Introduction

GLOW (Geometry Layout Oriented Workflow) is a Python package providing 2D unstructured geometries to the DRAGON5 lattice transport computer code for tracking calculations.

The DRAGON5 computer code provides several models, in terms of FORTRAN modules, to simulate the neutronic behaviour of the fuel elements in a nuclear reactor [2]. Among the available modules, some focus on solution techniques for the neutron transport equation, which describes the statistical behaviour of a large population of particles [1]. In particular, the SALT module can process a geometry mesh to compute the tracking information requested to solve the neutron transport equation. The geometry processed by DRAGON5 is of the surface type where cell mesh boundaries are described by equations.

Among the many methods for the solution of the neutron transport equation, the following two are used with DRAGON5:

  • the Method of Characteristics (MOC), an approach used to solve the neutron transport equation by tracing the paths (characteristics) of neutrons through a discretized geometry. It evaluates the neutron flux distribution by integrating the transport equation along the characteristics, which are straight lines in neutral particle transport, accounting for interactions with the material.

  • the Collision Probability Method (CPM), an approach that computes the probability of a neutron emitted in one region to have the first collision in another region. It generally assumes isotropic neutron emission.

Having a proper description of the geometry layout is essential to perform tracking calculations in DRAGON5. Geometries for fuel assemblies used in nuclear reactors are typically made of several fuel pins arranged according to regular patterns to form Cartesian or hexagonal lattices. Even though DRAGON5 can track any convex surface-type geometry without empty concavities, it can only natively handle the generation of structured geometries. These rely on regular basic surface elements, such as polygons and circles, repeated to create uniform patterns, which in DRAGON5 are built using a small set of predefined operations. This is done with the GEO module where the G2S module converts the geometry layout to a .dat description file used by the SALT tracking module. More complex cases deal with unstructured geometries which are made by general elements having different shapes and dimensions. The combination of surface elements can be obtained as the result of applying boolean operations, such as cuts and unions. Unstructured geometries require external tools based on Constructive Solid Geometry (CSG), a method for creating 2D/3D geometries from combinations of simpler primitive shapes.

So far, several applications have been developed to address the generation of unstructured surface geometries, among the main ones we can cite SALOMON and ALAMOS [4]. SALOMON is a Python2-based application developed in 2001 by EDF that exploits the GEOM module of the SALOME platform, an open-source environment offering 2D/3D CAD modelling capabilities [3]. SALOMON relies on an input file with a specific structure containing the description of the lattice’s geometry layout in terms of cells and materials associated to each region of the lattice. It allows users to construct a geometric description file in the format of the TDT solver of APOLLO2. Currently, SALOMON’s development seems no longer active, resulting in the tool becoming obsolete. ALAMOS is GUI-based application developed by CEA for APOLLO3 that exploits the Python libraries offered by the SALOME platform [4]. In particular, it makes use of the MEDCoupling module of SALOME to build the geometries and apply meshes to them. The regions of the geometry can also be characterised in terms of material property maps. While ALAMOS can handle any kind of complex geometry layout, SALOMON can only deal with lattices made of cartesian-type cells, thus greatly limiting its application. In addition, SALOMON does not provide any support to the construction of surface geometries by exploiting the most common boolean operations, which, instead, are supported by ALAMOS. Unfortunately, ALAMOS, despite being a more complete tool with respect to SALOMON, does not have an open-source distribution.

GLOW was developed to offer an open-source alternative for generating unstructured surface geometries, exploiting the Constructive Solid Geometry functionalities provided by the APIs of the GEOM module of the SALOME platform. In GLOW, complex surface geometry layouts are constituted by 2D areas bounded by closed sets of edges, which are referred to as regions. A cell is a base geometry layout built from a characteristic surface, such as a rectangle or a hexagon, subdivided in different areas, each representing a region. The repetition of adjacent cells in the 2D space constitutes a lattice. GLOW can handle lattices made of cartesian or hexagonal cells, including the option to frame the lattice in a box. Different types of properties, such as the material, can be assigned to each region. GLOW supports Euclidean geometric transformations, such as translation and rotation, and boolean operations, such as union, intersection, cut and partitioning, among geometric shapes.

Since calculations on smaller geometries are expected to be computationally cheaper, symmetries should be considered whenever possible. GLOW can perform cuts to extract parts from an existing layout, thereby isolating the minimum portion of the geometry required to describe the entire pattern. As an example for a Cartesian lattice with a 180 degree reflection symmetry, the cut part would be half of the lattice. Special symmetries according to the main lattice types are implemented:

  • Cartesian - half, quarter and eighth symmetries.

  • Hexagonal - third, sixth and twelfth symmetries.

GLOW does not only exploit the geometric functionalities offered by the GEOM module of SALOME; it also displays the built geometries in SALOME 3D viewer through its graphical user interface (GUI module). Additionally, if the basic functionalities offered by GLOW are not enough to construct specific layouts for cells and lattices, users can rely on the geometry-building functionalities offered by the GEOM module directly. The resulting custom layouts can still be used in GLOW.

In DRAGON5, tracking relies on a description file (.dat) of the geometry layout. GLOW allows the collection of geometric and property information from the built lattice layout, producing this .dat file in the format of the TDT solver of APOLLO2.

GLOW is developed by the Codes & Methods Department of newcleo and it is released under the LGPL-2.1 License.

DISCLAIMER

The DRAGON5 lattice code is not distributed with GLOW. Please, refer to http://merlin.polymtl.ca/version5.htm for downloading the latest version and for the installation instructions.