This release of OpenMC includes many bug fixes, performance improvements, and several notable new features. The major highlights of this release include a new openmc.deplete.R2SManager class that automates the workflow for rigorous 2-step (R2S) shutdown dose rate calculations, the ability to collect higher moments for tally results that can be used to test normality, a new uncertainty-aware criticality search method, a new collision tracking feature that enables detailed tracking of particle interactions, support for distributed cell densities, and several new tally filters. The random ray solver also continues to receive significant updates, including automatic setup capabilities, improved geometry handling, and better weight window support. Depletion capabilities have been expanded with thermochemical redox control, external transfer rates, and improved performance.

Compatibility Notes and Deprecations

MCPL has been changed from a build-time dependency to a runtime optional dependency, which means OpenMC will attempt to load the MCPL library at runtime when needed rather than requiring it at build time.

The openmc.mgxs.Library.add_to_tallies_file method has been renamed to openmc.mgxs.Library.add_to_tallies.

New Features

A new collision tracking feature enables detailed tracking of particle interactions (#3417)
Added openmc.Model.keff_search method for automated criticality searches (#3569)
Introduced automated workflow for mesh- or cell-based R2S calculations (#3508)
Ability to source electron/positrons directly for charged particle simulations (#3404)
Multi-group capability for kinetics parameter calculations with Iterated Fission Probability (#3425)
Introduced a new openmc.MeshMaterialFilter class (#3406)
Added support for distributed cell densities (#3546)
Implemented a openmc.WeightWindowsList class that enables export to HDF5 (#3456)
Added openmc.Material.mean_free_path method (#3469)
Introduced openmc.lib.TemporarySession context manager (#3475)
Added material depletion function for tracking individual material depletion (#3420)
Added methods on openmc.Material class for waste disposal rating / classification (#3366, #3376)
Support for thermochemical redox control transfer rates in depletion (#2783)
Support for external transfer rates source term in depletion (#3088)
Added combing capability for fission site sampling and delayed neutron emission time (#2992)
Ability to specify reference direction for azimuthal angle in openmc.stats.PolarAzimuthal distribution (#3582)
Allow spatial constraints on element sources within openmc.MeshSource (#3431)
Added VTK HDF (.vtkhdf) format support for writing VTK data (#3252)
Implemented filter weight capability (#3345)
Optionally collect higher moments for tallies (#3363)
Several random ray solver enhancements:
- Random Ray AutoMagic Setup for automatic configuration (#3351)
- Point source locator for random ray mode (#3360)
- Support for DAGMC geometries (#3374)
- Optimized mapping of source regions to tallies (#3465)
- Base source region refactor (#3576)

