IDOM NUCLEAR_NEUTRONICS

Neutronics and CFD Analysis MCNPX-ANSYS Fluent coupling

IDOM Nuclear Services has developed a software package that provides automatic coupling between MCNPX and ANSYS Fluent in multi-physics problems, where radiation-matter interaction is involved. This allows both a reduction of analysis time-cost and improved precision.

April 2013

Contents

Introduction 05 Approach 06 Potential Applications 07 Current stage of development and examples

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Next Steps 09 Reference Projects 10 References 14

Introduction During the past few years IDOM Nuclear Services (IDOM-NS) has participated in several projects in which analyzing the interactions between nuclear particles and fluids was necessary.

Back in 2008 IDOM undertook a feasibility study for integrating two particle accelerators in the Liquid Metal Laboratory of TECHNOFUSION in Madrid, in the frame of the international IFMIF/EVEDA project (International Fusion Materials Irradiation Facility / Engineering Validation Engineering Design Activities), aimed at creating the experimental conditions (neutron irradiation) for developing and testing the materials needed for future fusion reactors. This project included a computational fluid dynamics (CFD) simulation of the liquid lithium target, which was done using ANSYS Fluent, and at the same time the study of the impact of the accelerated particles on the surface of the target, assessed using the Monte Carlo nuclear simulation code MCNP5/MCNPX. Similarly, in the feasibility study of a double loop Pb15.7Li / pressurized He dual coolant system for fusion plant technology, an in-depth analysis was made of the different needs and options for tritium breeding blankets, and in different studies done for the Spallation Neutron Source and the European Spallation Source both the aforementioned codes have been used intensively.

Hence the question arose of whether the codes could be coupled so that they can iteratively interact with each other in a semi- or fully automated way: determining the physical quantities generated by radiation sources through a Monte Carlo code and imposing them automatically into a CFD code can shorten the analysis cycle, improve the accuracy of multi-physics calculations and can substantially improve the design of any fission or fusion component.

Therefore in a proof of concept exerciseIDOM-NS thoroughly studied the problem of how each program handles its input and output data in the specific case of calculating the volumetric heat deposition generated by a neutron source on a liquid metal flow. The normal methodology for such problems requires calculation of an approximate heat deposition and imputting it manually into the CFD software.

The promising results obtained led IDOM-NS to the definition of a full-scale R&D project aimed at achieving a more complete coupling of the two programs; this project has been partially supported by a grant of the Spanish Center for Industrial Technological Development (CDTI, Centro para el Desarrollo Tecnológico Industrial) of the Ministry of Economy and Competitiveness and developed in collaboration with academic institutions, such as UNED-Madrid (Universidad Nacional de Educación a Distancia), UPC Barcelona (Universidad Politécnica de Cataluña) and CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas) of the Ministry of Economy and Competitiveness.

Several clients of IDOM-NS have shown interest and support for the results and the potential of the coupling, among others Fusion for Energy, the European Domestic Agency for ITER.

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Approach In our approach, once the volumetric heat deposition is locally estimated by a MCNPX mesh tally, the values are mapped to the ANSYS Fluent mesh and inserted as a source term into ANSYS Fluent’s User Defined Memory (UDM) through the User Defined Function (UDF). The mapping process between the two meshes is performed by a commercial numerical library which has been incorporated into the UDF. The interpolations can be performed by four different algorithms and using different conditions, such as the connectivity of the cell, for which the user has the possibility to choose the best option. Besides, several numerical procedures, such as for instance the volumetric correction factors that take into account the difference between real and standard library mesh elements, have been implemented in order to achieve a smaller interpolation error. Once the best interpolation has been obtained, the UDM can be loaded and the ANSYS Fluent calculation performed. If the CFD temperature analysis shows a significant temperature difference from the MCNPX model, a new Monte Carlo calculation will be done in order to adjust the source terms. An iterative procedure between the codes can easily be done.

Three main checks have been implemented in the software in order to find out and evaluate errors and make future improvements easier.

Moreover the software accepts any type of geometry thanks to the utilization of a conversion module which allows importing components directly from CATIA v5, thus avoiding one of the fundamental difficulties of MCNPX, which is geometry definition.

