Advanced Safety Modeling

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Advanced Safety Modeling

• Thomas H. Fanning
• Engineering Simulation and Safety
  AnalysisNuclear Engineering DivisionArgonne
  National Laboratory
• AFCI NEAMS MeetingMay 19, 2009

2
Highlights

• Objective
• Provide high-fidelity reactor and plant safety
  analysis models for integration into the advanced
  simulation code framework
• FY08 Milestones
• 2/29 (M3) Status Report on Uncertainty
  Assessment Plan (ANL-AFCI-218)
• 4/30 (M3) Specify Advanced Modeling
  Requirements for Safety Modeling Assessment
  (ANL-AFCI-229)
• 9/30 (M2) Report on Initial Advanced Safety
  Modeling Capability and Prototypic Analyses
  Demonstrating Advanced Simulation Capabilities
  (ANL-AFCI-243)

3
Highlights

• FY09 Milestones
• 7/31 (M2) Coupling of High Fidelity and Integral
  Analysis Methods (on schedule).
• 9/30 (M3) Prototypic Analyses Demonstrating
  Coupled Safety Modeling (on schedule).
• FY09 Funding
• Initial funding level 450k.
• Funding increase 100k.
• 9/30 (M3) Global Sensitivity Metrics and
  Efficient Methods for Their Evaluation
  (beginning).

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Initial Advanced Safety Modeling Capabilities
(FY08)

• Advanced Safety Modeling Requirements
• Preserve extensive investment in safety modeling
  capabilities
• Transition to more modern code practices and
  frameworks
• Advanced Safety Modeling Capabilities
• Role of multi-resolution approach
• Whole-core subchannel transients
• Data visualization
• Comparisons of CFD (RANS) models with subchannel
  models
• Importance of higher fidelity plenum modeling
  capabilities

217-pin Subchannel (ABTR peak assembly at
steady-state)
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Advanced Safety Modeling Requirements

• Current modeling capabilities for fast reactor
  safety analyses are the result of several hundred
  person-years of code development effort supported
  by experimental validation.
• Broad spectrum of mechanistic and
  phenomenological models.
• Enormous amount of institutional knowledge
  needs to be maintained.
• Existing code architecture evolved from
  then-modern programming practices of the 1970s.
• Monolithic application with interdependent data.
• Requires significant knowledge of the
  complexities of the entire code in order for each
  component to be maintained.
• Current code demonstrates fast execution times.
• As we move forward we need to preserve the
  existing capabilities.

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SAS4A/SASSYS-1 is the Starting Point

• SAS4A/SASSYS-1 contains extensive modeling
  capabilities that include
• Multiple channel and subchannel core thermal
  hydraulics
• Point kinetics and spatial kinetics capabilities
  including decay heat and reactivity feedback
  models.
• Detailed mechanistic models for oxide and
  metallic fuel and cladding in a fast neutron
  spectrum.
• Two-phase coolant thermal hydraulics for
  low-pressure sodium boiling.
• Intra-pin oxide fuel melting and relocation,
  molten cladding dynamics and freezing,
  fuel-coolant interactions, fuel freezing and
  plating.
• Primary and intermediate loop reactor coolant
  systems models.
• Balance of plant thermal hydraulic modeling
  capabilities.
• Reactor control systems models.
• An earlier version (SAS3A) was used extensively
  in licensing FFTF.
• SAS4A was developed to support licensing of
  CRBRP.
• Oxide fuel deformation, disruption, and material
  relocation models.
• Exported to Germany, France, and Japan in the
  late 1980s.

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Where are we headed?

• Code development efforts focus on higher-order,
  higher-resolution tools which work together under
  a multi-physics, multi-scale framework.
• High fidelity neutronics codes model full 3-D
  detail of core region
• High fidelity thermofluids codes (DNS, LES, RANS,
  SC) model full 3-D detail of selected regions of
  reactor
• High fidelity structural mechanics codes model
  full 3-D detail of selected regions of reactor
• Lower fidelity codes to model whole-core
  transient behavior coupled to 1- or 2-D models in
  remaining reactor regions.

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How Do We Get There? Initial Focus is on Thermal
Hydraulic Modeling

• Thermal and hydraulic conditions dictate buoyant
  driving forces, natural circulation flow
  patterns, and flow channel temperature
  distributions, which are critical to safety
  performance.
• Correct prediction of thermal and hydraulic
  conditions is important not only for determining
  component performance, but also in determining
  reactivity feedback during whole-plant dynamics
  simulations.
• Temperature impacts on reactivity include
• Fuel Doppler
• Fuel, cladding, and coolant density variations.
• Three-dimensional subassembly temperature
  distributions and the impact on subassembly
  bowing and radial expansion.
• Plenum outlet temperature distributions and the
  impact on control-rod driveline expansion.
• Reactor vessel expansion causing core
  displacement relative to control-rod driveline
  positions.
• Inlet temperature distributions and grid plate
  expansion.

