OVERVIEW - RELAP/SCDAPSIM

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OVERVIEW - RELAP/SCDAPSIM
Presented Dr. Chris Allison
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Outline

• General modeling approaches
• Primary differences between RELAP/SCDAPSIM and
• RELAP/MOD3.3
• MAAP and MELCOR codes

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RELAP5 and SCDAP WERE ORIGINALLY DEVELOPED BY US
NRC

• RELAP5 developed for DBA analysis (Late 1970s)
• SCDAP (Severe Core Damage Analysis Package) added
  in 1980s for SA analysis)
• RELAP/SCDAPSIM developed by ISS/SDTP for
  commercial applications
• Advanced numerics and programming
• Standard RELAP5/MOD3.2/3.3 and SCDAP/RELAP/MOD3.2
  models

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RELAP/MOD3.2 and RELAP/MOD3.3 models used for
system TH analysis

• Non-equilibrium, two fluid models for
  hydrodynamics including transport of
  non-condensable gases
• 2D/3D capability provided through cross-flow
  options
• Convective and radiative heat transfer
• 1D heat conduction in system structures
• Point reactor kinetics
• External 3D kinetics provided through link to
  user supplied reactor kinetics packages
• Control system, trip logic, and special system
  components such as valves and pumps

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SCDAP components/models used for detailed vessel
and core behavior

• Detailed LWR core components
• Upper plenum structures
• Core debris and molten pools
• Lower plenum debris and vessel structures

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User selects representative fuel rod, control
rod/blade and other components for LWR core

• Bundle convective and radiative heat transfer
• Radiation absorption by fluid
• Bundle deformation/blockage/grid spacer effects
  on flow patterns
• 2D heat conduction
• Grid spacer heating and melting
• Bundle deformation/blockage formation
• Liquefaction and failure of core components
• Debris/void formation

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Representative components can have different
power levels
Fuel Rod 1
Fuel Rod 2
Control rod
Water Rod
User defines representative assembly for each
flow channel in core
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SCDAP fuel rod components use 2D models to
predict temperature (r,z), deformation, chemical
interactions and melting

Zr Cladding
UO2 Fuel Pellet
Gap
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SCDAP fuel rod components consider failure due to
spacer grid interactions, metallic and ceramic
melt relocation, and fragmentation

• 2D heat conduction
• Fission product buildup and release
• Cladding deformation and rupture
• Cladding oxidation and hydrogen production
• Effects of steam availability and vapor diffusion
  considered
• Zr spacer grid interactions
• UO2 dissolution by molten Zr
• Zr melting and relocation
• UO2/ZrO2 melting and relocation

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SCDAP control rod components use 2D models to
predict temperature (r,z), deformation, chemical
interactions and melting

Zr Guide Tube
SS Sheath
Ag-In-Cd/B4C Absorber
Gap
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SCDAP BWR control components use 3D models to
predict temperature (r,z), deformation, chemical
interactions and melting

Gap between absorber tube and sheath
Zr Guide Tube
SS Sheath
B4C Absorber
Interstitial Gap
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SCDAP Ag-In-Cd or B4C control rod/blade models
consider early failure of control structures

• 2D heat conduction
• Cladding oxidation and hydrogen production
• Effects of steam availability and vapor diffusion
  considered
• Zr/SS control material interactions
• Guide tube, cladding, control material melting
  and relocation

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SCDAP general 2D shroud model tracks behavior of
other core components

• LWR SCDAP general shroud model used to model core
  walls, experimental facility structures
• 2D heat conduction
• Zr layer oxidation and hydrogen production
• Effects of steam availability and vapor diffusion
  considered
• Melting and relocation

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SCDAP upper plenum models describe heating and
melting

• Oxidation
• Parabolic rate
• Steam starvation
• Heat conduction
• Lumped parameter
• Relocation of upper plenum structures into core
  or lower plenum

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SCDAP in-core debris/model pool models describe
later stages of core failure

