Advanced Safety Modeling

Advanced Safety Modeling

Thomas H. Fanning

Engineering Simulation and Safety AnalysisNuclear Engineering DivisionArgonne National Laboratory

AFCI NEAMS MeetingMay 19, 2009

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)

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).

217-pin Subchannel(ABTR peak assembly at steady-state)

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

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.

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.

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.

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.

CFD Plenum Model

iMesh

MOAB

High-Fidelity Decay Heat

Removal System Model

Stand-Alone Driver

Mesh Services

Parallel I/OMesh PartitioningMesh Refinement

Etc…

SAS4A/SASSYS-1

ROOT FPIN2TSCL0 CNTLSYSTSPK BOPPRIMAR-4 CLAPDEFORM-4 PLUTO2DEFORM-5 PINACLESSCOMP LEVITATE

Stand-Alone Driver

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.

Safety Modeling in the SHARP Framework

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

CFD Plenum Model

iMesh

MOAB

Coupled Advanced Safety Modeling Driver

ROOTTSCL0Etc…

PRIMAR-4

Mesh Services


Etc…

SAS4A/SASSYS-1




T.H. Fanning and T. J. Tautges, “Specification of Advanced Safety Modeling Requirements,” ANL-AFCI-229, April 2008.

Role of Multi-Resolution Capability in Safety Modeling

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

Multi-Resolution Thermal Hydraulic Simulation Hierarchy

BoundaryConditions

ModelingParameters

Reynolds Averaged Navier Stokes

BoundaryConditions

ModelingParameters

Large Eddy Simulation

BoundaryConditions

ModelingParameters

Direct Numerical Simulation

Incr

easi

ng

Res

olu

tion

Increasin

g Dom

ain S

ize

12

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.

13

SAS4A/SASSYS-1 Subchannel Temperature Results for SHRT-17

QuickTime™ and aH.264 decompressor

are needed to see this picture.

14

Comparison Between RANS and Subchannel Models

15

217-pin RANS217-pin Subchannel

(peak assembly at steady-state)

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.

16

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.

17 Steady State ULOF: t = 120 seconds

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.

18

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.

19

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.

20

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

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.

21

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).

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

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

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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Åú Pump inlet

Permeability 5%

Permeability 1%

Permeability 0%

Permeability 11.8%

Å° IHX inlet

IHX outlet

Sheildings

Core

Åõ Pump outlet

Impact of Stratification on IHX Inlet Temperatures

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.

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.

EBR-II Plant Inherent Control Tests

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.

CFD Plenum Model

iMesh

MOAB

Coupled Advanced Safety Modeling Driver

ROOTTSCL0Etc…

PRIMAR-4

Mesh Services


Etc…

SAS4A/SASSYS-1




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.

VHTRTH

Model

iMesh

MOAB

VHTR Neutronic

Model

Star-CD

Mesh Services


Etc…

SAS4A/SASSYS-1


DeCART

CFD Plenum Model

Star-CD

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.

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.

Documents

Advanced Safety Modeling