Maximum

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Fuel Type, Enrichment, Geometry
Fuel Type, Geometry
Thermal- Hydraulics
Neutronics (reactor physics)
Lecture 2 Design methodology
Power distribution
Maximum Power
Fuel Performance
Maximum Burnup
Economics
COE
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Reactor Physics (Hydride fuel)

• Input
• - wt U in fuel (45) core size (Rcore, L)
  Energy output Constraint
• - Negative reactivity feedback (H in fuel)
• Output
• - H/U235 in core (i.e., hydrogen-to-enrichment
  ratio)

Apellet fuel pellet cross section
Aflow coolant subchannel area
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LWR thermal-hydraulic characteristics
core thermal power, M W
L height of fuel in rods Nrod number of fuel
rods in a fuel assembly NFA number of fuel
assemblies (in BWRs, bundles) in core
flow rate of coolant (water or steam) per
subchannel, kg/s (a subchannel includes one fuel
rod and associated coolant)
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Linear heat rate (LHR)

LHR (kW/m) q Dpellet fuel pellet
diameter, mm (J/fiss)
volumetric heat rate, kW/m3 3.2x10-11eNU?fiss?
?fiss 5x10-22 cm2 NU U atom density
6x1023?U/238 ?U U mass density in fuel
material, g/cm3 e U-235 enrichment ?
thermal neutron flux, cm-2s-1
q 3.2x10-11?U?e
Example ? 2x1013 cm-2s-1 Dpellet 1.0
cm UO2 ?U 9.3 g/cm3 e 0.035 q
20.6 kW/m Hydride ?U 3.7 g/cm3 e 0.14
q 20.6 kW/m
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Burnup (BU, MWd/kgU)
initial mass of U per unit fuel
length, kgU/cm
E q x td x 10-5 thermal energy per unit fuel
length, MWd/cm
td irradiation time, days
Ex td 365 d q 50 kW/m Dpellet 0.7 cm
?U (g/cm3) 9.6 (UO2) 3.8
(hydride) BU (UO2) 12 MWd/kgU BU
(hydride) 30 MWd/kgU (thermal energy
produced is the same for both fuels)
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Peaking factors
Peaking factors are the maxima of the spatial
distributions of the LHR in the core. They are
obtained from the flux distribution

• PWR Since there is no duct around the FA, the
  core is effectively filled with fuel rods and
  associated coolant

rod-avg LHR at rcore
Radial peaking factor
Axial peaking factor
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BWR Peaking factors
1. Core average ? fuel-assembly average
assembly-average peaking factor ?FA(0)
? r/Rcore
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BWR Peaking factors (cont)
2. bundle-average ? bundle axial peak
qFA
qFA
local LHR of bundle qFA(z)
?(0) bundle peaking factor
-1/2 lt z/L lt 1/2
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PWR fuel-rod layout

• Channel unit cell containing 1 fuel element and
  associated water (also called subchannel)

Aflow P2 pD2/4
D

• Wetted/heated perimeter
• ? pD

P pitch

• Equivalent (hydraulic) diameter De 4Aflow/?

• Fuel rods in core

• e efficiency of filling core with fuel rods lt 1
  because
• Control rods occupy part of fuel assembly
  
• Cannot fill circle (core) with squares (fuel
  assemblies)

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BWR fuel-rod layout

• All fuel rods in the duct constitute the unit of
  flow

NFA fuel assemblies(FAs) in core Nrod rods
per FA
? Awater rods/Afuel
?h enthalpy rise over core, J/kg made the
same for each FA by orificing the inlet plates
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LWR Reference Cores
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Reference Fuel Elements (rods)
See NUREG-1754 Figs 2.7 2.8
D 2(?C ?g)
At startup, cold
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Reference-core thermal-hydraulic parameters