Note: OpenMC supports the recommended University of Wisconsin Unified Workflow (UWUW)

Code-Specific steps for OpenMC

There are two varieties of code-specific steps for OpenMC:

Defining attributes of the geometry using Cubit groups
Defining DAGMC runtime parameters using the OpenMC input files

Geometry metadata

The DAGMC geometry file can be used to define material assignments, boundary conditions, and temperatures.

Material Assignment

The generic workflow description includes details on Grouping volumes and surfaces, but a specific naming convention is required for OpenMC. To define materials, the material ID must be provided in the group name. The format for the group name is as follows: mat:[matid].

[matid] should be replaced by the material ID that will be specified in the materials.xml file.

For example, consider a problem where material 7 should be assigned to volumes 4 through 18. The following command should be used to specify that information:

CUBIT> group "mat:7" add vol 4 to 18

All volumes must belong to a group, if you want volumes to be filled with vacuum then add them to a group called, "mat:Vacuum":

CUBIT> group "mat:Vacuum" add vol 4 to 18

Assigning Materials by Name (recommended)

OpenMC materials can also be assigned by name:

CUBIT> group "mat:fuel" add vol 4 to 18

For this example, a material with the name attribute "fuel" must be present in OpenMC's materials.xml file:

<materials>
  <material id="40" name="fuel">
    <density units="g/cc" value="11" />
    <nuclide ao="1.0" name="U235" />
  </material>
</materials>

This method for assigning materials is recommended for use with OpenMC as it provides a more verbose description of the material definition and purpose.

This method of assignment also allows for straightforward model conversion to the UWUW workflow without material assignment modification, re-faceting, or re-sealing. To do this one can simply embed a PyNE material library in the original .h5m file at any point using uwuw_preproc. No modification to the material assignments is necessary, provided that materials with appropriate names and definitions are in the PyNE material library. After conversion to a UWUW format, the embedded PyNE material library will be used in place of the materials.xml file.

Note: material names must be unique in the materials.xml file for this style of material assignment to work properly.

Surface boundary conditions

The following boundary conditions available in OpenMC are supported in DAGMC. These are:

vacuum
reflective
transmission

For example, suppose you want to specify that surfaces 10 and 11 should be reflecting surfaces. This command would achieve that:

CUBIT> group "boundary:Reflecting" add surf 10 11

Note: surfaces without a specified boundary condition will be set to transmission .

Problem boundary

The DAGMC model should have a "containing volume" which bounds the volumes of interest. This volume represents a particle "graveyard" or a region where particles are removed from the simulation upon entry. This volume represents the boundary between the problem and the outside world. This volume should surround the entire geometry with a shell of finite thickness. Any geometric shape can be used, but a cubic shell is recommended to maximize performance.

To create a containing volume, make two volumes in Cubit with the same shape and same center with one slightly larger than the other, making sure that both bound the entire problem geometry. Then, subtract the smaller one from the larger one. The result is a containing volume for the problem.

For example, consider a geometry containing 99 volumes, all of which fit inside a cube of side length 99 cm centered at the origin. The following commands would create a valid graveyard for this problem:

CUBIT> create brick x 100             # This will be volume 100
CUBIT> create brick x 105             # This will be volume 101
CUBIT> subtract vol 100 from vol 101  # This will produce volume 102
CUBIT> group "mat:Graveyard" add volume 102

A volume in the mat:Graveyard group will be assigned a void material and its surfaces will be given vacuum boundary conditions when the OpenMC simulation is initialized.

Temperatures

Volume temperatures can be defined in OpenMC using a similar syntax to materials or boundary conditions but with "temp" as the keyword.

To assign a temperature of 900K to a volume one can use the following command.

CUBIT> group "temp:900" add vol x

Note: all temperatures are assumed to be in units of Kelvin in OpenMC.

Implicit complement materials

If you would like to assign a material to the implicit complement, a special procedure is needed. Since the implicit complement doesn't exist before running DAGMC and DAGMC can only recognize groups that contain an entity, the material for the implicit complement must be specified as if it were being specified for the graveyard volume. For example, if you would like the implicit complement to be modeled as material 9, and the graveyard volume is volume 102, the following command should be used:

CUBIT> group "mat:9_comp" add vol 102

DAGMC will recognize that volume 102 is the graveyard volume, and the _comp keyword will trigger it to assign the specified material and density to the implicit complement rather than the containing volume.

Running DAG-OpenMC

The command for running OpenMC is identical to an OpenMC run using native geometry. Certain modifications to the OpenMC input files are required, however. The DAGMC geometry must be available as a .h5m file and provided with dagmc_universe in the geometry.xml file. For more information on how to use DAGMC geometries in OpenMC, see here.