Hi!
Just wanted to start some basic tests in material decay calculations and got a platform-dependent (“Darwin”) error. The same calculation on Linux works without problems.
Similar cases have been reported (https://openmc.discourse.group/t/depletion-simulation-on-supercomputer-gets-stuck-between-depletion-steps/606/14, Problem running Pincell Depletion Example - #2 by paulromano), but since it is on only struggling on MacOS, I thought I give it a new thread.
My input is
import openmc
import openmc.deplete
import openmc.data
import numpy as np
import math
from pathlib import Path
openmc.deplete.pool.USE_MULTIPROCESSING = True
openmc.deplete.pool.NUM_PROCESSES = 1
t12=166344200.0
dc = math.log(2)/t12
Co60mat = openmc.Material()
Co60mat.add_nuclide('Co60', 1, 'ao')
Co60mat.set_density('atom/cm3',1/dc)
# Create 3D array for microscopic cross sections
micro_xs = openmc.deplete.MicroXS(np.empty((0, 0, 0)), [], [])
# Create depletion operator with no reactions
op = openmc.deplete.IndependentOperator.from_nuclides(
volume=1.0,
nuclides=Co60mat.get_nuclide_atom_densities(),
flux=0.0,
micro_xs=micro_xs,
chain_file=Path(__file__).parents[0]/ "chain_endfb81.xml",
normalization_mode='source-rate'
)
# Create time-integrator and integrate
integrator = openmc.deplete.PredictorIntegrator(
op, timesteps=[t12,t12], source_rates=[0.0, 0.0], timestep_units='s'
)
integrator.integrate(final_step=False)
# Get concentration
results = openmc.deplete.Results('depletion_results.h5')
mat_id = op.materials[0].id
_, newCo60 = results.get_atoms(f"{mat_id}", "Co60")
_, newNi60 = results.get_atoms(f"{mat_id}", "Ni60")
print("Co60:",newCo60)
print("Ni60:",newNi60)
print(results.get_activity(f"{mat_id}"))
and using OpenMC
openmc --version
OpenMC version 0.15.3-dev114
Commit hash: ca63da91b9b0fc0cd144766155b533318d4a71c3
Copyright (c) 2011-2025 MIT, UChicago Argonne LLC, and contributors
MIT/X license at <https://docs.openmc.org/en/latest/license.html>
Build type: RelWithDebInfo
Compiler ID: GNU 15.1.0
MPI enabled: no
Parallel HDF5 enabled: no
PNG support: yes
DAGMC support: no
libMesh support: no
MCPL support: no
Coverage testing: no
Profiling flags: no
UWUW support: no
on a MacBook with Silicon M1 chip using the gcc-suite from homebrew.
The error is (shortened)
uv run bug_pool.py
[openmc.deplete] t=0.0 s, dt=166344200.0 s, source=0.0
/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/uncertainties/core.py:1024: UserWarning: Using UFloat objects with std_dev==0 may give unexpected results.
warn("Using UFloat objects with std_dev==0 may give unexpected results.")
[openmc.deplete] t=0.0 s, dt=166344200.0 s, source=0.0
/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/uncertainties/core.py:1024: UserWarning: Using UFloat objects with std_dev==0 may give unexpected results.
warn("Using UFloat objects with std_dev==0 may give unexpected results.")
Traceback (most recent call last):
File "<string>", line 1, in <module>
from multiprocessing.spawn import spawn_main; spawn_main(tracker_fd=7, pipe_handle=18)
~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 122, in spawn_main
exitcode = _main(fd, parent_sentinel)
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 131, in _main
prepare(preparation_data)
~~~~~~~^^^^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 246, in prepare
_fixup_main_from_path(data['init_main_from_path'])
~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 297, in _fixup_main_from_path
main_content = runpy.run_path(main_path,
run_name="__mp_main__")
File "<frozen runpy>", line 287, in run_path
File "<frozen runpy>", line 98, in _run_module_code
File "<frozen runpy>", line 88, in _run_code
File "/Users/becker/Entwicklung/MakeChainE81/bug_pool.py", line 36, in <module>
integrator.integrate(final_step=False)
~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/abc.py", line 893, in integrate
proc_time, n_list, res_list = self(n, res.rates, dt, source_rate, i)
~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/integrators.py", line 56, in __call__
proc_time, n_end = self._timed_deplete(n, rates, dt, _i)
~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/abc.py", line 744, in _timed_deplete
results = deplete(
self._solver, self.chain, n, rates, dt, i, matrix_func,
self.transfer_rates, self.external_source_rates)
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/pool.py", line 190, in deplete
with Pool(NUM_PROCESSES) as pool:
~~~~^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/context.py", line 119, in Pool
return Pool(processes, initializer, initargs, maxtasksperchild,
context=self.get_context())
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/pool.py", line 215, in __init__
self._repopulate_pool()
~~~~~~~~~~~~~~~~~~~~~^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/pool.py", line 306, in _repopulate_pool
return self._repopulate_pool_static(self._ctx, self.Process,
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
self._processes,
^^^^^^^^^^^^^^^^
...<3 lines>...
self._maxtasksperchild,
^^^^^^^^^^^^^^^^^^^^^^^
self._wrap_exception)
^^^^^^^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/pool.py", line 329, in _repopulate_pool_static
w.start()
~~~~~~~^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 121, in start
self._popen = self._Popen(self)
~~~~~~~~~~~^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/context.py", line 289, in _Popen
return Popen(process_obj)
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/popen_spawn_posix.py", line 32, in __init__
super().__init__(process_obj)
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/popen_fork.py", line 20, in __init__
self._launch(process_obj)
~~~~~~~~~~~~^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/popen_spawn_posix.py", line 42, in _launch
prep_data = spawn.get_preparation_data(process_obj._name)
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 164, in get_preparation_data
_check_not_importing_main()
~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 140, in _check_not_importing_main
raise RuntimeError('''
...<16 lines>...