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Potential applications As previously mentioned, this coupling software can be very useful in the design/optimization of many fusion/fission components where heat deposition is present. Possible applications include components of the fusion experiments ITER, DEMO and IFMIF or spallation sources such as the SNS/ESS, whereas wet storage systems of spent fuel racks, vessel shielding or fuel assembly can be areas of interest in fission.

As an example, the coupling software can be used in those cases where the heat deposition is computed

by MCNPX through the mesh tally # 3, which integrates the contribution of all the particles produced (neutrons, photons, muons...). On the other hand, by using the MCNPX mesh tally #1, we are able to discriminate the contribution only of a single typology of particle, such as for instance photons. In this way, by discretizing over the different sources, it would be possible to optimize the shielding configuration (placement, composition and thickness).

In the specific case of a dry storage system, we envisage that the coupling software could be used to determine the temperature field, the mass and paths of the air flow, or the presence of hot spots. In this way, the product could be further optimized improving materials, thicknesses and other shielding mechanisms.

In this case, previously to the MCNPX simulation, the spent fuel composition should be computed by a depletion code such as ORIGEN-S contained inside the SCALE package

IDOM-NS model of a dry-storage container

In the case of pools containing spent fuel racks the same procedure can be used to compute heat transfer from spent fuel rods to the water of the pool and optimize the cooling of the pool or the design of the fuel rack.

IDOM-NS has the capability of fully executing such a process or on the other hand can offer support or collaboration to others in specific tasks.

Furthermore, in the near future, we are going to extend the use of the software to other parameters of MCNPX, such as flux and dose.

Moreover, the interpolation library can be easily connected to other engineering software, such as Abacus, ANSYS Mechanical, STAR-CCM+, broadening the spectrum of potential applications.

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Current stage of development and examples The software is currently in an advanced prototype stage.

It has been tested against several verification and validation cases, such as for instance the ITER Triangular Support, which allowed the validation of the algorithms; a series of internal checks evaluates the results/ error in each step, with the mapping and computational errors limited to fractions of a percent. Although the software is capable of dealing with all the MCNPX mesh tally typologies (i.e., rectangular, cylindrical and spherical), the interface boundaries and the geometric differences between the two models are currently the main error sources. However, thanks to a deep sensitivity analysis, which has covered the mapping algorithms, the mesh sensitivity, the typology of element and its connectivity, the whole power deposition error in the process is less than 0.5%.

The software has been tested against a verification case proposed by UNED consisting of a cylindrical tube of Eurofer filled with water. The tube is targeted by a neutron flux, which deposits heat energy within the tube. This neutron interaction is computed by MCNP. Heat deposition is transferred to ANSYS Fluent by the mapping software. In Figure 1 the result of the MCNPX simulation is presented together with the mapping to ANSYS Fluent using a coarse and a fine mesh.

Figure 1: result of the MCNPX simulation (left) and mapping to ANSYS Fluent (right)

The software has been developed to tackle the needs of various projects, some of which are presented in the “Reference projects� section.

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Next Steps The next steps in the development of the software require a refactoring of the code according to IDOM-NS nuclear quality standards (based, among others, on Spanish and international standards such as ASME NQA-1-2008 and IEEE 730-2002). New features have been also identified that can increase the number of potential applications for the software.

IDOM-NS engineers are available to test the software with verification/validation cases provided by external entities, show more in detail the features of the tool and/or to benchmark it against other tools or processes.

New features have been identified that need to be implemented to extend the capabilities of the software, such as:

>> Spherical Mesh tally/Wedge element: so far only the tetrahedral element has been used in the mapping. We shall test the wedge element in order to check for further improvements. To use this feature, some modifications in the connectivity and the nodes numeration are needed. >> Numerical sub-division of the mesh tally: thanks to the application of a scale factor in each dimension, we want to create more MT sub-cells, which may allow the optimization of the interpolation. Our intention is to have the two meshes as similar as possible to reduce the error. >> UDF Local Error Evaluation: implementing a procedure inside the UDF which allow us to measure the local error of the whole process, using initially the centroid of the ANSYS Fluent cell and then a cloud of user-defined points. >> Implementation of the C++ program inside the UDF and optimization of the UDF: once all the previous MCNPX post processing operations have been implemented inside a C++ program, we have to connect it to the UDF modifying and optimizing of some of its parts. >> CAD part of the process: during the whole development of the coupling project, we have been mostly concentrating on the interpolation and the ANSYS Fluent UDF. However, the CAD conversion part has to be analyzed in more depth. >> Installation Manual and User Documentation: a brief installation manual has been already created, but it needs to be improved and reviewed.

