Integrating STAR-CCM+ with a Systems Analysis Code for ...mdx2.plm.automation.siemens.com/sites/default/... · assemblies are grouped in to form a core. • RGG takes advantage of

Integrating STAR-CCM+ with a Systems Analysis Code for Nuclear Reactor Safety Simulations

Justin W. Thomas Nuclear Engineering Division Argonne National Laboratory STAR-American Conference Chicago, Illinois June 28, 2011

Why Sodium-cooled Fast Reactors (SFRs)?

§  All nuclear power plants currently operating the U.S. use water as their coolant –  But the first reactor to generate electricity was a fast reactor

§  Fast reactors get their name because, on average, neutrons are moving faster than in water reactors –  Changes the likelihood of the occurrence of various nuclear reactions

§  Fast reactors can be designed for: –  Actinide burning: Continue to produce energy from “used” nuclear fuel

from water reactors –  Breeding: Produce more fissile fuel than what consumed in the core

§  As a part of a strategy to recycle used nuclear fuel from water reactors, SFRs help to:

–  Extract more energy from uranium –  Reduce reliance on uranium enrichment –  Reduce the amount of used fuel

STAR-‐American Conference, Chicago Illinois, June 28, 2011

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Passive safety of SFRs

§  Nuclear power plant operators must convince regulators that their reactors will remain safe, even under accident scenarios and off-normal events

§  Even after the nuclear fission reactions have stopped in the reactor, a small amount of heat is still heat being generated – decay heat – which needs to be removed for an extended period of time

§  If the coolant pumps fail, SFRs can rely on natural circulation to drive coolant through the reactor’s core and remove heat

§  The potential for SFRs to survive severe accident initiators with no damage was demonstrated in a series of tests at the Experimental Breeder Reactor-II facility in the 1980s

–  Complete loss-of-flow and loss-of-heat-sink tests were performed –  Experimental results from this program will be used to validate the work

described here


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Modeling transients in SFRs

§  Argonne’s safety systems code SAS4A/SASSYS-1 models the dynamic response of a reactor during a posited transient scenario

§  Physics include: –  The core’s response to changes in its environment –  Structural mechanics –  Fuel performance –  Decay heat generation –  Fluid mechanics and heat transfer

•  Natural circulation, buoyancy-driven flow, thermal stratification

§  By including STAR-CCM+ in SAS4A/SASSYS-1 transient analyses, the goal is to improve the fluid mechanics/heat transfer solution while still maintaining the sophistication of the other models available in SAS4A/SASSYS-1

–  Specific cases where 3-D effects are important –  E.g., thermal stratification in large plena


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Example: Loss-of-flow in Toshiba’s 4S reactor

§  Argonne supported safety analysis for a small SFR concept developed by Toshiba

§  In a hypothetical loss-of-flow scenario: 1.  The pumps stop, reducing the flow rate

through the core 2.  The reactor scrams, stopping the nuclear

fission reactions but decay heat remains §  Because of #2, the sodium entering the

outlet plenum is now cooler than the bulk sodium in the plenum

§  Because of #1, the time for the cooler sodium to reach the Intermediate Heat Exchanger (IHX) can be significant à  Thermal stratification

§  This effects the natural circulation head in the system

4S Schematic (Not to scale)


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Example: Loss-of-flow in Toshiba’s 4S reactor

§  A model of the 4S outlet plenum was built with STAR-CCM+

–  2-D axisymetric for demonstration purposes §  Remainder of reactor system modeled with

the system code SAS4A/SASSYS-1 §  STAR-CCM+ and SAS4A/SASSYS-1

communicate at the flow boundaries §  For each core channel, SAS4A/SASSYS-1

sends the outlet temperature and mass flow rate

–  Temperature and fluid velocity distributed uniformly along STAR-CCM+ boundary

§  At the IHX inlet, STAR-CCM+ provides the average pressure and temperature

4S Schematic (Not to Scale)


Temperature predictions in the outlet plenum


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§  SAS4A/SASSYS-1 predicts the low flow rates and cooler temperatures from the core when the transient starts

§  Cool sodium slowly progresses upward through the plenum towards the heat exchanger

