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1 Introduction to Serpent Code Fusion neutronics workshop, Cambridge, UK, June 11-12, 2015 Jaakko Leppänen VTT Technical Research Center of Finland

2 Click to edit Master title Outline style Serpent overview Current Second status level and on-going work: Early development and applications Spatial Third homogenization level Coupled Fourth multi-physics level simulations Fifth level Photon transport mode Summary

3 Developed at VTT since 2004 Serpent overview Serpent is best characterized as a continuous-energy Monte Carlo reactor physics burnup calculation code: Public Third distribution level since 2009, licensed free of charge for non-commercial research and Fourth educational level use Typical users: universities Fifth level and research organizations working on various topics in reactor physics Technical details: Three-dimensional universe-based CSG geometry model Physics based on ACE format cross section libraries and ENDF format decay and fission yield data External source and k-eigenvalue criticality source simulation Built-in depletion solver based on the Chebyshev Rational Approximation method (CRAM)

4 Click Jaakko to edit Leppänen Master (misc. text stuff) styles Maria Pusa (CRAM, deterministic solvers for homogenization) Tuomas Third Viitanen level(tms temperature treatment routine) Fourth level Ville Valtavirta (multi-physics coupling) Fifth level Serpent overview Serpent is developed by a five-member core team at VTT: With: Toni Kaltiaisenaho (photon transport) Paula Sirén, VTT (D.Sc. thesis on fusion neutronics) Riku Tuominen, VTT (M.Sc. thesis on Serpent-OpenFOAM coupling) Mohammad Hessan, Aachen University, Germany (D.Sc. thesis on delayed neutron emission in dynamic simulations) and valuable contributions from the user community

5 Serpent overview User community in numbers: Click 440to registered edit Master users text in mailing styles list (44% Europe, 40% North America) 135 organizations (54% universities) 35 countries Third level Typical user: Fourth M.Sc. level or Ph.D. student performing academic Fifth level research and thesis work 50 Theses on Serpent-related topics since scientific journal and conference papers since 2005 Number of registered users Year Serpent website: Serpent discussion forum:

6 overview Click to edit MasterSerpent title style Click to edit Master text styles Second level Third level Fourth level Fifth level Figure 1: Group photos from International Serpent User Group Meetings: Dresden, 2011; Madrid, 2012; Berkeley, 2013; Cambridge, The 2015 meeting will be hosted by University of Tennessee in Knoxville, TN, USA, on October

7 Early development and applications Serpent was originally developed as a simplified lattice physics code for spatial homogenization, i.e. production of input parameters for deterministic core simulators. As the user community began to grow, so did the variety of applications: Monte Third Carlolevel simulation is inherently 3D, which allowed the modeling of complicated Fourth systems level Serpent had advanced Fifth level and versatile burnup calculation capabilities, which was not the case for most Monte Carlo codes at the time It turned out that most of the users were not running Serpent for spatial homogenization, but for a variety of reactor physics applications: fuel cycle studies, research reactor modeling, etc. Meeting the users needs became a major challenge for code development, originally started with a very specific application in mind.

8 Early development and applications Third level Fourth level Fifth level Figure 2: Example of research reactor modeling using Serpent (ATR test reactor operated by Idaho National Laboratory. Left: cross sectional view of core geometry. Right: fission rate and thermal flux distribution.

9 Early development and applications Problems with excessive memory usage and limitations in parallelization lead to the development of a new code version (Serpent 2) in The work is still under way, and currently focused on two major topics: i) Advanced Third methods level for spatial homogenization ii) Coupled multi-physics Fourth level calculations Fifth level The work on multi-physics coupling lead to the development of a photon transport mode and advanced geometry types. With these new capabilities it became possible to expand the use of Serpent into new fields beyond reactor physics: iii) Radiation shielding iv) Fusion neutronics Examples of the traditional, new and future applications of Serpent are introduced in the following

10 Spatial homogenization Modeling of an operating nuclear reactor is a complicated task: Full-scale solution to neutron transport problem is computationally expensive Transport problem becomes non-linear when feedbacks from material temperatures Third level and densities are taken into account (iteration between solvers) Same applies Fourth to changes level in fuel composition with increasing burnup Fifth level Due to the limited computer capacity, routine design and safety analyses rely on a multi-stage calculation scheme: The heterogeneous transport solution is obtained for a set of sub-problems covering the full range of reactor operating conditions The neutron interaction physics is condensed into a handful of homogenized few-group constants by preserving the reaction rate balance The group constants are used as the building blocks for a simplified full-scale model (typically based on diffusion theory), for which the solution can be obtained at an acceptable computational cost

