Assigning materials & densities¶

The generic workflow description includes details on Grouping volumes and surfaces, but a specific naming convention is required for DAG-Tripoli4. To define materials, the Tripoli composition name must be provided in the group name. The format for the group name is as follows:

comp_[compname]

[compname] is replaced by the composition name that will be specified in the Tripoli4 input file. [density] is replaced by either the atomic density or the mass density. Like the MCNP cell cards, positive values are atomic densities in [atoms/barn-cm] and negative values are mass densities in [g/cc].

For example, suppose UO2 is composition Uoxide and volumes 4 through 18 consist of this material in the problem. To assign materials to these volumes, the following command would be used:

CUBIT> group "comp_Uoxide" add vol 4 to 18

Note: If a volume is not assigned to a specific group, when run in DAGMC it will be treated as a void; the material for that cell will be zero. This can actually become a fairly useful debugging tool to identify volumes that were not assigned to their appropriate group.

If you would like to assign a material to the implicit complement, you can use the same mechanism, but add the string _comp to the end of the group name. Since DAGMC only recognizes those groups that contain an entity, it is necessary to add a volume to this group, recognizing that this material will NOT be assigned to that volume. (_It is often convenient to assign the graveyard volume (see below) to the implicit complement material group to minimize confusion._) For example, if you would like the explicit complement to be modeled with the composition named “air”:

CUBIT> create group "comp_air_comp"

Defining boundary conditions¶

There are two general classes of boundary condition supported by DAG-Tripoli4. a vacuum boundary and reflecting surfaces, and they are implemented in different ways.

Defining the “graveyard”: vacuum boundaries

A vacuum boundary condition is typically defined in Tripoli4 by simply having a surface with no defined volume on its other side. Since DAGMC’s implicit complement is also defined this way, it is not possible to use this convention in DAG-Tripoli4. Instead, volumes are created on the vacuum side of those same surfaces, but those volumes are placed in a special group, known as the “graveyard” to change their behavior. Any geometric shape can be used for this; however, a cubic shell is often preferred. This shell volume will represent the outside world and will be the cell where all of the particles are terminated (thus it will have an importance of zero).

[need more on implicit complement in context of Tripoli]

To create this volume create two volumes in CUBIT with the same shape, same center, and one slightly larger than the other. Subtract the smaller from the larger. The remaining volume is the graveyard.

Like the material definitions and boundary conditions discussed in the previous section. The graveyard is defined by assigning it a specific group name, one of the following keywords:

graveyard
outside.world
rest.of.world

Consider a geometry with 99 volumes that all fit within a cube centered at the origin with side-length 99 cm. To create a graveyard for this problem in CUBIT, you could issue the following commands:

CUBIT> create brick x 100
CUBIT> create brick x 105
CUBIT> subtract vol 100 from vol 101
CUBIT> group "graveyard" add vol 102

When DAG-Tripoli4 is run, the surfaces of volume 102 (or any other volumes included in the group) will be defined as having only one volume - the one on the OTHER SIDE relative to volume 102. Note: this assumes that the two create brick commands generate volumes numbered 100 and 101, respectively, and that the Boolean subtraction results in a new volume number 102.

If you have boundary conditions (reflecting, white, or periodic) it is not required that you surround them with the bounding volume, but is not incorrect to do so. Only areas where particles should escape need to be enclosed. However, it is often easiest to simply create a single graveyard that covers all directions and volumes of the entire system.

Surface boundary conditions: reflection

Surface boundary conditions are similarly enforced by specifying a group name. This type of attribute (surface boundary condition) is only required if reflective or white boundary conditions are used in the problem. If not, this section may be skipped. Note that periodic boundary conditions are not yet supported.

Specifying reflecting and white boundary conditions are fairly straightforward. The group names for reflecting and white are respectively:

spec.reflect
white.reflect

Suppose surfaces 10 and 11 are reflecting boundary conditions. To specify these as reflecting surfaces, the following group would be created:

CUBIT> group "spec.reflect" add surf 10 11

DAGMC runtime parameters¶

The DAGMC-Tripoli input file is formatted just like any other Tripoli input file but using the DAGMC_GEOMETRY block to indicate the geometry. This block has the following parameters:

These parameters are described in the section on additional_parameters. In addition to many other Tripoli input blocks, it is important to define the material compositions that have been assigned in the previous step.

Running DAG-Tripoli4¶

Running DAGMC-Tripoli is identical to running the standard Tripoli.