MCNP6 and SERPENT2 performances with unstructured mesh geometry

Alberto Talamo (Corresponding Author), Yousry A. Gohar, Jaakko Leppänen

    Research output: Contribution to journalArticle in a proceedings journalScientificpeer-review


    Traditionally, Monte Carlo codes model a nuclear reactor by combinatorial geometry. In this model, volume primitives (e,g, spheres, cylinders, parallelepipeds...) are combined together by Boolean operations (e.g. union, intersection, and complement) and replicated by lattice definitions to build the core geometry. Recently, high performances computing platforms allowed using unstructured mesh geometry in Monte Carlo codes. When this option is chosen, any type of core geometry can be modeled since the restriction of using volume primitives is removed. In an unstructured mesh geometry, Boolean operations and lattice definitions are not used since each volume is defined by triangular surfaces (facets). This study focuses on two Monte Carlo codes: MCNP (version 6.1.1 beta) developed at Los Alamos National Laboratory since 1957 and written in FORTRAN90, C, and C++ (267,000 total source code lines), and SERPENT (version 2.1.27) developed at VTT Technical Research Centre since 2004 and written in C (195,000 total source code lines). These two codes follow different approaches for the unstructured mesh geometry modeling: MCNP uses ABAQUS format, whereas SERPENT uses the STL format. In the STL (STereoLithographic) geometry model only the surfaces of the volume are meshed; the interior part of the volumes is not meshed. In the STL geometry model, the whole geometry is defined by the sequence of SOLID and ENDSOLID sections. Each volume of the geometry must have a SOLID and ENDSOLID section. The STL file contains no information specifying the materials of the volumes. MCNP and SERPENT can model any kind of complicated geometry using unstructured meshes. MCNP and SERPENT use the ABAQUS and STL file formats, respectively. The STL file format does not have tetrahedrons and has only triangular facets on the volumes boundary. This allows SERPENT simulations to run 10 times faster than MCNP. However, if Monte Carlo results have to be coupled to thermal-hydraulics codes, the interior of the geometry volumes must be meshed, which may reduce SERPENT performances.
    Original languageEnglish
    Pages (from-to)1111-1113
    JournalTransactions of the American Nuclear Society
    Issue number2
    Publication statusPublished - 1 Jan 2017
    MoE publication typeA4 Article in a conference publication
    EventANS 2017, Annual Meeting - San Francisco, United States
    Duration: 11 Jun 201715 Jun 2017


    This work is supported by the U.S. Department of Energy, Office of Material Management and Minimization (M3), National Nuclear Security Administration. The submitted manuscript has been created by UChicago Argonne LLC, Operator of Argonne National Laboratory ("Argonne"). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.


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