11 Spatial homogenization Experimental measurements and nuclear models Isotopic micro-group cross sections Third level Fourth level Fifth level Transport calculation at at fuel assembly level Homogenized few-group constants Coupled full-core simulation Figure 3: Left: Multi-stage calculation scheme in reactor analysis. Right: Typical 2D BWR assembly geometry used for spatial homogenization

12 Spatial homogenization Spatial homogenization is traditionally handled using deterministic 2D transport codes, and Serpent is one of the first Monte Carlo codes specifically designed for this task. Advantages of using continuous-energy Monte Carlo simulation for homogenization: Thetransport Third level simulation is inherently three-dimensional, possibility to account for axial heterogeneities Fourth level and other 3D effects The best available Fifth interaction level data can be used almost as-is, no need for additional processing to account for self-shielding effects, etc. The same code and cross section library can be used for modeling any fuel or reactor type... and the limitations: Burnup calculations and covering all state points requires a lot of CPU time Calculation of certain parameters requires special tricks Serpent has been successfully used for producing input data for fuel cycle and transient simulation codes, and the work continues within the SAFIR 2018 research programme

13 Spatial homogenization 0 1 R P N M L K J H G F E D C B A Click to edit Master text styles Second level Third level Fourth level Fifth 19.8 level R P N M L K J H G F E D C B A Relative power Serpent 3D ARES Axial node Figure 4: HFP radial and axial power distributions in a PWR core calculated using Serpent-ARES code sequence, compared to reference 3D Serpent calculation. Left: Assembly powers (MW). Center: Relative differences in assembly powers (%). Right: Axial power peaking. (MIT BEAVRS Benchmark)

14 Boron concentration (ppm) Third level Fourth level Fifth level Spatial homogenization BEAVRS Benchmark ARES Time (days) Difference in boron concentration (ppm) Time (days) Figure 5: Boron dilution curve calculated by Serpent-ARES for the first operating cycle compared to experimental measurements. Left: Critical boron concentration. Right: Relative difference to measured data. (MIT BEAVRS Benchmark)

15 main Click development to edit Master goals for text Serpent styles 2 Coupled multi-physics simulations Coupling of high-fidelity numerical methods for the direct simulation of feedback-effects is a hot research topic in reactor analysis, and multi-physics coupling is also one of the The multi-physics coupling scheme in Serpent 2 operates at two levels: Third level i) Internal light-weight Fourth level solvers for thermal hydraulics and fuel behavior ii) External coupling Fifth via level a universal multi-physics interface The methodology relies heavily on the separation of state-point information from the actual geometry model: For the code user this means that no modifications are needed in the geometry input, as density and temperature distributions are handled using separate structures For the Monte Carlo tracking routine this means that density and temperature distributions can be handled efficiently using a rejection sampling based algorithm

16 COSY a three-dimensional system/component scale thermal hydraulics solver based on a porous-medium three-field flow model (not coupled to Serpent yet) Third level FINIX a thermo-mechanical Fourth level fuel behavior module for the modeling of temperature feedback inside fuel pins in steady-state and transient conditions Coupled multi-physics simulations Internal multi-physics coupling is based on two light-weight solvers, integrated to Serpent 2 at source code level: Fifth level The internal solvers are intended to provide sufficiently accurate solutions to the coupled problem at a low computational cost External coupling to high-fidelity solvers is handled via sequential exchange of input and output files. The work is currently focused on two interface types: Fuel performance code interface for thermo-mechanical fuel behavior coupling Unstructured mesh based thermal hydraulics interface for coupling with OpenFOAM CFD calculations

17 Coupled multi-physics simulations Third level Fourth level Fifth level Figure 6: Results of Serpent-FINIX transient simulations (reactivity excursion, 2D pin-cell model) Top: a) linear power, b) pellet center-line and surface temperatures as function of time. Bottom: a) radial temperature distribution, b) displacement of radial nodes at different times.

