The Use of RELAP5-3D for Subchannel Analysis of SFR Fuel Assemblies

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The Use of RELAP5-3D for Subchannel Analysis of
SFR Fuel Assemblies

• Matthew J Memmott
• Supervisors Jacopo Boungiorno, Pavel Hejzlar
• Massachusetts Institute of Technology
• RELAP Users Group Meeting
• November 19, 2008

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Outline

• Background
• Subchannel Model
• Benchmarks
• Application
• Results/Conclusions
• Future Work

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Background

• SFR is currently the fast reactor of choice under
  GNEP (ABR1000)
• MIT is investigating innovative fuel designs
  (annular fuel, bottle shaped, etc.)
• Subchannel analyses required fuel temps, clad
  temps, coolant velocities, etc.
• Current subchannel codes cannot handle
  nonstandard geometries

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Subchannel Model

• 4 Primary Considerations
• Subchannel Geometry
• Cross-Flow
• Sodium Conduction
• Turbulent Mixing

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Subchannel Geometry (I)
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Subchannel Geometry (II)

• Composed of multiple 22 segment pipes
• 2 segments for entrance/exit and shielding
• 5 segments for plenum
• 15 segments for core
• Fuel rods divided into 6 segments (no peaking)
• Allows for connection to each subchannel
• Cannot evaluate azimuthal conduction in rods
• Inlet conditions controlled by time dependent
  volume/junction

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Subchannels only (I)
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Subchannels Only (II)
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Cross-Flow (I)

• Multiple junctions Each volume of the pipe is
  connected to the adjacent subchannel volume of
  the same number
• Transverse bundle flow resistances1
• Area equal to transverse area of the subchannel
• Volume lengths in y and z input for each volume
• VERY little mixing, almost no mixing due to tight
  pitch of the core/uniform velocity profile.

1. I. E. Idelchik, Handbook of Hydraulic
Resistance Second Edition, pg. 608, Hemisphere
Publishing Corporation, New York, USA, 1986.
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Cross-Flow (II)

• Pressure-gradient driven
• Modeled in RELAP5 using horizontal junctions
• Creates an outlet temperature profile

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Sodium Conduction

• Heat transfer due to conduction
• Axial Conduction
• Radial Conduction
• Fouriers Law
• Control variables (CVs) in RELAP5-3D
• Requires over 5,000 CVs limit is 10,000

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Axial Conduction (full power)
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Axial Conduction (4 power)
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Radial Conduction (full power)
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Radial Conduction (4 flow)
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Turbulent Mixing (I)

• Mixing due to wire-wrap
• Two regions of turbulent mixing
• Inner channels
• Edge and Corner Channels
• Mixing modeled by two parameters
• e - Effective Eddy Diffusivity
• C1L Swirl Ratio

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Turbulent Mixing (II)
e - Effective Eddy Diffusivity

• Induced coolant swirling caused by wire wrap
  enhances mixing
• Total mass flow in/out of each subchannel face is
  zero
• Enhanced mixing flattens temperature profile in
  interior

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Turbulent Mixing (III)

• C1L Swirl Ratio

• Unidirectional orientation of wire-wrap causes a
  transverse flow in edge channels
• This flow results in a circular flow inside duct
  wall
• C1L is the ratio of transverse to axial flow in
  the edge channels
• Effect of edge swirl velocity is to equalize
  edge/corner channel temperatures

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Effect of Mixing
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Constraints

• Limiting feature of model is control variables
• 6 control variables per volume
• 22 volumes per subchannel
• 51 subchannels for 9 ring, 1/12 assembly model
• 6732 Control variables out of 10,000 possible

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SUPERENERGY II

• Developed by Chen and Todreas
• Only valid for solid pin, hexagonal assemblies
• Calculates sodium properties at single T
• Can only evaluate 1-8 ring assemblies
• Created a SUPERENERGY II model matching an 8 ring
  RELAP model
• Outlet temperature profile comparison

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Benchmark I

• Excellent Agreement 3C
• Sodium properties are temperature dependent
• The difference is due to the sodium properties
  calculated in the RELAP5 model
• RELAP5 more accurate

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Benchmark II ORNL 19-Pin Test
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ORNL 19-Pin Benchmark
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Example Application of RELAP as a Subchannel
Analysis Code

• Cold assembly dimensions
• Rod-duct gap in edge channels is 1.21 mm
• Wire-wrap diameter is 0.805 mm
• Edge to interior subchannel flow area ratio 2.0
• This results in a LARGE temperature distribution
  (70C)
• Hot dimensionsstill large T distribution (35C)
• Flattened profile can be achieved by
• Decreasing edge channel area
• Increasing flow resistance in edge channels

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Assembly Duct Ribs
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RELAP5-3D Advantages

• Non-standard fuel geometries (annular, inverse,
  thermal expansion, etc.)
• Various assembly designs (wire-wrap, grid spaced,
  duct ribs, etc.)
• Temperature dependent coolant properties
• Detailed fuel rod analysis
• Steady State or Transient analyses

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RELAP5-3D Disadvantages

• Huge input file (gt23,000 lines of code)
• Control Variable limited (13 rings)
• Long runtime (3 to 26 hours, depending on size)
• Can take significant time to modify input file
  due to size and complexity

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Results/Conclusions

• RELAP5-3D subchannel model capable of modeling
  wide variety of fuel types, sodium cooled
  assembly geometries, and conditions
• RELAP5-3D model within acceptable performance
  region dictated by benchmarking of traditional
  subchannel SFR codes with ORNL19 pin
• Scope of RELAP5-3D would be greatly enhanced by
  the addition of a wire-wraped sodium subchannel
  mixing option

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Thank you!

• Questions?

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Extra Slides
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Annular Fuel

• Decrease number of rods
• Increase rod diameter
• Create inner coolant channel, providing internal
  and external cooling of rod
• Decrease clad surface temp
• Decrease fuel max temp
• Can safely increase power density

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Bottle Neck Fuel

• Maintain constant plenum volume by decreasing
  plenum radius and increasing height
• Decreases pressure drop in assemblies
• Ideal plenum radius minimizes pressure drop at
  low ?H
• Potential for power uprate or decreased pump sizes

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