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Use of MCAM in creating 3D neutronics model for ITER building
Fusion Engineering and Design 87 (2012) 1273–1276
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Use of MCAM in creating 3D neutronics model for ITER building Qin Zeng a,b , Guozhong Wang b,∗ , Tongqiang Dang b , Pengcheng Long a,b , Michael Loughlin c a
Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, Hefei, Anhui 230031, China School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, Anhui 230027, China c ITER Organization, Route de Vinon sur Verdon, 13115 St. Paul-Lz-Durance, France b
a r t i c l e
i n f o
Article history: Available online 27 March 2012 Keywords: Monte Carlo Neutronics modeling International Thermonuclear Experimental Reactor Building Monte Carlo Automatic Modeling
a b s t r a c t The three dimensional (3D) neutronics reference model of International Thermonuclear Experimental Reactor (ITER) only defines the tokamak machine and extends to the bio-shield. In order to meet further 3D neutronics analysis needs, it is necessary to create a 3D reference model of the ITER building. Monte Carlo Automatic Modeling Program for Radiation Transport Simulation (MCAM) was developed as a computer aided design (CAD) based bi-directional interface program between general CAD systems and Monte Carlo radiation transport simulation codes. With the help of MCAM version 4.8, the 3D neutronics model of ITER building was created based on the engineering CAD model. The calculation of the neutron flux map in ITER building during operation showed the correctness and usability of the model. This model is the first detailed ITER building 3D neutronics model and it will be made available to all international organization collaborators as a reference model. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In order to conduct consistent neutronics analyses of International Thermonuclear Experimental Reactor (ITER), the international organization (IO) started the neutronics strategy to create series of neutronics reference models [1]. ITER IO distributed the detailed 3D neutronics reference calculation model of the ITER tokamak machine, named A-lite, in April, 2009. However, the model only extends to the bio-shield. In order to meet future neutronics analysis needs, for example during operation the accessibility and the security of the sensitive electronic equipment inside the building, it is necessary to create a reference model of the ITER building outside the bio-shield. In the frame of the ITER China service contract (ITER/CT/09/4100001055), the 3D neutronics model of ITER building was created based on the engineering computer aided design (CAD) model. The engineering CAD model of ITER building was created employing CATIA V5 software by ITER IO. This model describes very detailed geometry of the buildings from the bio-shield to the outside wall, including the Tokamak Building, the Diagnostic Building and the Tritium Building. The size is 118 m in length, 82 m in width and 74 m in height. It would be time-consuming to manually create the corresponding Monte Carlo (MC) simulation model. The CAD-based modeling approach has been developed by the participants of the ITER project to produce the models
for nuclear analysis for ITER [2,3]. Monte Carlo Automatic Modeling Program for Radiation Transport Simulation (MCAM) [4–6] was developed by FDS Team, China, as a bi-directional interface program between general commercial CAD systems and MC radiation transport codes (e.g. MCNP [7], TRIPOLI [8]). On one hand, CAD models can be converted into the input geometry suitable for the MC codes conveniently and rapidly. On the other hand, the existing MC models can be converted into CAD model and visualized for verification and further update. MCAM also supports some supplementary functions such as creation and repair of CAD models and analysis of physics properties. MCAM has been widely used in the neutronics modeling of many nuclear devices [9–17]. The neutronics simulation model of ITER building for MCNP simulation was created using MCAM4.8 version based on the engineering CAD model. This model is the first detailed ITER building 3D neutronics model and it will be made available to all IO collaborators as a reference model. To illustrate applications of this neutronics model, the neutron flux map inside the ITER building during operation was calculated. This calculation showed the correctness and usability of the model.
2. Creating neutronics model for ITER building 2.1. MCAM improvements and new capabilities
∗ Corresponding author. Tel.: +86 551 5593631. E-mail address: [email protected] (G. Wang). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.02.119
Compared with the previous version 4.7, MCAM4.8 offers many enhancements and new capabilities.
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Fig. 1. The ITER building CAD models in CATIA V5 (left) and in MCAM4.8 (right).
