1. OVERVIEW - RELAP/SCDAPSIM
Presented Dr. Chris Allison

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

3. 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

4. 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

5. 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

6. Bundle convective and radiative heat transfer
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

7. User defines representative assembly for each flow channel in core
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

8. SCDAP fuel rod components use 2D models to predict temperature (r,z), deformation, chemical interactions and melting
Zr Cladding
UO2 Fuel Pellet
Gap

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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)

16. 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

17. 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

18. 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

19. 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

20. 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 < 1500 K (2200 oF)
Limited cladding oxidation (< embrittlement)
RELAP5/MOD3.3 radiation exchange heat transfer model neglects absorption by fluid

21. 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

22. Primary differences between RELAP/SCDAPSIM and RELAP/MOD3.3
RELAP5/MOD3.3’s 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)

23. Primary differences between RELAP/SCDAPSIM and RELAP/MOD3.3
RELAP5/MOD3.3’s 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

24. 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 times faster than real time
Typical SA transients run 1-5 times faster than real time

25. 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

26. SCDAP/RELAP5 Nodalization of RCS
TML with AM and HPI
13:
Cold Leg
12:
Crossover
Leg 9:
Hot Leg
10:
Tubes
Up Flow
11:
Down Flow 5: 4: 3:
Pressurizer 7: 6:
Crossover Leg 8:
Downcomer
Broken Loop
Intact Loop 2:
Upper
Plenum 1:
Core
MAAP4 Nodalization
of RCS
SCDAP/RELAP5 Nodalization of RCS

27. RELAP/SCDAP nodalization of 4-Loop RPV
2D connections allow for cross flow due to natural circulation or loss of geometry

28. RELAP/SCDAPSIM models generally more detailedVS
MAAP/MELCOR
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)

29. 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

30. 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

31. RELAP/SCDAPSIM models generally more detailedVS
MAAP/MELCOR
Reflood
Oxide spalling
Accelerated heating, oxidation, melting
Reflood
Oxide spalling (MELCOR)
Accelerated heating, oxidation, melting
MAAP does not consider oxide spalling

32. Oxide spalling during reflood critical to predict H2 and melt formation

33. RELAP/SCDAPSIM models generally more detailedVS
MAAP/MELCOR
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)

34. Assumptions on lower plenum debris will impact vessel failure
Layers formed by debris/melt relocation
Molten pool (mixture)
Gap cooling
MELCOR
Layers formed by debris/melt relocation
SCDAP
Structural material
MAAP
Corium

35. 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

36. 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