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1 New features in Serpent 2 for fusion neutronics 5th International Serpent UGM, Knoxville, TN, Oct , 2015 Jaakko Leppänen VTT Technical Research Center of Finland

2 Click to edit Master title Outline style Background Click Serpent to edit overview Master text styles New Second applications level CAD-based Third geometry leveltype in Serpent 2 Fourth ITER C-Lite calculations Fifth level First results: M&C 2015 New results: shut-down dose rate calculations Ideas for future work

3 Click to edit Master title Background style Serpent is a continuous-energy Monte Carlo reactor physics burnup calculation code, developed at VTT Technical Research Centre of Finland since 2004: 1 Originally developed for lattice physics, although currently used for a variety of applications in fission reactor analysis Third level Three-dimensional Fourth level universe-based CSG geometry model, particle transport based on the combination of surface- and delta-tracking Fifth level Cross sections read from ACE format data libraries ( laws of physics shared with MCNP), decay and fission yield data from standard ENDF files Built-in depletion solver (CRAM 2 and automated calculation routines for burnup calculation and spatial homogenization Parallelization by OpenMP and MPI Serpent is distributed by the OECD/NEA Data Bank and RSICC, and has about 460 users in 142 organizations in 36 countries around the world. 1 For a more complete description, see project website 2 M. Pusa. "Numerical Methods for Nuclear Fuel Burnup Calculations," D.Sc. Thesis, Aalto University, 2013

4 Click to edit Master title Background style A major part of Serpent development is currently devoted to coupled multi-physics simulations, which involves two-way coupling to fuel performance, thermal hydraulics and Click CFD to (OpenFOAM) edit Master codes: text 3 styles Based on internal light-weight solvers (FINIX and COSY) and external coupling Third level Passing of Fourth state-point level information is handled via a multi-physics interface, without any modifications to main geometry input Fifth level Simulation of gamma heating requires a photon transport mode, which was introduced in the latest update (2.1.24): Photon energies ranging from 1 kev to 100 MeV, reaction cross sections from ACE format libraries, additional data for interaction physics (photo-atomic) Built-in response functions for dose rates, radioactive decay source mode easily combined with burnup or activation calculation Development of a coupled neutron-photon transport mode will begin in late J. Leppänen, et al. "The Numerical Multi-Physics Project (NUMPS) at VTT Technical Research Centre of Finland," Ann. Nucl. Energy 84 (2015)

5 Click to edit Master title Background style Work on the multi-physics interface and CFD code coupling also lead to the development of two new geometry types: 1) Unstructured mesh-based geometry type 4 Geometry Third composed level of a polyhedral unstructured volume mesh Based on the Fourth OpenFOAM level file format Fifth level 2) CAD-based geometry type 5 Solid bodies formed using unstructured triangulated surface mesh Based on the STL file format Photon transport capability and advanced geometry types offer the possibility to extend the scope of Serpent applications to new fields beyond reactor analysis, including radiation shielding and fusion neutronics. 4 J. Leppänen, et al. "Unstructured Mesh Based Multi-physics Interface for CFD Code Coupling in the Serpent 2 Monte Carlo Code," In Proc. PHYSOR 2014, Kyoto, Japan, Sept Oct. 3, J. Leppänen. "Development of a CAD Based Geometry Model in Serpent 2 Monte Carlo Code," Trans. Am. Nucl. Soc. 111 (2014)

6 Overview of methodology: CAD-based geometry type in Serpent 2 Conversion from advanced formats to triangulated STL can be accomplished using most CAD tools, no further processing or conversion to CSG required Both ASCII Third and level binary STL supported STL geometries Fourth are level handled as separate universes, individual solids as cells Fifth level Geometries can be nested inside each other no need to specify void space, can be combined with OpenFOAM mesh-based geometry type Cell search routine is based on ray tests, designed to cope with small holes between surface triangles (caused, e.g. by limited numerical precision) Adaptive N-tree search mesh used to speed up the cell search routine Performance of delta-tracking is not strongly dependent on the resolution of the geometry model The STL format was chosen for Serpent 2 because of its simplicity no third-party libraries or source code used in the implementation.

