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adding trigger to mgxs convert #3925

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bb65889
adding trigger to mgxs convert
shimwell Apr 21, 2026
daee6d4
Merge remote-tracking branch 'origin/develop' into trigger-mgxs-gener…
jon-proximafusion Jun 23, 2026
9f1dea8
Auto-tune MGXS generation: nparticles=None converges via a tally trigger
jon-proximafusion Jun 23, 2026
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97 changes: 68 additions & 29 deletions openmc/model/model.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1871,6 +1871,7 @@ def differentiate_mats(self, diff_volume_method: str = None, depletable_only: bo
def _auto_generate_mgxs_lib(
model: openmc.model.Model,
groups: openmc.mgxs.EnergyGroups,
nparticles: int | None,
correction: str | None,
directory: PathLike,
) -> openmc.mgxs.Library:
Expand All @@ -1882,8 +1883,9 @@ def _auto_generate_mgxs_lib(
----------
groups : openmc.mgxs.EnergyGroups
Energy group structure for the MGXS.
nparticles : int
Number of particles to simulate per batch when generating MGXS.
nparticles : int or None
Number of particles to simulate per batch when generating MGXS, or
None to auto-tune the count via a cross-section convergence trigger.
mgxs_path : str
Filename for the MGXS HDF5 file.
correction : str
Expand Down Expand Up @@ -1933,6 +1935,35 @@ def _auto_generate_mgxs_lib(
# Check the library - if no errors are raised, then the library is satisfactory.
mgxs_lib.check_library_for_openmc_mgxs()

# Determine particle count / convergence behaviour. When nparticles is
# None the generation run is auto-tuned: rather than a fixed (guessed)
# particle count, a relative-error trigger is attached to every
# cross-section tally so the continuous-energy run extends batches until
# the group constants are statistically converged (capped internally).
# This keeps nparticles off the user knob list while avoiding both
# under-resolved and needlessly over-resolved libraries.
if nparticles is None:
# Internal auto-tune defaults (not user-facing arguments):
seed_particles = 2000 # particles per batch to start from
target_rel_err = 1.0e-2 # converge cross-section tallies to 1%
max_batches = 1000 # safety cap on the batch count
model.settings.particles = seed_particles
# ignore_zeros so structurally-empty bins (e.g. nu-fission in
# non-fissile materials or unpopulated fast groups) neither trip the
# trigger prematurely nor stall convergence. The trigger is applied
# (via Library.tally_trigger, before build_library) to the flux and
# reaction-rate tallies that form each group constant, so converging
# both converges the cross section.
mgxs_lib.tally_trigger = openmc.Trigger(
'rel_err', target_rel_err, ignore_zeros=True)
model.settings.trigger_active = True
# settings.batches is the minimum batch count; bound the upper end
# so a poorly-converging material cannot run unbounded.
model.settings.trigger_max_batches = max(
max_batches, model.settings.batches)
else:
model.settings.particles = nparticles

# Construct all tallies needed for the multi-group cross section library
mgxs_lib.build_library()

Expand Down Expand Up @@ -2037,7 +2068,7 @@ def _create_mgxs_sources(
def _isothermal_infinite_media_mgxs(
material: openmc.Material,
groups: openmc.mgxs.EnergyGroups,
nparticles: int,
nparticles: int | None,
correction: str | None,
directory: PathLike,
source: openmc.IndependentSource,
Expand All @@ -2053,8 +2084,9 @@ def _isothermal_infinite_media_mgxs(
The material to generate MGXS for
groups : openmc.mgxs.EnergyGroups
Energy group structure for the MGXS.
nparticles : int
Number of particles to simulate per batch when generating MGXS.
nparticles : int or None
Number of particles to simulate per batch when generating MGXS, or
None to auto-tune the count via a cross-section convergence trigger.
correction : str
Transport correction to apply to the MGXS. Options are None and
"P0".
Expand Down Expand Up @@ -2084,7 +2116,6 @@ def _isothermal_infinite_media_mgxs(

# Settings
model.settings.batches = 100
model.settings.particles = nparticles

model.settings.source = source

Expand All @@ -2104,7 +2135,7 @@ def _isothermal_infinite_media_mgxs(

# Generate MGXS
mgxs_lib = Model._auto_generate_mgxs_lib(
model, groups, correction, directory)
model, groups, nparticles, correction, directory)

if temperature is not None:
return mgxs_lib.get_xsdata(domain=material, xsdata_name=name,
Expand All @@ -2115,7 +2146,7 @@ def _isothermal_infinite_media_mgxs(
def _generate_infinite_medium_mgxs(
self,
groups: openmc.mgxs.EnergyGroups,
nparticles: int,
nparticles: int | None,
mgxs_path: PathLike,
correction: str | None,
directory: PathLike,
Expand Down Expand Up @@ -2150,8 +2181,9 @@ def _generate_infinite_medium_mgxs(
----------
groups : openmc.mgxs.EnergyGroups
Energy group structure for the MGXS.
nparticles : int
Number of particles to simulate per batch when generating MGXS.
nparticles : int or None
Number of particles to simulate per batch when generating MGXS, or
None to auto-tune the count via a cross-section convergence trigger.
mgxs_path : str
Filename for the MGXS HDF5 file.
correction : str
Expand Down Expand Up @@ -2314,7 +2346,7 @@ def _create_stochastic_slab_geometry(
def _isothermal_stochastic_slab_mgxs(
stoch_geom: openmc.Geometry,
groups: openmc.mgxs.EnergyGroups,
nparticles: int,
nparticles: int | None,
correction: str | None,
directory: PathLike,
source: openmc.IndependentSource,
Expand All @@ -2331,8 +2363,9 @@ def _isothermal_stochastic_slab_mgxs(
The stochastic slab geometry.
groups : openmc.mgxs.EnergyGroups
Energy group structure for the MGXS.
nparticles : int
Number of particles to simulate per batch when generating MGXS.
nparticles : int or None
Number of particles to simulate per batch when generating MGXS, or
None to auto-tune the count via a cross-section convergence trigger.
correction : str
Transport correction to apply to the MGXS. Options are None and
"P0".
Expand Down Expand Up @@ -2364,7 +2397,6 @@ def _isothermal_stochastic_slab_mgxs(
# Settings
model.settings.batches = 200
model.settings.inactive = 100
model.settings.particles = nparticles
model.settings.output = {'summary': True, 'tallies': False}
model.settings.temperature = temperature_settings