The computer memory consumption was greatly reduced in both the conversion process from CAD models to MCNP models and the reverse conversion (also called inversion) process by more than 60%. For example, only 600 megabyte (MB) memory is needed for the inversion of the whole ITER Alite4 MCNP model, while it would consume more than 2 gigabyte (GB) memory for MCAM4.7 as the limit of a 32 bit platform program. And so was the conversion of the whole ITER CAD model generated from the Alite4 MCNP model, about 800 MB. It is able to convert much larger models without splitting them into separate ones benefited from this improvement. A more accurate method of the General Quadric (GQ) surfaces was developed. In the inversion process from MCNP geometry to CAD geometry, it is needed to identify the surface type (cylinder, cone or ellipsoid) of a GQ surface description and resolve the mathematic parameters (base center, radius, axis direction, etc.). The eigen value and eigenvector related algorithm were adopted and the solution was thereby more accurate. The automatic filling of the internal void space of pipe-like CAD solid geometry was developed. Traditionally, the completion of the CAD model with void cells was implemented by boolean subtracting the solids from a series of regular boxes. The CAD feature recognition technology was taken here. The algorithm firstly traversed the geometry and topology of a pipe-like CAD solid, extracted the features (holes, etc.) and saved the boundary information (vertices, edges, loops and faces) of the internal space. Then the algorithm constructed the internal space solid geometry based on the saved boundary information. Compared with the boolean subtraction method, this method is more rapid and has a higher success rate. Some other new features of MCAM4.8 are as follows: (1) Offers Undo and Redo operation for CAD geometry creation and modification. (2) Supports more syntax of MCNP input. For example, the macrobody description in repeated structure geometry, the TRCL card in surface description. (3) 2D section view function was improved from gray to colorful to be more intuitive. (4) Supports electron importance. (5) Offers a slice through CAD solid geometry with cylindrical and spherical surface.
truss structure. The crane was designed to be placed in the space between the roof and the tokamak. The Diagnostic Building and the Tritium Building are located at the two sides of the Tokamak Building and both consist of eight levels. Based on the CATIA format CAD model, the neutronics model for MCNP simulation was created by MCAM. Firstly, the model was imported into MCAM and preprocessed. The purpose of preprocess is to prepare an available CAD model from the engineering CAD model before converting it into a MCNP input file. The preprocessing of the ITER building model includes format exchange, irrelevant details removal and simplification and overlap removal. Then the physics properties were edited graphically. Finally, the model was converted into the text format MCNP input file with automatic generation of void space description. 2.2.1. Format exchange Since the engineering CAD model was created by CATIA software, it was firstly exported as the neutral file format STEP (Standard Exchange of Product data model), and then imported into MCAM. Due to limitations of data transfer through neutral file format STEP, the model imported may be imprecise and have problems such as gaps between entities, and the absence of topology connectivity information. The heal function of MCAM was used to detect and fix the errors that existed in the imported model. Additionally, since the CATIA CAD system uses millimeters as the unit of dimension while MCNP models use centimeter instead, the imported model was scaled with a ratio of 0.1. The model was then saved as SAT format for reuse. Fig. 1 is a south-west corner view in CATIA and in MCAM4.8. 2.2.2. Removal and simplification of irrelevant details The engineering CAD model was designed for the machining and manufacturing with abundant irrelevant small details on the doors and the ladders. The removal of these details had little influence on the accuracy of neutronics analysis and would make the creation of neutronics model and neutronics analysis much faster. The steel structure of the roof model contains large numbers of staves and overlaps. The steel structure was simplified to one solid body which will make a little influence on the simulation results. The simplified roof model was kept the same mass and occupied volume as original model with mixture material of void and steel. The mixture material density of simplified roof model is 0.02894 g/cm3 .