§  Important to predict the time delay required for cooler sodium to reach the heat exchanger

§  Note: These results are preliminary and should not be considered to represent the actual performance of the 4S reactor

From Core

To Heat Exchanger

Reactor system response

§  The natural circulation driving head depends on the temperature difference between the heat exchanger (heat sink) and the core (heat source)

§  Some interesting phenomena were predicted during the coupled simulations of SAS4A/SASSYS-1 and STAR-CCM+


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Secondary-side becomes a heat source rather than a heat sink due to flow stagnation

Long delay before IHX senses cooler core temperatures

Tem

pera

ture

Implementation of STAR-CCM+ coupling

§  Coupling with the SAS4A/SASSYS-1 code is implemented through the STAR-CCM+ client via a Java macro –  Portions of Fortran routines developed for STAR-CD coupling preserved –  Java calls the Fortran functions via Java Native Access (JNA)

§  Communication between SAS4A/SASSYS-1 and STAR-CCM+ via file I/O

§  Synchronize each SAS4A/SASSYS-1 time step –  SAS4A/SASSYS-1 determines its time step size using its normal

approach •  Monitors temperature changes and other conditions, user-input tolerances

–  STAR-CCM+ time step is the smaller of: •  ½ the SAS4A/SASSYS-1 time step •  The user-input value in the simulation

–  Linear interpolation performed as needed


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Implementation of STAR-CCM+ coupling (cont)

§  At the end of its time step, SAS4A/SASSYS-1 prints for each inlet flow boundary

–  Mass flow rate –  Temperature

§  STAR-CCM+ assumes a uniform velocity and temperature profile at each flow boundary, computed from the SAS4A/SASSYS-1 data

§  Just before the next SAS4A/SASSYS-1 time step, STAR-CCM+ prints for all flow boundaries

–  Area-averaged absolute pressure –  Mass-flow averaged temperature

§  STAR-CCM+ annotates a plot with the current time (from SAS4A/SASSYS-1) and prints for the animation

§  Heat transfer at boundaries to be implemented soon –  Exchange heat flux or temperature at wall boundaries


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Reports

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Future Work: EBR-II Analysis

§  But are these predictions accurate? §  Measured data from the EBR-II tests provides a validation exercise of

whole-plant response to a loss-of-flow scenario –  Cold pool tank modeled with STAR-CCM+ –  Remainder of the cooling system modeled with SAS4A/SASSYS-1

§  Seven flow boundaries that connect to SAS4A/SASSYS-1


EBR-II Initialization


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RGG: Reactor Geometry and Mesh Generator

•  A set of tools to generate reactor assembly, core geometry and mesh models.

•  Fuels and other rods are grouped in to form assemblies and lattice of assemblies are grouped in to form a core.

•  RGG takes advantage of information about repeated structures in both assembly and core lattices.

•  Provides a balance between lattice-guided automation and opportunities for user interaction at key points of the process.

•  Supports rectangular and hexagonal lattices. •  Operates in 3 stages:

1. AssyGen 2. Meshing 3. CoreGen

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•  In this step assembly model and mesh script are created.

•  Keyword based input file is used to define assembly geometry.

•  AssyGen supports rectangular and hexagonal assembly types.

•  AssyGen created mesh script, MeshKit algorithms or user defined mesh script can be used to for meshing the assembly geometry.

•  Side skin surface of all the assemblies forming the core must have matching nodes.

•  CoreGen copy-move-merges assemblies to form the core.

•  Metadata propagation from individual assembly meshes to the core

•  Core geometry/mesh can be exported into several file formats

•  Several symmetry options available

STAGE 1: ASSYGEN STAGE 2: MESHING STAGE 3: COREGEN

MONJU reactor, full core model: 9.7M hexes, 99k vols takes 4.3GB RAM and 176 mins. 715 assemblies.

RGG: Reactor Geometry and Mesh Generator

Thank you


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Integrating STAR-CCM+ with a Systems Analysis Code for ...mdx2.plm.automation.siemens.com/sites/default/... · assemblies are grouped in to form a core. • RGG takes advantage of