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Project Description

Analysis of the Triangular Support of the ITER Vacuum Vessel (Fusion for Energy, ongoing) The Vacuum Vessel (VV) of the ITER tokamak is located inside the magnet system and inside the cryostat and houses the In-Vessel components. It is a double wall structure that surrounds and provides a first shielding for the plasma, maintaining at the same time the necessary vacuum conditions and extracting a heat deposition close to 650MW, with several Ports for Diagnostics, Heating and Remote Handling access.

The triangular support is an element of the VV and has the functions of extracting heat, stabilizing the plasma and contributing to the structural integrity of the VV. It is composed of a steel frame and a copper plate on the side facing the plasma and allows circulation of the cooling water. The nuclear heat load produced over the in-vessel components will be very elevated especially in the blanket module and in the triangular support due to the proximity of the plasma. The accurate design of these components requires an interaction between CFD and neutronic software.

In collaboration with F4E, IDOM-NS is validating the MCNPX/ANSYS Fluent automatic coupling software over the triangular support. The presence of three different materials, the 15 000 000 ANSYS Fluent meshing cells and the consequent high computational cost, and the difference between the neutronic and the CFD model are the main challenges encountered.

The active collaboration with the F4E neutronic team has shown the possibility of improvements in some of the coupling software features, such as the determination of a punctual error between the MCNPX map and ANSYS Fluent value or the development of a new mapping algorithm. Further ongoing activities aim at testing other parameters options in order to decrease punctual and integral errors.

Figure 3: 3D model of the triangular support

Figure 4 results of the MCNPX simulation of the ITER triangular support for three materials (steel, water, copper) and for the entire piece (from top to bottom and left to right)

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Project

Analysis of the Irregular Sector of the ITER Vacuum Vessel (Fusion for Energy, ongoing)

Description

The vacuum vessel (VV) of ITER’s tokamak is composed of nine sectors, of which some are referred to as “irregular” due to the modifications introduced to house the Neutral Beam Injectors (NBI). Some of the nuclear heat data are available as an MCNPX mesh tally, which requires a data format conversion to introduce the data into the CFD model.

For this reason, the MCNPX/ANSYS Fluent automatic coupling system will be used for the currently ongoing VV thermal hydraulic analysis, aimed at increasing the accuracy of the data introduced, a more accurate determination of related parameters such as mass flow rate, pressure drop and hot-spots, while at the same time decreasing the analysis effort.

Moreover, the know-how acquired during the Triangular Support validation case will be very useful for scaling up the use of the software to a whole system such as the VV PHTS (vacuum vessel primary heat transfer system).

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Project Description

Analysis of heat removal from a HiPER section (UNED, ongoing) HiPER is a project based on the “direct drive� ignition concept for fusion reactor technology: 48 laser beams simultaneously hit a deuterium/tritium target inside a spherical chamber, producing a controlled fusion reaction. The energy generated by the reaction is transferred to the liquid lead/lithium coolant flowing through the 16 Eurofer elements of the chamber, as shown in the figure.

In this project the need for coupling MCNP and CFD calculations was solved by imposing the source terms from a text input file through a UDF and using a simplified 2D model of HiPER, demonstrating the feasibility of the coupling on the ANSYS Fluent side.

IDOM-NS is using this study as a reference case for the new developments of the MCNPX/ANSYS Fluent coupling project in collaboration with UNED. At the moment we are analyzing the possibility of using a 3D model and a MNCPX mesh tally results in collaboration with the UNED neutronic team, applying the whole methodology which has been developed, ie: MCNPX mesh tally calculation, mapping process, introduction of data inside UDM from UDF, coupling calculations.

Figure 2: a section of the HiPER chamber

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Project Description

Liquid breeder module of IFMIF (CIEMAT, ongoing) IFMIF, the International Fusion Materials Irradiation Facility, is an international scientific research program designed to test candidate materials for suitability for use in a fusion reactor. IFMIF will use a particle accelerator-based neutron source to produce a large neutron flux, in a suitable quantity and time period to test the long-term behavior of materials under conditions similar to those expected at the inner wall of a fusion reactor.