2.2. Creating neutronics model The ITER building from the bio-shield to the outside wall includes the Tokamak Building, the Diagnostic Building and the Tritium Building. The Tokamak Building consists of 9 levels and the tokamak machine was designed to be installed at its center. The top part of the Tokamak Building is a steel roof of staggered
2.2.3. Check and removal of overlaps There were many overlaps between neighboring entities in the engineering CAD model. These overlaps may not influence the machining and manufacturing but overlaps are not be accepted by MCNP calculations. All the overlaps were detected and removed in MCAM. All the overlaps had very small volumes compared to
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the volume and the mass density (Eqs. (1) and (2)). The volumes were calculated by MCAM on the basis of analytical calculations.
Fig. 2. The fifth floor of the Tokamak Building.
Fig. 3. Three columns overlapped with the external wall. The overlapped part of external wall was removed.
the whole model so that the removal of overlaps would have little influence to the simulation results. For example, in the fifth floor (see Fig. 2) of the Tokamak Building, three columns overlapped with the external wall (see Fig. 3). The overlapped part of external wall was removed. 2.2.4. Physics modeling After preprocessing, the CAD model should be assigned with physics properties. Firstly, the components were grouped by material numbers. Then the neutron importance and photon importance, material density and other physical properties were assigned onto the cells. Finally, the material definition and source, tally specification were edited graphically. The components of doors were assigned SS304 stainless steel with the density 8.03 g/cm3 . The roof was also assigned the mixture material of SS304 and void with the density 0.02894 g/cm3 . The rest of this building was assigned the concrete with density 2.2 g/cm3 . The nuclide composition information of concrete and SS304 were directly obtained from the ITER A-lite4 neutronics reference model. 2.2.5. Conversion After previous procedure, the model was converted into text format MCNP input file. The geometry (cell card surface card) description in the file was produced from the CAD geometry. The void space geometry description was also automatic produced during the conversion process. The material, source, tally and other physical information were appended after the geometry description. With these procedures, the full-formed MCNP input file was produced. 2.3. Model verification The mass comparison and MCNP simulation were performed to verify the MCNP input file. 2.3.1. Mass comparison The mass of the original CAD model and the modified neutronics model was compared. The mass was calculated as the product of
m1 = concrete V11 + steel V12
(1)
m2 = concrete V21 + steel V22 + roof V23
(2)
where m1 is the mass of the original CAD model, m2 is the mass of the modified CAD model, V11 is the volume of the original whole building except the roof and the doors, V12 is the volume of the original roof and the doors, V21 is the volume of the modified whole building except the roof and the doors, V22 is the volume of the modified doors, V23 is the volume of the modified roof, concrete is the mass density of concrete (2.2 g/cm3 ), steel is the mass density of SS304 (8.03 g/cm3 ), roof is the mass density of the simplified roof (0.02894 g/cm3 ). The mass of the original CAD model m1 is 2.72E+11 g and the mass of modified CAD model m2 is 2.71E+11 g. The difference is 0.4%. This result shows that the modification was reasonable. 2.3.2. Geometry check MCNP was used to check the geometry correctness of the MCNP input file. The model was set as vacuum (no materials) with “VOID” card so that many particles could be run in a short time and more particles could be simulated to increase the chance of particles going to most part of the geometry. A point source was set at the center. After 1e8 particle histories, the result showed that no particles lost. Based on this result, the probability of error occurred in per unit volume (no particle passed through this unit volume) was less than 1E−4. This value was evaluated as following: Concerning a point source, the probability of one particle passing through per unit volume P1 can be rudely estimated as: P1 =
1 4r 3
(3)
where r is the farthest distance from any one point in the model to the point source. Then the probability of one particle not passing through per unit volume P2 is: P2 = 1 − P1 = 1 −
1 4r 3
(4)
Therefore, the probability of none of the particles passing through per unit volume P3 is: P3 =
1−
1 4r 3
˛
(5)
where ˛ is the number of the simulated particles. Then the geometry error in per unit volume can be estimated as: = P3 =
1−
1 4r 3
˛
(6)
3. Neutron flux map in the building During the D-T operation of the ITER, neutrons stream surrounding the machine will make radiation outside the bio-shield. Assessment of radiation distribution inside the building during operation is required to identify the hot locations where human accessibility is prohibited. The neutron flux map during operation was generated everywhere in the building. Due to the large size of the tokamak and the building, it was necessary to break down the problem into two models where neutrons transport is carried out in the tokamak model to generate the required external source needed as input to the building model. The calculation was performed with MCNP and the fusion evaluated nuclear data library FENDL [18]. The calculation procedures include two steps:
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Fig. 4. The 3D geometry model and neutron flux map (n/cm2 /s) of the Lower Level (B1) of the Tokamak Building.
