Example of use#

The following is an example of how to use GEOUNED to convert a CAD file to a CSG file for use in MCNP. It includes the methodology and rationale behind the process.

Warning

It is assumed that GEOUNED has been properly installed as described in the Quick install guide.

  1. CAD simplification

  2. Export as STEP

  3. Conversion to CSG

  4. Suspicious solids and debug

  5. Void check verification

CAD simplification#

The CAD model of the system should undergo a process of simplification. In this process, features that do not affect the radiation transport should be removed. The level of detail is usually decreased. Surfaces that are not allowed like splines are substituted by simpler surfaces like planes or cylinders.

Export as STEP#

Once the CAD is simplified, it is exported as a STEP file to be used as input by GEOUNED.

Conversion to CSG#

The below code is run in a Python environment with GEOUNED installed. In the comments of the code there is an explanation of the parameters used.

import geouned

settings = geouned.Settings(
    debug=True,  # This will create a folder with the CAD bodies and their decomposition
    startCell=12001,  # The first cell id
    startSurf=12001,  # The first surface id
    compSolids=False,  # False so the different solids of a component
                       # are not combined into a single MCNP cell
    voidGen=True,  # Generate the void cells
)

geo = geouned.CadToCsg(settings=settings)
geo.load_step_file(filename=f"Geometry/model.stp")  # Path to the STEP file
geo.start()
print("Start exporting")
geo.export_csg(
    geometryName="My model",  # Name of the model to appear in the MCNP file
    outFormat=("mcnp",),  # The output format
    UCARD=50,  # Makes the model a filler universe with filler id 50
    volCARD=True,  # The volume card is included in the cell definition
    volSDEF=True,  # Generate a SDEF card
)
print("Exporting finished")

Suspicious solids and debug#

Running the above code will generate a debug folder that contains each of the converted solids, both the original and the decomposed ones. Those bodies can be opened and examined in a CAD software to compare them to the original model. If the decomposed model doesn’t look right, it is possible to manually decompose a specific solid and redo the conversion.

Also, it is possible that a folder called Suspicious solids is created. This folder contains the same solids as the debug folder but only the ones that have been flagged as potentially problematic. This can be useful to quickly identify the problematic solids.

After applying any fix necessary and exporting to STEP, the conversion can be run again.

Void check verification#

Once the conversion is finished, it is recommended to stocastically check the volume of the solids and to make some plots.

In this example, the UCARD was used to make the model a filler universe. For these initial checks the U=50 cards should be commented out as we want to test the standalone model. Plots with the MCNP plotter should be done to check that the model looks as expected.

Also, a void check simulation should be run. The volSDEF=True card automatically generated a spherical inward-oriented source with the correct weight to calculate volumes stochastically. The tallies for the solid MCNP cells were also automatically generated. The user can run the simulation directly and the results of the tallies should all be close to 1.0. If a MCNP cell tally shows a result far from 1.0 it would mean that the volume of the solid is not correct.

Warning

The void check simulation should be run with a high number of particles to achieve an acceptable statistical error. An usually number is 1e9 particles. Always check that the statistical error of the tallies is below 0.1.