18 Temperature and density fields passed into Serpent tracking routine using an unstructured 3D polyhedral mesh Third level Similar unstructured Fourth level 3D mesh can be used for modeling complicated irregular geometry shapes Fifth level Future applications The implementation of the OpenFOAM multi-physics interface required development of new geometry routines: Some of the same subroutines were also used in the development of a new CAD-based geometry type Development of photon transport capability for the purpose of gamma heating in multiphysics simulations was started in 2014 In addition to multi-physics calculations, the developed features will enable broadening the use of Serpent into new applications beyond reactor physics The CAD based geometry type and fusion neutronics applications will be covered in a separate presentation

19 Even though the main motivation for expanding to photon transport is to simulate gamma heating in multi-physics calculations, the capability can also be used for radiation shielding Third applications level Fourth level For example, combined burnup / neutron activation and photon transport calculation: Fifth level Photon transport mode The work on photon transport mode is now completed (M.Sc. Thesis of Toni Kaltiaisenaho), and the complete physics model will be included in the next update (2.1.24) 1) Neutron transport simulation with material activation or burnup calculation to obtain radioactive material compositions 2) Photon transport simulation with radioactive decay source and photon emission spectra from ENDF decay files The same input can be used in both calculations, conversion from isotopic (neutron transport) to elemental (photon transport) material compositions and source rate normalization done automatically Built-in photon mass-energy attenuation coefficients (NIST) for dose rate calculations

20 Photon transport mode: interaction data Photon interaction physics for elements from Z = 1 to 98 and photons from 1 kev to 100 MeV, with reaction cross sections read from ACE format libraries NJOY cannot generate all the required interaction data in the ACE format, so additional data is read from separate files: Third level Most of the Fourth interaction level data is from ENDF-B-VII.1 (form factors, incoherent scattering functions, photoelectric cross sections and atomic relaxation data) Fifth level Other sources for data not included in ENDF-B-VII.1 (Compton profiles, bremsstrahlung data and electron stopping powers) Matlab script is used for preprocessing of some of the data and for converting the data to a simpler format The data scheme may be revised at some point

21 Photon transport mode: interaction physics Rayleigh scattering: (elastic scattering from the electron cloud of an atom) Direction is sampled using the form factor approximation Compton scattering: (inelastic scattering from an atomic electron) Direction Third is sampled level using the incoherent scattering function approximation Fourth level Doppler broadening Fifth level of the photon energy is taken into account (caused by the momentum distribution of the electron), important below 1 MeV Photoelectric effect: Electron shell is selected with a probability given by its cross section, all sub-shells are included Pair production: The energies of the electron and positron are sampled from the theoretical differential cross section given by Davies, Bethe and Maximon, with some extensions and approximations used in PENELOPE and Geant4 Positron annihilation at rest

22 Atomic relaxation: Photon transport mode: secondary photons Compton scattering and photoelectric effect cause vacancies in electron shells Relaxation cascade through radiative (fluorescence) and non-radiative (Auger, Coster Kronig) Third level transitions Transitions Fourth are sampled level according to the probabilities given by ENDF/B-VII.1, all possible transitions Fifth level are included Thick-target bremsstrahlung approximation: Electrons are generated through Compton scattering, photoelectric effect, pair production and non-radiative transitions Bremsstrahlung photon production is important especially for high-z atoms at energies above 1 MeV The number of bremsstrahlung photons and their energies are sampled from the distributions given by the continuous slowing down approximation (CSDA) Angular distribution is omitted; the direction of the bremsstrahlung photon is equal to the direction of the electron

23 Photon.ux spectrum (a.u.) Relative di,erence/error Click to edit Master title style Photon transport mode 10 Click to edit Master 2 Serpent MCNP6 text styles Relative difference Relative error (Serpent) 10 Third 1 level Fourth level 10 0 Pb, E = 0.1 MeV Fifth level Photon energy (MeV) Figure 7: Comparison between Serpent 2 and MCNP kev photon source in lead.

24 Photon transport mode: first applications The work on photon transport continues with the development of a coupled neutronphoton transport mode for the purpose of gamma heating in multi-physics simulations Radiation shielding applications are currently limited by the lack of efficient variance reduction techniques, which is another important topic for future work Third level First planned applications Fourth level for photon transport mode: Comparison to gamma Fifth level spectrum measurements to be carried out by a summer student at Aalto University Radiation shielding calculations in a dry spent fuel storage (KATVE project in the SAFIR 2018 Finnish National Research Programme) Radiation shielding calculations in fusion applications (combined to material activation)

25 Click to edit Master title Summary style The Serpent Monte Carlo code has been developed at VTT since The code has a five-member developer team and an active user community of about 440 users working on various topics in the field of reactor analysis Serpent was originally designed for spatial homogenization, but the applications have considerably Third diversified level along with the growing user community Fourth level The on-going work is focused on two major topics: Fifth level i) Advanced methods for spatial homogenization ii) Coupled multi-physics applications The work on multi-physics coupling has lead to the development of new features: Advanced geometry types Photon transport simulation mode These enable broadening the scope of applications beyond reactor physics, in particular to fusion neutronics and radiation shielding

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