(1) Firstly, the internal boundary source (IBS) at the outer surface of the cryostat was calculated and obtained from 40◦ toroidal sector Alite4 model. The D-T fusion plasma source included in the distributed Alite4 model was selected as neutron source for neutrons transport calculation in the tokamak machine. (2) Then the IBS was used in the subsequent calculation for the building model. As an example, the neutron flux map in the Lower Level (B1) of the Tokamak Building is showed in Fig. 4. The neutron flux map inside the zone from the inner surface of the cryostat to the center of the tokamak was not given here. These neutron flux maps were visualized by SVIP [19] software, which is a visualization tool for neutronics analysis codes such as MCNP. The detailed calculation and analysis will be performed later, for example, during operation the accessibility inside the building and the security of the sensitive electronic equipment installed at the four corners of the Tokamak Building. This model was used to assess the biological dose maps in the building during the activated divertor moving in the Tokamak building to the hot cell for refurbishment. The details can be found in Ref. [17]. 4. Conclusions With the help of MCAM4.8, based on the engineering CAD model, the detailed neutronics simulation model for MCNP code of ITER building was created. This model is the first detailed ITER building 3D neutronics model and it will be made available to all IO collaborators as a reference model. This model can be used to assess the accurate radiation map during the maintenance scenarios by transporting the activated components (e.g. divertor) from inside of the cryostat to the servicing halls and galleries in the building for assisting the shielding design for the personnel and sensitive equipment. The automatic generation of ITER building MCNP model from the CAD model demonstrated the capability of MCAM4.8 to be applied to the nuclear device with large scale complex geometry. To illustrate applications of the ITER building neutronics model, the neutron flux inside the building during operation was calculated. This calculation showed the correctness and usability of the model. The detailed calculation and analysis will be performed later.
Acknowledgments This work was performed under the ITER service contract (ITER/CT/09/4100001055). The authors would like to express their appreciation to ITER Organization for helpful support. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. References [1] M. Loughlin, P. Batistoni, P. Konno, et al., ITER nuclear analysis strategy and requirements, Fusion Sci. Technol. 56 (2009) 566–572. [2] P.P.H. Wilson, R. Feder, U. Fischer, et al., State-of-the-art 3-D radiation transport methods for fusion energy systems, Fusion Eng. Des. 83 (2008) 824–833. [3] U. Fischer, H. Iida, Y. Li, et al., Use of CAD generated geometry data in Monte Carlo transport calculations for ITER, Fusion Sci. Technol. 56 (2009) 702–709. [4] Y. Wu, FDS Team, CAD-based interface programs for fusion neutron transport simulation, Fusion Eng. Des. 84 (2009) 1987–1992. [5] Y. Li, L. Lu, A. Ding, et al., Benchmarking of MCAM 4.0 with the ITER 3D model, Fusion Eng. Des. 82 (2007) 2861–2866. [6] L. Lu, Y.K. Lee, J. Zhang, et al., Development of Monte Carlo automatic modeling functions of MCAM for TRIPOLI-ITER application, Nucl. Instrum. Methods Phys. Res. Sect. A 605 (2009) 384–387. [7] J.F. Briesmeister, MCNP – A General Monte Carlo N-Particle Transport Code, version 4C, Los Alamos National Laboratory, USA, 2000. [8] C.M. Diop, E. Dumonteil, F.X. Hugot, et al., An overview on the Monte Carlo particle transport code TRIPOLI-4, T, Am. Nucl. Soc. 97 (2007) 694–695. [9] Q. Zeng, L. Lu, A. Ding, et al., Update of ITER 3D basic neutronics model with MCAM, Fusion Eng. Des. 81 (2006) 2773–2778. [10] G. Wang, J. Xiong, D. Wang, et al., Progress and applications of MCAM: Monte Carlo automatic modeling program for particle transport simulation, Prog. Nucl. Sci. Technol. 