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Thermal Hydraulic Modeling in the SAS4A/SASSYS-1
Code

• Recent additions to SAS4A/SASSYS-1 include
  detailed subchannel modeling capabilities for
  in-core treatment.
• PRIMAR-4 implements most of the ex-core TH
  modeling capabilities of SAS4A/SASSYS-1.

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Safety Modeling in the SHARP Framework

• Long-range goal is to couple SAS4A/SASSYS-1 into
  the SHARP simulation framework through PRIMAR-4

T.H. Fanning and T. J. Tautges, Specification of
Advanced Safety Modeling Requirements,
ANL-AFCI-229, April 2008.
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Role of Multi-Resolution Capability in Safety
Modeling
Multi-Resolution Thermal Hydraulic Simulation
Hierarchy

• Fast-running low resolution methods
• To provide rapid turn around for engineering
  design and safety analyses.
• Highly-scalable high-order RANS/LES/DNS
• To provide modeling parameters for improved
  modeling results at lower fidelities
• DNS-informed LES models
• LES-informed RANS models
• RANS-informed subchannel models

Subchannel Models
Modeling Parameters
Boundary Conditions
Reynolds Averaged Navier Stokes
Modeling Parameters
Increasing Resolution
Increasing Domain Size
Boundary Conditions
Large Eddy Simulation
Modeling Parameters
Boundary Conditions
Direct Numerical Simulation
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Whole-Core Subchannel Analysis Capabilities

• LES/RANS modeling capabilities are not generally
  suitable for whole-core (whole-plant) safety
  analysis.
• Subchannel modeling capabilities have been
  demonstrated for multiple assemblies, and can
  readily be scaled to full-core simulations.
• The EBR-II SHRT-17 test (protected loss of flow
  at full power) provided subchannel level
  temperature distributions within the instrumented
  subassembly XX09.
• Advanced visualization capabilities have been
  added to SAS4A/SASSYS-1 to support analysis of
  large transient simulation data sets.

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SAS4A/SASSYS-1 Subchannel Temperature Results for
SHRT-17
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Comparison Between RANS and Subchannel Models
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Comparison Between RANS and Subchannel Models

• Comparisons have been carried out between RANS
  and the SAS4A/SASSYS-1 subchannel model.
• Comparisons disabled cross-pin conduction in the
  subchannel model and evaluate cross flow and
  temperature distributions.

217-pin RANS
217-pin Subchannel (peak assembly at steady-state)
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Cross-Pin Conduction

• In addition to subchannel cross flows, cross-pin
  conduction is also important in determining
  subchannel temperature distributions.
• Current capabilities have difficulty meshing the
  full geometry needed to model the conjugate heat
  transfer problem for a 217 pin assembly.
• Cross-pin conduction terms in SAS4A/SASSYS-1 are
  defined by modeling approximations or by
  comparisons with (limited) experimental data.
• Classic example of how higher-fidelity methods
  can provide modeling parameters for
  lower-fidelity models.

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Importance of Cross Pin Conduction During a
Transient

• Cross pin conduction is less important under
  steady-state, high-flow conditions.
• Under low flow conditions, cross-pin conduction
  becomes an important heat transfer mechanism to
  the assembly duct wall.
• Duct wall temperature distributions are important
  in determining assembly bowing and related
  reactivity feedback.

Steady State
ULOF t 120 seconds
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Subchannel Temperature Profile

• Peak power-to-flow assembly represented by 438
  subchannels (coolant, fuel, cladding and
  structure)
• Whole-plant model includes core, primary coolant
  loop, pumps, IHXs, secondary coolant loop, steam
  generators, decay heat removal systems, etc.
• Peak fuel temperatures occur at approximately 15
  seconds into the transient (right figure).
• Much of the fuel is cooler than at steady state.
• Cladding, coolant, and structure temperatures
  have increased.
• Detailed transient temperature distributions are
  critical for determining reactivity feedback.

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RANS Temperature Profile at Pin Bundle Exit

• High-fidelity RANS results show impact of wire
  wrap on assembly temperature distributions.
• Local effects between adjacent subchannels
• Global effects across the whole pin bundle
• These effects are not characterized by the
  subchannel model.

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Comparison Between RANS and Subchannel Results

• Axially-independent cross flow terms used in the
  subchannel model are not able to resolve the
  axial periodicity in the temperature due to the
  wire wraps (see arrows).
• Temperature distribution is symmetric in the
  subchannel results, but skewed in the RANS
  results. (Unanticipated bias)
• Cross flow terms from higher-fidelity modeling
  would result in better agreement between
  subchannel and RANS.