7 C-Lite calculations first results The C-Lite model (V1 Rev ) of ITER was used as the test case for the new CAD-based geometry type in a paper presented at M&C 2015 in April: 6 11 components / 1,548 solids / 1,842,576 points / 614,192 triangular facets Conversion Third from levelstep to STL using FreeCAD The calculations Fourth were limited level by the lack of material data and a realistic neutron source distribution, so the main Fifth level purpose of this study was to see if the new geometry type could handle large and complicated systems: The geometry was tested by running a large number of consistency checks Volumes were calculated by Monte Carlo sampling and compared to values given by FreeCAD Computational performance tested by running a transport simulation The calculations revealed that conversion to STL using FreeCAD produced a large hole in one of the solids, which was later fixed by switching to SpaceClaim. 6 J. Leppänen. "CAD-Based Geometry Type in Serpent 2: Application to Fusion Neutronics," In Proc. SNA + M&C 2015, Nashville, TN, USA, April 19-23, 2015.

8 C-Lite calculations first results Table 1 : C-Lite model of the ITER fusion reactor. Number of points, triangular facets and solids in the STL geometry model, and the size of the adaptive search mesh. Cryostat, control and correction coils and central solenoid were modeled in the same universe to reduce the number of geometry levels, and they share the same search mesh. STL Geometry Model Adaptive Search Mesh Component Third level Points Facets Solids Cells Memory Blanket Fourth level 108,459 36, ,473, MB Divertor Fifth level 161,232 53, ,869, MB Vacuum vessel ports 10,164 3, ,985, MB Vacuum vessel 967, , ,050, MB Toroidal field coils 105,753 35, ,173, MB Thermal shields 166,836 55, ,528, MB Poloidal field coils 34,758 11, , MB Biological shield 1, ,799, MB Cryostat 264,456 88, , MB Control and correction 13,332 4, Central solenoid 8,748 2, Total 1,842, , ,056, GB

9 C-Lite calculations first results Third level Fourth level Fifth level Figure 1 : Serpent geometry plot showing a cross-sectional view of the ITER C-Lite geometry model. Separate STL solids are plotted with different colors.

10 C-Lite calculations first results Third level Fourth level Fifth level Figure 2 : Serpent geometry plot showing a cross sectional view of a divertor cassette. Separate STL solids are plotted with different colors and the search mesh is shown adapted around the boundaries.

11 C-Lite calculations first results Third level Fourth level Fifth level Figure 3 : Left: Large hole in one of the STL solids forming the cryostat caused by conversion to STL using FreeCAD (used in the calculations of the M&C 2015 paper). Right: Same solid converted to STL using SpaceClaim (used in the calculations of this study).

12 1) Neutron transport simulation C-Lite calculations new results Since the M&C 2015 paper the work has continued to testing the capabilities for activation and shut-down dose rate calculations with a two-stage calculation scheme: Constant Third power level operation at 500 MW for 400 seconds, 50/50 D/T mix Source distribution Fourth level from plasma simulation (See Paula s presentation) Fifth level Result: material activation using the built-in depletion routine in Serpent 2, each STL solid handled as a separate material zone (1,548 zones) 2) Photon transport simulation Radioactive decay source obtained from the neutron activation calculation Result: absorbed dose rates at various cooling times calculated using built-in response functions (NIST mass-energy attenuation data) NOTE: As in the M&C 2015 paper, the results cannot be considered physically realistic because of the lack of accurate material data. 7 7 Biological shield was modeled as concrete, all other components as stainless steel.

13 C-Lite calculations new results 10 Second 12 level Neutron flux(1/cm 2 s) Third level Fourth level Fifth level Neutron flux(1/cm 2 s) Figure 4 : Results of neutron transport simulation: Volume-averaged flux. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle.

14 C-Lite calculations new results Click to edit Master text styles Totalcollision density (1/cm 3 s) Third level 10 9 Fourth level 10 8 Fifth level Totalcollision density (1/cm 3 s) Figure 5 : Results of neutron transport simulation: Volume-averaged total collision density. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle.

15 C-Lite calculations new results 10 7 Second level Photonemission density (1/cm 3 s) 10 8 Third 10 6 level Fourth level 10 5 Fifth level Photonemission density (1/cm 3 s) Figure 6 : Results of photon transport simulation: Volume-averaged photon emission density. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle. One minute after shut-down.

16 C-Lite calculations new results 10 0 Second level Photon dose rate (Gy/h) Third 1 level Fourth level 10 2 Fifth level Photon dose rate (Gy/h) Figure 7 : Results of photon transport simulation: Volume-averaged dose rate. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle. One minute after shut-down.

17 C-Lite calculations new results 10 7 Second level Photonemission density (1/cm 3 s) 10 8 Third 10 6 level Fourth level 10 5 Fifth level Photonemission density (1/cm 3 s) Figure 8 : Results of photon transport simulation: Volume-averaged photon emission density. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle. One hour after shut-down.