2 (2011) 821–825. [11] H. Hu, Y. Wu, M. Chen, et al., Benchmarking of SNAM with the ITER 3D model, Fusion Eng. Des. 82 (2007) 2867–2871. [12] Y. Wu, Conceptual design activities of FDS series fusion power plants in China, Fusion Eng. Des. 81 (2006) 2713–2718. [13] Y. Wu, FDS Team, Fusion-based hydrogen production reactor and its material selection, J. Nucl. Mater. 386–388 (2009) 122–126. [14] Y. Wu, FDS Team, Conceptual design and testing strategy of a dual functional lithium-lead test blanket module in ITER and EAST, Nucl. Fusion 47 (2007) 1533–1539. [15] Y. Wu, FDS Team, Design analysis of the China dual-functional lithium lead (DFLL) test blanket module in ITER, Fusion Eng. Des. 82 (2007) 1893– 1903. [16] Y. Wu, Z. Xie, U. Fischer, A discrete ordinates nodal method for one-dimensional neutron transport numerical calculation in curvilinear geometries, Nucl. Sci. Eng. 133 (1999) 350–357. [17] D. Ying, Q. Zeng, Y. Qiu, et al., Assessment of radiation maps during activated divertor moving in the ITER building, Fusion Eng. Des. 86 (2011) 2087–2091. [18] D.L. Aldama, A. Trkov, Evaluated Nuclear Data Library for Fusion Applications, INDC (NDS)-467, 2004. [19] Y. Luo, P. Long, G. Wu, et al., SVIP-N 1.0: an integrated visualization platform for neutronics analysis, Fusion Eng. Des. 85 (2010) 1527–1530.
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Use of MCAM in creating 3D neutronics model for ITER building Qin Zeng a,b , Guozhong Wang b,∗ , Tongqiang Dang b , Pengcheng Long a,b , Michael Loughlin c a
Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, Hefei, Anhui 230031, China School of Nuclear Science and Technology, University of Science and Technology of China, Hefei, Anhui 230027, China c ITER Organization, Route de Vinon sur Verdon, 13115 St. Paul-Lz-Durance, France b
a r t i c l e
i n f o
Article history: Available online 27 March 2012 Keywords: Monte Carlo Neutronics modeling International Thermonuclear Experimental Reactor Building Monte Carlo Automatic Modeling
a b s t r a c t The three dimensional (3D) neutronics reference model of International Thermonuclear Experimental Reactor (ITER) only defines the tokamak machine and extends to the bio-shield. In order to meet further 3D neutronics analysis needs, it is necessary to create a 3D reference model of the ITER building. Monte Carlo Automatic Modeling Program for Radiation Transport Simulation (MCAM) was developed as a computer aided design (CAD) based bi-directional interface program between general CAD systems and Monte Carlo radiation transport simulation codes. With the help of MCAM version 4.8, the 3D neutronics model of ITER building was created based on the engineering CAD model. The calculation of the neutron flux map in ITER building during operation showed the correctness and usability of the model. This model is the first detailed ITER building 3D neutronics model and it will be made available to all international organization collaborators as a reference model. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In order to conduct consistent neutronics analyses of International Thermonuclear Experimental Reactor (ITER), the international organization (IO) started the neutronics strategy to create series of neutronics reference models [1]. ITER IO distributed the detailed 3D neutronics reference calculation model of the ITER tokamak machine, named A-lite, in April, 2009. However, the model only extends to the bio-shield. In order to meet future neutronics analysis needs, for example during operation the accessibility and the security of the sensitive electronic equipment inside the building, it is necessary to create a reference model of the ITER building outside the bio-shield. In the frame of the ITER China service contract (ITER/CT/09/4100001055), the 3D neutronics model of ITER building was created based on the engineering computer aided design (CAD) model. The engineering CAD model of ITER building was created employing CATIA V5 software by ITER IO. This model describes very detailed geometry of the buildings from the bio-shield to the outside wall, including the Tokamak Building, the Diagnostic Building and the Tritium Building. The size is 118 m in length, 82 m in width and 74 m in height. It would be time-consuming to manually create the corresponding Monte Carlo (MC) simulation model. The CAD-based modeling approach has been developed by the participants of the ITER project to produce the models