Differences Between Steady-State Subchannel and
RANS Coolant Temperature Distributions in a
217-Pin Fuel Bundle.
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Conclusions from FY08 Work

• Results of the comparison reveal three
  significant observations
• Subchannel model predicts peak (coolant)
  temperatures that are 15 degrees higher than the
  RANS model. May be resolved through better
  selection of cross-flow mixing terms.
• Subchannel model is unable to resolve details of
  the axial temperature dependence, which is
  important for subassembly bowing.
• RANS model is limited in its ability to
  characterize a long-term transient. Whole-core
  and whole-plant transients are presently beyond
  the capabilities of current and foreseeable
  computing architectures.
• These observations emphasize the need for a
  multi-resolution approach.
• Future developments will need to include
• A more capable subchannel model (e.g. one that
  includes a forcing function or distributive
  resistance model).
• Conjugate heat transfer in the RANS model (fuel
  and structure).

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FY09 Scope of Work

• Scope of work package is to accomplish the
  coupling of high fidelity RANS/CFD
  thermal-hydraulics analysis capabilities with an
  existing integral safety analysis computer code.
  The coupling will initially be applied to
  multidimensional simulation of reactor coolant
  flow in ex-core volumes (plenums).
• Increased fidelity for coolant flow simulation in
  ex-core regions will yield improved predictions
  of natural circulation heat removal in shutdown
  and accident transients by being able to better
  resolve multidimensional temperature and flow
  fields.
• Thermal stratification (outlet plenum or cold
  pool)
• Impacts natural circulation driving forces,
  reactor vessel expansion, control-rod driveline
  expansion, IHX performance, pump inlet
  conditions, bypass flow paths, etc.
• Current transient safety capabilities limited to
  coarse, 1-D treatment

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Tasks and Milestones

• Definition of the coupling technique
• Implementation of coupling mechanisms
• Demonstration of the coupled capability with
  prototypic application
• Identified Phenix EOL Natural Convection test for
  demonstration
• Integrates well with the International Passive
  Safety work package.
• Incomplete benchmark specifications affect
  ability to develop realistic models.
• Obtained permission from Toshiba (through CRIEPI)
  to use older 4S plenum design description.
• Milestone Reports
• July 2009 Coupling of High Fidelity and Integral
  Analysis Methods Report
• September 2009 Report on Prototypic Analyses
  Demonstrating Coupled Safety Modeling

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Phenix End of Life Testing

• Natural convection test will provide data on
  primary system natural circulation flow rates
  following a steam generator dryout accident with
  manual scram and pump trip.
• SAS4A/SASSYS-1 will be used to evaluate flow
  conditions as part of the IAEA CRP benchmark.
• Axial thermocouple probes will be inserted in
  both the hot and cold pools prior to the test.
• Provides an opportunity to compare
  higher-fidelity plenum modeling results with
  actual plant data.
• Axial temperature distributions.
• Impact of stratification on natural circulation
  development.

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Toshiba 4S Outlet Plenum Stratification

• Previous work with CRIEPI compared system-wide
  results from PLOF and ULOF accident sequences.
• Plenum results from the 2-D treatment (CERES)
  fall between SAS4A/SASSYS-1 stratified model
  (blue) and a perfect mixing model (red) during a
  PLOF.
• More detailed 3-D treatment may reveal better
  mixing than 2-D treatment provides.

Impact of Stratification on IHX Inlet Temperatures
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Monju Startup Testing

• Shutdown transients showed that inner barrel
  bypass holes influenced thermal stratification.

• Previous international passive safety work
  performed evaluations of this test, but did not
  include a whole-plant (or even core) model.
• Additional core and primary system modeling
  information would be needed.

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EBR-II Cold Pool Stratification

• Thermocouple probes present in the EBR-II cold
  pool during PICT testing showed thermal
  stratification during normal operations.
• Thermal stratification gradient begins to
  increase near the primary pump inlet.
• Behavior of the stratified layer during a
  transient may affect passive safety performance
  by impacting core inlet temperatures.
• Natural circulation flow rates.
• Core radial expansion.

EBR-II Plant Inherent Control Tests
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Safety Modeling in the SHARP Framework

• Long-range goal is to couple SAS4A/SASSYS-1 into
  the SHARP simulation framework through PRIMAR-4
  in order to provide whole-plant capabilities to
  support development of advanced methods.

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Initial Plenum Model Coupling

• Initial coupling between SAS4A/SASSYS-1 and
  Star-CD will be separate from the SHARP
  framework.
• Coupling will eventually leverage ongoing work to
  couple Star-CD with the SHARP framework under the
  VHTR program.

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Summary

• Completion of FY08 work revealed areas for
  improvement in current subchannel and RANS models
  and the role that a multi-resolution approach can
  play in safety modeling.
• Ongoing work in FY09 will demonstrate initial
  coupling with a higher-fidelity plenum modeling
  capability.
• Also ties in with international passive safety
  work package.
• Leverages framework coupling activities in the
  VHTR program.