Material Tagging

Material Tagging#

Material tags identify each volume in the DAGMC geometry. They connect your CAD geometry to materials in your neutronics simulation.

Tagging Methods#

Method

Description

Best For

Manual Tags

Explicit list of tag strings

Simple models, STEP files

Assembly Names

Use CadQuery assembly part names

CadQuery assemblies

Assembly Materials

Use cq.Material objects

CadQuery 2.6.1+

GMSH Physical Groups

Extract from GMSH mesh

Pre-existing meshes

Quick Comparison#

Feature

Manual Tags

Assembly Names

Assembly Materials

GMSH Groups

Requires ordering

Yes

No

No

No

Auto from geometry

No

Yes

Yes

Yes

Works with STEP

Yes

Yes

Yes

No

CadQuery version

Any

Any

> 2.6.1

Any

How Tags Are Stored#

Material tags are stored in the h5m file with a mat: prefix:

  • "tungsten" becomes "mat:tungsten" in the h5m file

  • "steel" becomes "mat:steel" in the h5m file

When using in OpenMC, reference materials without the prefix:

import openmc

# Material name matches the tag (without "mat:" prefix)
tungsten = openmc.Material(name="tungsten")
tungsten.add_element("W", 1.0)
tungsten.set_density("g/cm3", 19.3)

Tag Name Guidelines#

Warning

Reserved name: Do not use "vacuum" as a material tag. DAGMC and OpenMC treat volumes tagged with "vacuum" as true vacuum (void space with no material). Particles entering a vacuum region will be lost. If you need to model low-density gas, use a different name like "air" or "helium".

Warning

DAGMC truncates material tag names longer than 28 characters. Keep your tags concise.

Recommended practices:

  • Use descriptive but short names: "fuel", "coolant", "shield"

  • Avoid special characters

  • Avoid using "vacuum" (reserved for true void)

  • Use lowercase for consistency

  • Match names to your OpenMC material definitions

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