for nuclear analysis for ITER [2,3]. Monte Carlo Automatic Modeling Program for Radiation Transport Simulation (MCAM) [4–6] was developed by FDS Team, China, as a bi-directional interface program between general commercial CAD systems and MC radiation transport codes (e.g. MCNP [7], TRIPOLI [8]). On one hand, CAD models can be converted into the input geometry suitable for the MC codes conveniently and rapidly. On the other hand, the existing MC models can be converted into CAD model and visualized for verification and further update. MCAM also supports some supplementary functions such as creation and repair of CAD models and analysis of physics properties. MCAM has been widely used in the neutronics modeling of many nuclear devices [9–17]. The neutronics simulation model of ITER building for MCNP simulation was created using MCAM4.8 version based on the engineering CAD model. This model is the first detailed ITER building 3D neutronics model and it will be made available to all IO collaborators as a reference model. To illustrate applications of this neutronics model, the neutron flux map inside the ITER building during operation was calculated. This calculation showed the correctness and usability of the model.
2. Creating neutronics model for ITER building 2.1. MCAM improvements and new capabilities
∗ Corresponding author. Tel.: +86 551 5593631. E-mail address: [email protected] (G. Wang). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.02.119
Compared with the previous version 4.7, MCAM4.8 offers many enhancements and new capabilities.
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Fig. 1. The ITER building CAD models in CATIA V5 (left) and in MCAM4.8 (right).
The computer memory consumption was greatly reduced in both the conversion process from CAD models to MCNP models and the reverse conversion (also called inversion) process by more than 60%. For example, only 600 megabyte (MB) memory is needed for the inversion of the whole ITER Alite4 MCNP model, while it would consume more than 2 gigabyte (GB) memory for MCAM4.7 as the limit of a 32 bit platform program. And so was the conversion of the whole ITER CAD model generated from the Alite4 MCNP model, about 800 MB. It is able to convert much larger models without splitting them into separate ones benefited from this improvement. A more accurate method of the General Quadric (GQ) surfaces was developed. In the inversion process from MCNP geometry to CAD geometry, it is needed to identify the surface type (cylinder, cone or ellipsoid) of a GQ surface description and resolve the mathematic parameters (base center, radius, axis direction, etc.). The eigen value and eigenvector related algorithm were adopted and the solution was thereby more accurate. The automatic filling of the internal void space of pipe-like CAD solid geometry was developed. Traditionally, the completion of the CAD model with void cells was implemented by boolean subtracting the solids from a series of regular boxes. The CAD feature recognition technology was taken here. The algorithm firstly traversed the geometry and topology of a pipe-like CAD solid, extracted the features (holes, etc.) and saved the boundary information (vertices, edges, loops and faces) of the internal space. Then the algorithm constructed the internal space solid geometry based on the saved boundary information. Compared with the boolean subtraction method, this method is more rapid and has a higher success rate. Some other new features of MCAM4.8 are as follows: (1) Offers Undo and Redo operation for CAD geometry creation and modification. (2) Supports more syntax of MCNP input. For example, the macrobody description in repeated structure geometry, the TRCL card in surface description. (3) 2D section view function was improved from gray to colorful to be more intuitive. (4) Supports electron importance. (5) Offers a slice through CAD solid geometry with cylindrical and spherical surface.
