Advanced Safety Modeling Thomas H. Fanning Engineering Simulation and Safety Analysis Nuclear Engineering Division Argonne National Laboratory AFCI NEAMS Meeting May 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).

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)

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.

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. CFD Plenum Model iMesh MOAB High-Fidelity Decay Heat Removal System Model Stand-Alone Driver Mesh Services Parallel I/O Mesh Partitioning Mesh Refinement Etc… SAS4A/SASSYS-1 ROOT FPIN2 TSCL0 CNTLSYS TSPK BOP PRIMAR-4 CLAP DEFORM-4 PLUTO2 DEFORM-5 PINACLE SSCOMP LEVITATE

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 ROOT TSCL0 Etc… PRIMAR-4 Mesh Services Parallel I/O Mesh Partitioning Mesh Refinement SAS4A/SASSYS-1 ROOT FPIN2 TSCL0 CNTLSYS TSPK BOP PRIMAR-4 CLAP DEFORM-4 PLUTO2 DEFORM-5 PINACLE SSCOMP LEVITATE High-Fidelity Decay Heat Removal System Model 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 Multi-Resolution Thermal Hydraulic Simulation Hierarchy Subchannel Models Boundary Conditions Modeling Parameters Reynolds Averaged Navier Stokes Increasing Resolution Boundary Conditions Modeling Parameters Increasing Domain Size Large Eddy Simulation The thermofluids component is an integrated multi-scale simulation capability which includes (read bullets on slide). The Argonne code Nek5000 is the primary development platform. The commercial CFD code Star-CD is being used to benchmark results of Nek5000 LES and DNS simulations and evaluate the range of applicability of RANS-based methods. LES is large eddy simulation. DNS is direct numerical simulation, and RANS is Reynolds Averaged Navier Stokes. Boundary Conditions Modeling Parameters Direct Numerical Simulation

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. 12 12

SAS4A/SASSYS-1 Subchannel Temperature Results for SHRT-17 13 13

Comparison Between RANS and Subchannel Models

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) 15 15

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.

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

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. 18

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.

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.

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. 24

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

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 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 27

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. CFD Plenum Model iMesh MOAB Coupled Advanced Safety Modeling Driver ROOT TSCL0 Etc… PRIMAR-4 Mesh Services Parallel I/O Mesh Partitioning Mesh Refinement SAS4A/SASSYS-1 ROOT FPIN2 TSCL0 CNTLSYS TSPK BOP PRIMAR-4 CLAP DEFORM-4 PLUTO2 DEFORM-5 PINACLE SSCOMP LEVITATE High-Fidelity Decay Heat Removal System Model

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. VHTR TH Model iMesh MOAB VHTR Neutronic Model Star-CD Mesh Services Parallel I/O Mesh Partitioning Mesh Refinement Etc… SAS4A/SASSYS-1 ROOT FPIN2 TSCL0 CNTLSYS TSPK BOP PRIMAR-4 CLAP DEFORM-4 PLUTO2 DEFORM-5 PINACLE SSCOMP LEVITATE DeCART CFD Plenum Model

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.