''')
RuntimeError:
An attempt has been made to start a new process before the
current process has finished its bootstrapping phase.
This probably means that you are not using fork to start your
child processes and you have forgotten to use the proper idiom
in the main module:
if __name__ == '__main__':
freeze_support()
...
The "freeze_support()" line can be omitted if the program
is not going to be frozen to produce an executable.
To fix this issue, refer to the "Safe importing of main module"
section in https://docs.python.org/3/library/multiprocessing.html
[openmc.deplete] t=0.0 s, dt=166344200.0 s, source=0.0
/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/uncertainties/core.py:1024: UserWarning: Using UFloat objects with std_dev==0 may give unexpected results.
warn("Using UFloat objects with std_dev==0 may give unexpected results.")
Traceback (most recent call last):
File "<string>", line 1, in <module>
from multiprocessing.spawn import spawn_main; spawn_main(tracker_fd=7, pipe_handle=18)
~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 122, in spawn_main
exitcode = _main(fd, parent_sentinel)
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 131, in _main
prepare(preparation_data)
~~~~~~~^^^^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 246, in prepare
_fixup_main_from_path(data['init_main_from_path'])
~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 297, in _fixup_main_from_path
main_content = runpy.run_path(main_path,
run_name="__mp_main__")
File "<frozen runpy>", line 287, in run_path
File "<frozen runpy>", line 98, in _run_module_code
File "<frozen runpy>", line 88, in _run_code
File "/Users/becker/Entwicklung/MakeChainE81/bug_pool.py", line 36, in <module>
integrator.integrate(final_step=False)
~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/abc.py", line 893, in integrate
proc_time, n_list, res_list = self(n, res.rates, dt, source_rate, i)
~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/integrators.py", line 56, in __call__
proc_time, n_end = self._timed_deplete(n, rates, dt, _i)
~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/abc.py", line 744, in _timed_deplete
results = deplete(
self._solver, self.chain, n, rates, dt, i, matrix_func,
self.transfer_rates, self.external_source_rates)
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/pool.py", line 190, in deplete
with Pool(NUM_PROCESSES) as pool:
~~~~^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/context.py", line 119, in Pool
return Pool(processes, initializer, initargs, maxtasksperchild,
context=self.get_context())
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/pool.py", line 215, in __init__
self._repopulate_pool()
~~~~~~~~~~~~~~~~~~~~~^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/pool.py", line 306, in _repopulate_pool
return self._repopulate_pool_static(self._ctx, self.Process,
~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
self._processes,
^^^^^^^^^^^^^^^^
...<3 lines>...
self._maxtasksperchild,
^^^^^^^^^^^^^^^^^^^^^^^
self._wrap_exception)
^^^^^^^^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/pool.py", line 329, in _repopulate_pool_static
w.start()
~~~~~~~^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/process.py", line 121, in start
self._popen = self._Popen(self)
~~~~~~~~~~~^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/context.py", line 289, in _Popen
return Popen(process_obj)
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/popen_spawn_posix.py", line 32, in __init__
super().__init__(process_obj)
~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/popen_fork.py", line 20, in __init__
self._launch(process_obj)
~~~~~~~~~~~~^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/popen_spawn_posix.py", line 42, in _launch
prep_data = spawn.get_preparation_data(process_obj._name)
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 164, in get_preparation_data
_check_not_importing_main()
~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/spawn.py", line 140, in _check_not_importing_main
raise RuntimeError('''
...<16 lines>...
''')
RuntimeError:
An attempt has been made to start a new process before the
current process has finished its bootstrapping phase.
This probably means that you are not using fork to start your
child processes and you have forgotten to use the proper idiom
in the main module:
if __name__ == '__main__':
freeze_support()
...
The "freeze_support()" line can be omitted if the program
is not going to be frozen to produce an executable.
To fix this issue, refer to the "Safe importing of main module"
section in https://docs.python.org/3/library/multiprocessing.html
^CTraceback (most recent call last):
File "/Users/becker/Entwicklung/MakeChainE81/bug_pool.py", line 36, in <module>
integrator.integrate(final_step=False)
~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/abc.py", line 893, in integrate
proc_time, n_list, res_list = self(n, res.rates, dt, source_rate, i)
~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/integrators.py", line 56, in __call__
proc_time, n_end = self._timed_deplete(n, rates, dt, _i)
~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/abc.py", line 744, in _timed_deplete
results = deplete(
self._solver, self.chain, n, rates, dt, i, matrix_func,
self.transfer_rates, self.external_source_rates)
File "/Users/becker/Entwicklung/MakeChainE81/.venv/lib/python3.13/site-packages/openmc/deplete/pool.py", line 191, in deplete
n_result = list(pool.starmap(func, inputs))
~~~~~~~~~~~~^^^^^^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/pool.py", line 375, in starmap
return self._map_async(func, iterable, starmapstar, chunksize).get()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/pool.py", line 768, in get
self.wait(timeout)
~~~~~~~~~^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/multiprocessing/pool.py", line 765, in wait
self._event.wait(timeout)
~~~~~~~~~~~~~~~~^^^^^^^^^
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/threading.py", line 659, in wait
signaled = self._cond.wait(timeout)
File "/opt/homebrew/Cellar/python@3.13/3.13.7/Frameworks/Python.framework/Versions/3.13/lib/python3.13/threading.py", line 359, in wait
waiter.acquire()
~~~~~~~~~~~~~~^^
KeyboardInterrupt
When I set openmc.deplete.pool.USE_MULTIPROCESSING to False, no problem occurs.
Should I raise a bug report at Github?