• Oxidation
• Parabolic rate
• Steam starvation
• Heat conduction
• Lumped parameter (in rubble)
• 1D (in metallic blockages)
• 1D (molten pool crust perimeter)

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SCDAP in-core debris/molten pool models describe
formation, growth, and failure of in-core molten
pools

• Molten pool behavior
• Radial and axial spreading
• Crust thinning and mechanical failure
• Side wall versus top surface
• Transient natural circulation
• Interactions with shroud wall

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SCDAP in-core debris/model pool models describe
formation, growth, and failure of in-core molten
pools

• Material relocation
• Void formation
• Molten pool upper crust collapse
• Mixing of debris/molten pool
• Relocation of upper plenum structures into core
• Molten pool slumping

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SCDAP uses a detailed 2D model to describe
behavior of lower plenum debris/vessel

• Heat conduction
• 2D finite element
• gap resistance (solid/melt)
• 1D model at crust boundary perimeter
• Molten pool behavior
• Transient natural circulation
• Interactions with vessel wall

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SCDAP uses detailed 2D model to describe behavior
of lower plenum debris/vessel

• Creep rupture failure of vessel wall
• Material relocation
• Relocation of upper plenum structures
• Relocation of core component materials
• Molten pool slumping
• Ex-vessel flooding

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Primary differences between RELAP/SCDAPSIM and
RELAP/MOD3.3

• RELAP5/MOD3.3 limited to transients that will not
  result in core damage
• Peak fuel cladding temperatures lt 1500 K (2200
  oF)
• Limited cladding oxidation (lt embrittlement)
• RELAP5/MOD3.3 radiation exchange heat transfer
  model neglects absorption by fluid

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Primary differences between RELAP/SCDAPSIM and
RELAP/MOD3.3

• RELAP/SCDAPSIM has detailed core component models
  for typical LWR/HWR designs
• LWR fuel rod
• Ag-In-Cd/B4C control rod
• BWR control blade model
• Electrically-heated fuel rod simulator
• RELAP/SCDAPSIM has upper and lower plenum models
  for typical LWR designs
• Detailed 2D finite element model to describe
  lower head
• RELAP5/MOD3.3 uses general 1D heat structure
  model to describe all structures including core
  and vessel

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Primary differences between RELAP/SCDAPSIM and
RELAP/MOD3.3

• RELAP5/MOD3.3s 1D heat conduction model to
  ignores important phenomena for fuel elements or
  electrically heated fuel element simulators
• Axial conduction
• Temperature-dependent electrical resistivity
  changes on power profile
• Burnup/thermal cycling influence on thermal
  properties
• Influence of changes in gap dimensions, fuel rod
  internal pressure, and fission product release on
  fuel-cladding gap conductance
• Steam starvation and vapor diffusion limits for
  cladding oxidation
• Zircaloy cladding embrittlement
• Fission product release
• Note Boiloff.i sample problem demonstrates
  differences between RELAP5 and SCDAP fuel rod
  models (plot)

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Primary differences between RELAP/SCDAPSIM and
RELAP/MOD3.3

• RELAP5/MOD3.3s 1D heat conduction model to
  ignores important phenomena for fuel elements or
  electrically heated fuel element simulators
• Axial conduction
• Temperature-dependent electrical resistivity
  changes on power profile
• Burnup/thermal cycling influence on thermal
  properties
• Influence of changes in gap dimensions, fuel rod
  internal pressure, and fission product release on
  fuel-cladding gap conductance
• Steam starvation and vapor diffusion limits for
  cladding oxidation
• Zircaloy cladding embrittlement
• Fission product release
• Note See boiloff example in Practical Examples
  of Severe Accident Analysis for demonstration of
  differences between RELAP5 and SCDAP fuel rod
  models

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Primary differences between RELAP/SCDAPSIM and
more simplified SA integral codes

• RELAP/SCDAPSIM limited to in-vessel behavior
• Source term and containment provided through
  links to IMPACT/SAMPSON Modules from NUPEC
• RELAP/SCDAPSIM/MOD4 being extended for integrated
  source term and containment response
• RELAP/SCDAPSIM computation times are longer than
  MAAP and comparable to MELCOR
• DBA transients typically run 10-20 times faster
  than real time
• Typical SA transients run 1-5 times faster than
  real time

