OVERVIEW - RELAP/SCDAPSIM Presented Dr. Chris Allison

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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

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

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)

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

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

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

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

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

RELAP/SCDAPSIM models generally more detailed VS 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)

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

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

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

Oxide spalling during reflood critical to predict H2 and melt formation

RELAP/SCDAPSIM models generally more detailed VS 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)

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

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

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