The LBVM, Liquid Breeder Validation Module, is included in the design of IFMIF to try to fill a gap in relation to the needs of functional materials for concept blanket liquids and particularly of PbLi. The PbLi is targeted by a deuterium beam, which deposits heat energy on the liquid metal. This heat energy is dissipated by a purge gas (helium) flowing at low velocity within the rig. More energy is dissipated by the faster flowing gas within the container.

In this project we identified the need to have a more accurate computation using MCNPX of the interaction of the deuterium with the lead-lithium, since the CFD simulation was first performed by imposing temperature of the liquid metal as the boundary condition.

IDOM-NS is using this study as a reference case for the development of the MCNPX/ANSYS Fluent coupling project in collaboration with CIEMAT.

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References The approach and preliminary results have been presented at the 38th meeting of the Spanish Nuclear Society and a poster presentation is planned at the upcoming ISFNT in September 2013.

The abstracts are presented below:

>>> Synopsis of the communication at the 38th meeting of the Spanish Nuclear Society (Caceres, October 2012) Clara Colomer, Raheel Ahmed, Agustín Alemán, Marco Fabbri, Jordi Salellas Advances in the development of the interaction between the MCNPX and ANSYS Fluent codes and its applications for fusion (original in Spanish).

ITER, IFMIF, DEMO and spallation sources such as the Spallation Neutron Source (SNS) or the future European Spallation Source (ESS) incorporate into their design key components where a nuclear thermal deposition occurs upon fluids, both conventional coolants, like water, and liquid metals, such as the eutectic Pb15.7Li in the case of ITER, pure lithium in case of IFMIF or mercury in spallation sources.

IDOM is developing an interaction code between MCNPX for nuclear calculations and ANSYS Fluent for fluid dynamic and thermo-hydraulic calculations. Correct coupling of both codes, with the corresponding validation and verification (not yet developed), will significantly reduce multiphysics analysis and calculation cycles.

Advances will be presented today in the project funded by the CDTI for the development of a code of MCNPX and the ANSYS Fluent interaction. Following the workflow carried out during the development of the project, the study of the most appropriate re-meshing algorithms between both codes will be exposed.

The selection and implementation of methods to verify the correct transmission of the variables involved between both screens will also be explained. The selection of cases for verification and validation of the interaction between both codes in each of the possible fields of application will be exposed.

>>> Abstract of the communication at the 11th International Symposium on Fusion Nuclear Technology (Barcelona, September 2013) - accepted Fabbri Marco, Colomer Clara, Alemán Agustín, Raheel Ahmed, Sallelas Jordi MCNPX/ANSYS Fluent automatic coupling software Determining the volumetric power deposition generated by radiation sources through a Monte Carlo code and imposing it into a CFD code, with high accuracy, is a great challenge but can substantially improve the design of any fission or fusion component. The IDOM team has successfully coupled MCNPX and ANSYS Fluent, which are two reference codes for ITER, building an automatic interface software written mostly in C/C++.

Basically, once the volumetric power heat deposition is estimated by a MCNPX mesh tally, the values are mapped with the ANSYS Fluent mesh and inserted as source term in the User Defined Memory through the User Defined Function or UDF. The mapping process is performed by the MapLib numerical library of the Fraunhofer-Institut which is incorporated into the UDF. If the CFD temperature analysis shows a significant temperature difference from the MCNPX model, a new Monte Carlo calculation will be done varying the

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cross sections, the temperature and the density of the materials in order to adjust the source terms. A simple iterative procedure between the codes can be carried out.

The analysis of several verification and validation cases, such as for instance the ITER Triangular Support, has allowed the validation of the procedures and a series of internal checks to evaluate the results/error in each step.

The interface boundaries and the geometric differences between the two models are the main error sources despite the capacity of the software to deal with all the MCNPX mesh tally typologies (i.e., rectangular, cylindrical and spherical). However, thanks to a deep sensitivity analysis, which has covered the mapping algorithms, the mesh sensitivity, the typology of element and its connectivity, the whole power deposition error in the process is less than 0.5%.

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