Expand All @@ -2378,7 +2410,7 @@ def _isothermal_stochastic_slab_mgxs(

# Generate MGXS
mgxs_lib = Model._auto_generate_mgxs_lib(
model, groups, correction, directory)
model, groups, nparticles, correction, directory)

# Fetch all of the isothermal results.
if temperature is not None:
Expand All @@ -2396,7 +2428,7 @@ def _isothermal_stochastic_slab_mgxs(
def _generate_stochastic_slab_mgxs(
self,
groups: openmc.mgxs.EnergyGroups,
nparticles: int,
nparticles: int | None,
mgxs_path: PathLike,
correction: str | None,
directory: PathLike,
Expand All @@ -2420,8 +2452,9 @@ def _generate_stochastic_slab_mgxs(
----------
groups : openmc.mgxs.EnergyGroups
Energy group structure for the MGXS.
nparticles : int
Number of particles to simulate per batch when generating MGXS.
nparticles : int or None
Number of particles to simulate per batch when generating MGXS, or
None to auto-tune the count via a cross-section convergence trigger.
mgxs_path : str
Filename for the MGXS HDF5 file.
correction : str
Expand Down Expand Up @@ -2521,7 +2554,7 @@ def _generate_stochastic_slab_mgxs(
def _isothermal_materialwise_mgxs(
input_model: openmc.Model,
groups: openmc.mgxs.EnergyGroups,
nparticles: int,
nparticles: int | None,
correction: str | None,
directory: PathLike,
temperature_settings: dict,
Expand All @@ -2541,8 +2574,9 @@ def _isothermal_materialwise_mgxs(
The model to use when computing material-wise MGXS.
groups : openmc.mgxs.EnergyGroups
Energy group structure for the MGXS.
nparticles : int
Number of particles to simulate per batch when generating MGXS.
nparticles : int or None
Number of particles to simulate per batch when generating MGXS, or
None to auto-tune the count via a cross-section convergence trigger.
correction : str
Transport correction to apply to the MGXS. Options are None and
"P0".
Expand Down Expand Up @@ -2572,13 +2606,12 @@ def _isothermal_materialwise_mgxs(
# Settings
model.settings.batches = 200
model.settings.inactive = 100
model.settings.particles = nparticles
model.settings.output = {'summary': True, 'tallies': False}
model.settings.temperature = temperature_settings

# Generate MGXS
mgxs_lib = Model._auto_generate_mgxs_lib(
model, groups, correction, directory)
model, groups, nparticles, correction, directory)

# Fetch all of the isothermal results.
if temperature is not None:
Expand All @@ -2596,7 +2629,7 @@ def _isothermal_materialwise_mgxs(
def _generate_material_wise_mgxs(
self,
groups: openmc.mgxs.EnergyGroups,
nparticles: int,
nparticles: int | None,
mgxs_path: PathLike,
correction: str | None,
directory: PathLike,
Expand All @@ -2617,8 +2650,9 @@ def _generate_material_wise_mgxs(
----------
groups : openmc.mgxs.EnergyGroups
Energy group structure for the MGXS.
nparticles : int
Number of particles to simulate per batch when generating MGXS.
nparticles : int or None
Number of particles to simulate per batch when generating MGXS, or
None to auto-tune the count via a cross-section convergence trigger.
mgxs_path : PathLike
Filename for the MGXS HDF5 file.
correction : str
Expand Down Expand Up @@ -2688,7 +2722,7 @@ def convert_to_multigroup(
self,
method: str = "material_wise",
groups: str | Sequence[float] | openmc.mgxs.EnergyGroups = "CASMO-2",
nparticles: int = 2000,
nparticles: int | None = None,
overwrite_mgxs_library: bool = False,
mgxs_path: PathLike = "mgxs.h5",
correction: str | None = None,
Expand All @@ -2712,8 +2746,13 @@ def convert_to_multigroup(
(e.g., ``"CASMO-2"``), or a sequence of floats specifying energy
bin boundaries in eV (e.g., ``[0.0, 1e6]`` for a single group).
Defaults to ``"CASMO-2"``.
nparticles : int, optional
Number of particles to simulate per batch when generating MGXS.
nparticles : int or None, optional
Number of particles to simulate per batch when generating MGXS. By
default (``None``) the generation run is auto-tuned: a relative-error
trigger is attached to the cross-section tallies so the simulation
runs until the group constants are statistically converged, rather
than for a fixed (guessed) number of particles. Pass an integer to
instead run a fixed number of particles per batch.
overwrite_mgxs_library : bool, optional
Whether to overwrite an existing MGXS library file.
mgxs_path : str, optional
Expand Down
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