18 C-Lite calculations new results 10 0 Second level Photon dose rate (Gy/h) Third 1 level Fourth level 10 2 Fifth level Photon dose rate (Gy/h) Figure 9 : Results of photon transport simulation: Volume-averaged dose rate. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle. One hour after shut-down.

19 C-Lite calculations new results 10 7 Second level Photonemission density (1/cm 3 s) 10 8 Third 10 6 level Fourth level 10 5 Fifth level Photonemission density (1/cm 3 s) Figure 10 : Results of photon transport simulation: Volume-averaged photon emission density. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle. One day after shut-down.

20 C-Lite calculations new results 10 0 Second level Photon dose rate (Gy/h) Third 1 level Fourth level 10 2 Fifth level Photon dose rate (Gy/h) Figure 11 : Results of photon transport simulation: Volume-averaged dose rate. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle. One day after shut-down.

21 C-Lite calculations new results 10 7 Second level Photonemission density (1/cm 3 s) 10 8 Third 10 6 level Fourth level 10 5 Fifth level Photonemission density (1/cm 3 s) Figure 12 : Results of photon transport simulation: Volume-averaged photon emission density. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle. One week after shut-down.

22 C-Lite calculations new results 10 0 Second level Photon dose rate (Gy/h) Third 1 level Fourth level 10 2 Fifth level Photon dose rate (Gy/h) Figure 13 : Results of photon transport simulation: Volume-averaged dose rate. Left: XY-plot averaged over axial dimension. Right: RZ-plot averaged over full rotation angle. One week after shut-down.

23 Running times: C-Lite calculations new results All test calculations were run on a 12-core 3.47 GHz Intel Xeon workstation Geometry processing takes about 2 minutes (reading of STL data, forming the adaptive Third search level mesh, etc.) Activation calculations Fourth level with 100 million neutron histories run for 6 hours (analog capture) / 11 hours Fifth level (implicit capture) Running time for depletion solver was negligible (seconds) Photon transport simulations with 100 million histories run for minutes (implicit source biasing) 8 / minutes (analog sampling) 9 Parallel scalability was practically linear up to 12 OpenMP threads. 8 Source points sampled uniformly, statistical weight adjusted according to local emission probability 9 Source points sampled uniformly, rejection sampling based on local emission probability

24 Lessons learned from this study: C-Lite calculations new results The STL-based geometry type seems to work with photons as well, although delta-tracking runs into problems with very short mfp s of low energy (E 1 kev) photons Third level Handling 1,548 Fourth separate level material zones was not a problem for activation calculation or radioactive Fifth level decay source, but the spatial sub-division is clearly not sufficient for some of the larger solids, e.g in the biological shield It was possible to use the same geometry model in both neutron and photon transport calculation with only a few minor modifications in the input Use of implicit capture for neutrons improves the statistics in the outer parts of the geometry, but also leads to significant increase in running time (FOM s not yet compared) Simple source biasing routine works well with the radioactive decay source, although this is probably not the case in geometries where high- and low-active parts are not as well shielded from each other

25 Ideas for future work The C-Lite model will be used as the test case for future studies as well: Accurate material compositions needed for physically realistic simulations (defining the compositions of 1500 material zones is not a trivial task) Performance Third level comparison to CSG model (Serpent input exists, but the geometry is not a 100% Fourth match level with the CAD model) Calculations with Fifth FENDL level or TENDL data (instead of ENDF/B-VII) to account for missing nuclides and high-energy reaction channels in activation calculations Moving to more complicated models: Test calculations with refined geometries for individual components Test calculations with hybrid STL solid / OpenFOAM mesh-based geometries Finding the practical limitations: what is the impact of model complexity on processing time, running time and memory consumption

26 Other future plans: reaction rates Ideas for future work Studies on the effects of local variation in source distribution on neutron-induced Validation Third bylevel comparison to experiments (JET) Other potential fusion Fourth neutronics level applications for Serpent: Fifth level Heat deposition, helium production and material DPA calculations, etc. can be done with standard tallies Tritium breeding calculations can be done using built-in depletion routines Multi-physics simulations Serpent-OpenFOAM interface exists and is already used for fission applications Fusion power plant (DEMO) simulations with Serpent coupled to the APROS system code

27 Topics for Serpent development: project of Paula Sirén Ideas for future work Development of a high-fidelity neutron source model continues as the Ph.D. Development Third level of a coupled neutron-photon transport mode will be started in the near future Fourth level Development of Fifth effective level variance reduction techniques is a necessity, especially for radiation shielding calculations Validation of photon and 14 MeV neutron physics with benchmark calculations (SINBAD)

28 Thank you for your attention! Questions? - Jaakko.Leppanen@vtt.fi Third level Fourth level Fifth level

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