-K
hgap hrad hfill h solid
solid-solid contact
gap filler (He or lm)
radiation
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hrad ??( )(TS TCi)
hfill kfill/ kfill thermal
conductivity of gap filler If gas tgap
(g gC) (? ?C)
zmean
open
closed
tgap calculated open-gap size
g, gC jump distances pi interfacial pressure
(solid-solid) ?, ?C surface roughnesses
hsolid f(kmean,pi/Hm,zmean)
zmean (? ?C)1/2
Hm Meyer hardness of cladding
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FRAPCON flow diagram for fuel temp.