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RELAP/SCDAPSIM allows much more detailed
representation of RCS/vessel

• RCS/Vessel nodalization more detailed than
  historical DBA analysis using RELAP/TRAC
• 2D/3D core/vessel
• 2D lower plenum/vessel
• Detailed 2D core component modeling
• Typical SA input models use
• Several hundred TH volumes and RCS heat
  structures
• Five representative assemblies with 2 or more
  SCDAP components
• Several hundred volumes in 2D lower plenum/vessel
  mesh

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TML with AM and HPI
MAAP4 Nodalization of RCS
SCDAP/RELAP5 Nodalization of RCS
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RELAP/SCDAP nodalization of 4-Loop RPV
2D connections allow for cross flow due to
natural circulation or loss of geometry
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RELAP/SCDAPSIM models generally more detailed
RELAP/SCDAPSIM
MAAP/MELCOR
VS

• 6 equation, non-equilibrium hydro
• 2 D heat conduction
• Relocation of Zr-In, Zr-U-O, (U-Zr)-O2
• Grid spacer interactions
• Molten pool (U-Zr)-O2 formation, growth, and
  relocation
• Radial, axial (bypass lower metallic layers)

• quasi-equilibrium hydrodynamics
• 1D lumped parameter
• Relocation of Zr-U-O
• Core slumping (user defined temperature)
• Axial
• User defined (MAAP)

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SCDAP will predict melting over wide range of
temperatures
Melting of (U-Zr)-O2
MAAP/MELCOR will predict core slumping at user
specified temperature
Liquefaction of Zr-O-U
Liquefaction of Structural and Control Material
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SCDAP can predict molten pool relocation into
lower plenum even if core plate and lower core
intact
TMI-2 End State
MAAP/MELCOR Lower core and plate must slump
before upper material can relocate
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RELAP/SCDAPSIM models generally more detailed
RELAP/SCDAPSIM
MAAP/MELCOR
VS

• Reflood
• Oxide spalling
• Accelerated heating, oxidation, melting

• Reflood
• Oxide spalling (MELCOR)
• Accelerated heating, oxidation, melting
• MAAP does not consider oxide spalling

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Oxide spalling during reflood critical to predict
H2 and melt formation
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RELAP/SCDAPSIM models generally more detailed
RELAP/SCDAPSIM
MAAP/MELCOR
VS

• Reflood
• Debris formation
• Exterior cooling of molten pool crusts
• Transient 2D lower plenum debris/vessel heat
  conduction and molten pool convection
• Stratified formation
• Homogenous molten pool

• Reflood
• Debris formation (user)
• Exterior cooling of debris beds (user)
• Steady state analytic/lumped parameter lower
  plenum debris/vessel
• Stratified formation
• Stratified metallic/ceramic (MAAP)

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Assumptions on lower plenum debris will impact
vessel failure
Layers formed by debris/melt relocation
Molten pool (mixture)
MELCOR
Gap cooling
Layers formed by debris/melt relocation
SCDAP
Structural material
MAAP
Corium
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RELAP/SCDAPSIM user defined parameters are
intentionally limited

• System defined through TH nodalization, selection
  of representative core and plenum components and
  nodalization
• RELAP5 and SCDAP user guidelines and training
• RELAP5 modeling parameters used to control flow
  regimes
• Established through RELAP5 validation activities
• SCDAP modeling parameters limited to critical
  areas of modeling uncertainties
• Recommended defaults set through validation
  activities

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MAAP/MELCOR make extensive use of modeling
parameters to adjust basic processes

• Extensive use of user defined parameters make
  evaluation of trends very difficult
• Scaling of code-to-data comparison results to
  plant behavior is unclear
• Modeling parameters are unique to facility
• Conservatism or non-conservatism may be
  influenced by user choices