Even though the website says to use the docker, we'll make use of apptainer, because is more friendly and is available on all platforms (LCLRS and MERLIN). Docker works as well, but I find it cumbersome.

This will make a sif image for the container to run from, localy in the folder

apptainer pull openmc.sif docker://openmc/openmc:latest

To run use this script to call apptainer opemnc container scripts/openmc_run.sh. NOTE: Update SIF and LIB_DIR paths as needed !

# ================================================================
#  OpenMC + Python Runner (Apptainer-based)
# ================================================================
#  Description:
#    This script runs either:
#      (1) OpenMC directly inside an Apptainer container, or
#      (2) a Python script using the same container environment.
#
#  Usage:
#    ./run_openmc.sh                → Run OpenMC in the container
#    ./run_openmc.sh script.py args → Run Python script with args
#    ./run_openmc.sh -h | --help    → Show help
#
#  Behavior:
#    - The current working directory is mounted into /workspace inside the container.
#    - The OpenMC library directory is mounted into /libs.
#    - The output log is saved in the same directory as the run:
#         * openmc_run.log  for OpenMC runs
#         * <scriptname>.log for Python runs
#
#  Example:
#    ./run_openmc.sh               # Run OpenMC normally
#    ./run_openmc.sh input.py      # API can be called inside the script

Making OpenMC work on WSL/LCLRS/MERLIN via Apptainer coverted from Docker Image

You can do it on your own machine (WSL)

The long way

docker pull -it --name=develop-dagmc-libmesh openmc/openmc:develop-dagmc-libmesh
docker save openmc/openmc:develop-dagmc-libmesh -o openmc_develop_dagmc_libmesh_docker_img.tar
apptainer build openmc_from_tar.sif docker-archive:openmc_develop_dagmc_libmesh_docker_img.tar

The short way

apptainer build openmc_develop_dagmc_libmesh_from_docker_img.sif docker://openmc/openmc:develop-dagmc-libmesh

LCLRS

# use the scratch disk so apptainer does not throw errors
BUILDROOT=/scratch/$USER/apptainer_openmc_build
mkdir -p "$BUILDROOT"/{tmp,cache}

export APPTAINER_TMPDIR="$BUILDROOT/tmp"
export APPTAINER_CACHEDIR="$BUILDROOT/cache"

# build 
apptainer build openmc_develop_dagmc_libmesh_from_docker_img.sif docker://openmc/openmc:develop-dagmc-libmesh

Merlin 7

nope

Tests

Use these test files here Tests If you need a quick lib, I left one on the scrach at S:\stafie_c\openmc_libs.

OpenMC libraries

Nuclear library status

NOTE: this is a work in progress, and currently not hosted on a public folder.

Setting the Cross Section Library in OpenMC

By default, OpenMC locates the cross section library using the environment variable OPENMC_CROSS_SECTIONS. But avoid doing this inside long-term scripts — it’s better to keep the path explicit in your model setup.

import os
os.environ['OPENMC_CROSS_SECTIONS'] = f"/libs/e80_hdf5/cross_sections.xml"

However, you can specify the library path directly in your Python script or XML files — no need to rely on environment variables.

Option 1 — Use Materials.cross_sections

You can embed the path into your exported materials.xml file:

materials.cross_sections = '/absolute/path/to/cross_sections.xml'
materials.export_to_xml()

This adds the following line inside your generated XML: <cross_sections>/absolute/path/to/cross_sections.xml</cross_sections>

Option 2 — Use openmc.config

For newer OpenMC versions, you can configure the cross-section file at runtime:

openmc.config['cross_sections'] = '/absolute/path/to/cross_sections.xml'

This temporarily overrides any environment variable and is useful for workflows or notebooks where you don’t want to modify global settings.

NOTE: Depending on the OpenMC version, openmc.config['cross_sections'] may not be automatically written to XML, in that case, use option 1 to ensure reproducibility.

Find more here

OpenMC Documentation – Using Cross Section Data

Python API: openmc.Materials.cross_sections

Tutorial alternatives

The tutorial on the website is nice but there are some alternatives:

Hands-on OpenMC introduction by ICTP Science, Technology and Innovation. It has 4 videos that are quite useful, you can find the workshop resources here:
- ICTP website
- Git with the files
OpenMC demonstration by the Centre for Energy Research & Institute of Nuclear Techniques.

README.md Unescape Escape

OpenMC

Make your own env and compile it

Quick Install Guide

Making OpenMC work on WSL/LCLRS/MERLIN via Apptainer