OpenMC Guide

❮ TalliesStatePoint Files ❯

Running Simulations in OpenMC

Running Your First Simulation

The simplest way to run a simulation is through OpenMC's Model class:

Basic Run

python
# Create model from components we made earlier
model = openmc.Model(
    geometry=geometry,     # From geometry creation
    materials=materials,   # From materials definition
    settings=settings,     # From settings setup
    tallies=tallies       # From tallies setup
)

# Run the simulation
model.run()
caalh/ReactorMCBrowse files

Tutorial snippet — no separate file in examples repo

Tip: Always start with a small number of particles (e.g., 1000) to test your model before running long simulations. This helps catch errors quickly.

Common Run Options

Several run-time options are available:

Debug Mode

python
# Run with geometry debugging
model.run(output=True)  # Show output
model.run(geometry_debug=True)  # Check geometry
caalh/ReactorMCBrowse files

Tutorial snippet — no separate file in examples repo

Parallel Run

python
# Run with 4 processors
model.run(mpi_args=['mpiexec', '-n', '4'])

# Set thread count
model.run(threads=2)
caalh/ReactorMCBrowse files

Tutorial snippet — no separate file in examples repo

Understanding Output

OpenMC produces several output files:

Key Files

  • statepoint.*.h5: Results
  • tallies.out: Tally data
  • summary.h5: Model info

Optional Output Files

  • particle_*.h5: Particle tracks (only created when particle tracking is enabled via settings.track = [...])

Common Issues

  • Geometry errors
  • Lost particles
  • Memory limits
  • Convergence issues

Reading Results

python
# Load results (from previous section)
sp = openmc.StatePoint('statepoint.100.h5')

# Get k-effective (for criticality runs)
keff = sp.keff
print(f'k-effective = {keff.nominal_value:.5f} ± {keff.std_dev:.5f}')

# Get tally results
tally = sp.get_tally(name='flux')
mean = tally.mean
rel_err = tally.std_dev / tally.mean
caalh/ReactorMCBrowse files

Tutorial snippet — no separate file in examples repo

Best Practices

Running Tips

  • Start with few particles to test setup
  • Use geometry debug mode to check for errors
  • Monitor memory usage for large models
  • Save statepoints periodically for long runs
  • Use parallel processing for speed when needed

Additional Resources

Related Topics

❮ TalliesStatePoint Files ❯