truss structure. The crane was designed to be placed in the space between the roof and the tokamak. The Diagnostic Building and the Tritium Building are located at the two sides of the Tokamak Building and both consist of eight levels. Based on the CATIA format CAD model, the neutronics model for MCNP simulation was created by MCAM. Firstly, the model was imported into MCAM and preprocessed. The purpose of preprocess is to prepare an available CAD model from the engineering CAD model before converting it into a MCNP input file. The preprocessing of the ITER building model includes format exchange, irrelevant details removal and simplification and overlap removal. Then the physics properties were edited graphically. Finally, the model was converted into the text format MCNP input file with automatic generation of void space description. 2.2.1. Format exchange Since the engineering CAD model was created by CATIA software, it was firstly exported as the neutral file format STEP (Standard Exchange of Product data model), and then imported into MCAM. Due to limitations of data transfer through neutral file format STEP, the model imported may be imprecise and have problems such as gaps between entities, and the absence of topology connectivity information. The heal function of MCAM was used to detect and fix the errors that existed in the imported model. Additionally, since the CATIA CAD system uses millimeters as the unit of dimension while MCNP models use centimeter instead, the imported model was scaled with a ratio of 0.1. The model was then saved as SAT format for reuse. Fig. 1 is a south-west corner view in CATIA and in MCAM4.8. 2.2.2. Removal and simplification of irrelevant details The engineering CAD model was designed for the machining and manufacturing with abundant irrelevant small details on the doors and the ladders. The removal of these details had little influence on the accuracy of neutronics analysis and would make the creation of neutronics model and neutronics analysis much faster. The steel structure of the roof model contains large numbers of staves and overlaps. The steel structure was simplified to one solid body which will make a little influence on the simulation results. The simplified roof model was kept the same mass and occupied volume as original model with mixture material of void and steel. The mixture material density of simplified roof model is 0.02894 g/cm3 .
2.2. Creating neutronics model The ITER building from the bio-shield to the outside wall includes the Tokamak Building, the Diagnostic Building and the Tritium Building. The Tokamak Building consists of 9 levels and the tokamak machine was designed to be installed at its center. The top part of the Tokamak Building is a steel roof of staggered
2.2.3. Check and removal of overlaps There were many overlaps between neighboring entities in the engineering CAD model. These overlaps may not influence the machining and manufacturing but overlaps are not be accepted by MCNP calculations. All the overlaps were detected and removed in MCAM. All the overlaps had very small volumes compared to
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the volume and the mass density (Eqs. (1) and (2)). The volumes were calculated by MCAM on the basis of analytical calculations.
Fig. 2. The fifth floor of the Tokamak Building.
Fig. 3. Three columns overlapped with the external wall. The overlapped part of external wall was removed.
the whole model so that the removal of overlaps would have little influence to the simulation results. For example, in the fifth floor (see Fig. 2) of the Tokamak Building, three columns overlapped with the external wall (see Fig. 3). The overlapped part of external wall was removed. 2.2.4. Physics modeling After preprocessing, the CAD model should be assigned with physics properties. Firstly, the components were grouped by material numbers. Then the neutron importance and photon importance, material density and other physical properties were assigned onto the cells. Finally, the material definition and source, tally specification were edited graphically. The components of doors were assigned SS304 stainless steel with the density 8.03 g/cm3 . The roof was also assigned the mixture material of SS304 and void with the density 0.02894 g/cm3 . The rest of this building was assigned the concrete with density 2.2 g/cm3 . The nuclide composition information of concrete and SS304 were directly obtained from the ITER A-lite4 neutronics reference model. 2.2.5. Conversion After previous procedure, the model was converted into text format MCNP input file. The geometry (cell card surface card) description in the file was produced from the CAD geometry. The void space geometry description was also automatic produced during the conversion process. The material, source, tally and other physical information were appended after the geometry description. With these procedures, the full-formed MCNP input file was produced. 2.3. Model verification The mass comparison and MCNP simulation were performed to verify the MCNP input file. 2.3.1. Mass comparison The mass of the original CAD model and the modified neutronics model was compared. The mass was calculated as the product of
m1 = concrete V11 + steel V12
(1)
m2 = concrete V21 + steel V22 + roof V23
(2)
where m1 is the mass of the original CAD model, m2 is the mass of the modified CAD model, V11 is the volume of the original whole building except the roof and the doors, V12 is the volume of the original roof and the doors, V21 is the volume of the modified whole building except the roof and the doors, V22 is the volume of the modified doors, V23 is the volume of the modified roof, concrete is the mass density of concrete (2.2 g/cm3 ), steel is the mass density of SS304 (8.03 g/cm3 ), roof is the mass density of the simplified roof (0.02894 g/cm3 ). The mass of the original CAD model m1 is 2.72E+11 g and the mass of modified CAD model m2 is 2.71E+11 g. The difference is 0.4%. This result shows that the modification was reasonable. 2.3.2. Geometry check MCNP was used to check the geometry correctness of the MCNP input file. The model was set as vacuum (no materials) with “VOID” card so that many particles could be run in a short time and more particles could be simulated to increase the chance of particles going to most part of the geometry. A point source was set at the center. After 1e8 particle histories, the result showed that no particles lost. Based on this result, the probability of error occurred in per unit volume (no particle passed through this unit volume) was less than 1E−4. This value was evaluated as following: Concerning a point source, the probability of one particle passing through per unit volume P1 can be rudely estimated as: P1 =
1 4r 3
(3)
where r is the farthest distance from any one point in the model to the point source. Then the probability of one particle not passing through per unit volume P2 is: P2 = 1 − P1 = 1 −
1 4r 3
(4)
Therefore, the probability of none of the particles passing through per unit volume P3 is: P3 =
1−
1 4r 3
˛
(5)
where ˛ is the number of the simulated particles. Then the geometry error in per unit volume can be estimated as: = P3 =
1−
1 4r 3
˛
(6)
3. Neutron flux map in the building During the D-T operation of the ITER, neutrons stream surrounding the machine will make radiation outside the bio-shield. Assessment of radiation distribution inside the building during operation is required to identify the hot locations where human accessibility is prohibited. The neutron flux map during operation was generated everywhere in the building. Due to the large size of the tokamak and the building, it was necessary to break down the problem into two models where neutrons transport is carried out in the tokamak model to generate the required external source needed as input to the building model. The calculation was performed with MCNP and the fusion evaluated nuclear data library FENDL [18]. The calculation procedures include two steps:
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Fig. 4. The 3D geometry model and neutron flux map (n/cm2 /s) of the Lower Level (B1) of the Tokamak Building.
(1) Firstly, the internal boundary source (IBS) at the outer surface of the cryostat was calculated and obtained from 40◦ toroidal sector Alite4 model. The D-T fusion plasma source included in the distributed Alite4 model was selected as neutron source for neutrons transport calculation in the tokamak machine. (2) Then the IBS was used in the subsequent calculation for the building model. As an example, the neutron flux map in the Lower Level (B1) of the Tokamak Building is showed in Fig. 4. The neutron flux map inside the zone from the inner surface of the cryostat to the center of the tokamak was not given here. These neutron flux maps were visualized by SVIP [19] software, which is a visualization tool for neutronics analysis codes such as MCNP. The detailed calculation and analysis will be performed later, for example, during operation the accessibility inside the building and the security of the sensitive electronic equipment installed at the four corners of the Tokamak Building. This model was used to assess the biological dose maps in the building during the activated divertor moving in the Tokamak building to the hot cell for refurbishment. The details can be found in Ref. [17]. 4. Conclusions With the help of MCAM4.8, based on the engineering CAD model, the detailed neutronics simulation model for MCNP code of ITER building was created. This model is the first detailed ITER building 3D neutronics model and it will be made available to all IO collaborators as a reference model. This model can be used to assess the accurate radiation map during the maintenance scenarios by transporting the activated components (e.g. divertor) from inside of the cryostat to the servicing halls and galleries in the building for assisting the shielding design for the personnel and sensitive equipment. The automatic generation of ITER building MCNP model from the CAD model demonstrated the capability of MCAM4.8 to be applied to the nuclear device with large scale complex geometry. To illustrate applications of the ITER building neutronics model, the neutron flux inside the building during operation was calculated. This calculation showed the correctness and usability of the model. The detailed calculation and analysis will be performed later.
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