Bug Fixes and Small Changes

Add two MPI barriers in R2S workflow (#3646)
Fix a few warnings, rename add_to_tallies_file (#3639)
Fix typo in DAGMC lost particle test (#3634)
Avoid multiprocessing Pool when running depletion tests with MPI (#3633)
Support MPI parallelism in R2SManager (#3632)
Update documentation for particle tracks (#3627)
Adding variance of variance and normality tests for tally statistics (#3454)
Avoid divide-by-zero in from_multigroup_flux when flux is zero (#3624)
Write particle states as separate lines in track VTK files (#3628)
Reset DAGMC history when reviving from source (#3601)
Add energy group structure: SCALE-999 (#3564)
Fix bug in normalization of tally results with no_reduce (#3619)
Enable nuclide filters with get_decay_photon_energy (#3614)
Update check_type calls to accept both str and os.PathLike objects (#3618)
Speed up apply_time_correction by reducing file I/O and deepcopies (#3617)
FW-CADIS Disregard Max Realizations Setting (#3616)
Random Ray Geometry Debug Mode Fix (#3615)
Don't write reaction rates in depletion results by default (#3609)
Allow Path objects in MGXSLibrary.export_to_hdf5 (#3608)
Clip mixture distributions based on mean times integral (#3603)
Allow V0 in atomic_mass function (for ENDF/B-VII.0 data) (#3607)
Re-run flaky tests when needed (#3604)
Ability to load mesh objects from weight_windows.h5 file (#3598)
Switch to using coveralls github action for reporting (#3594)
Add user setting for free gas threshold (#3593)
Speed up time correction factors (#3592)
Fix caching issue when using NCrystal materials (#3538)
Fix random ray source region mesh export when using model.export_to_xml() (#3579)
Ensure weight_windows_file information is read from XML (#3587)
Add missing documentation on <source> in depletion chain file format (#3590)
Adding tally filter type option to statepoint get_tally (#3584)
Optional separation of mesh-material-volume calc from get_homogenized_materials (#3581)
Fix IFP implementation (#3580)
Remove several TODOs related to C++17 support (#3574)
Fix performance regression in libMesh unstructured mesh tallies (#3577)
Update find_package calls in OpenMCConfig.cmake (#3572)
Ensure n_dimension_ attribute is set for unstructured meshes (#3575)
Allow newer Sphinx version and fix docbuild warnings (#3571)
Fixed a bug when combining TimeFilter, MeshFilter, and tracklength estimator (#3525)
PowerLaw raises an error if sampling interval contains negative values (#3542)
depletion: fix performance of chain matrix construction (#3567)
Do not apply boundary conditions when initialized in volume calculation mode (#3562)
Bump up tolerance for flaky activation test (#3560)
Fixed a bug in plotting cross sections with S(a,b) data (#3558)
Change test order to run unit tests first (#3533)
adding ecco 33 (#3556)
Refactor endf_data to be a fixture (#3539)
Revert "fix broken CI" (#3554)
fix broken CI (#3551)
Leverage particle.move_distance in event advance (#3544)
fix tests that accidentaly got broken (#3543)
not printing nuclides with 0 percent to terminal (option 2 ) (#3448)
Fix a bug in time cutoff behavior (#3526)
Avoid duplicate materials written to XML (#3536)
Use cached property for openmc.data.Decay.sources (#3535)
more helpful error message for dose_coefficients (#3534)
Adding 616 group structure (#3531)
Remove unused special accessors for tallies (#3527)
Consistent XML parsing using functions from _xml module (#3517)
Add stat:sum field to MCPL files for proper weight normalization (#3522)
Remove reorder_attributes from openmc._xml (#3519)
fixed a bug in MeshMaterialFilter.from_volumes (#3520)
Fixed a bug in distribcell offsets logic (#3424)
Add test for FW-CADIS based WW generation on a DAGMC model (#3504)
Fix for Weight Window Scaling Bug (#3511)
Fix: materials, plots, and tallies cannot be passed as lists (#3513)
Allow already-initialized openmc.lib in TemporarySession (#3505)
Update DAGMC and libMesh precompiler definitions (#3510)
Avoid adding ParentNuclideFilter twice when calling prepare_tallies (#3506)
Enabling MCPL source files to be read when using surf_source_read (#3472)
Boundary info accessors (#3496)
automatically finding appropriate dimension when making regular mesh from domain (#3468)
Add accessor methods for LocalCoord (#3494)
Make MCPL a Runtime Optional Dependency (#3429)
Use auto-chunking for StepResult HDF5 writing (#3498)
Provide a way to get ID maps from plot parameters on the Model class (#3481)
Update OSX install instructions to point to x64 platform (#3501)
Update conda install instructions for macOS Apple silicon (#3488)
Only show warning if in restart mode (#3478)
Add flag to CMakeLists to use submodules instead of searching (#3480)
Added citation metadata file (#3409)
fix zam parsing (#3484)
Support flux collapse method in get_microxs_and_flux (#3466)
Stabilize Adjoint Source (#3476)
Refactor and Harden Configuration Management (#3461)
Updated Docs to Not Give Specific Python Version Requirement (#3473)
Parallelization of Weight Window Update (#3467)
Limit Random Ray Weight Window Generation to Final Batch (#3464)
Fix Dockerfile DAGMC build (#3463)
Fix Weight Window Infinite Loop Bug (#3457)
Weight Window Birth Scaling (#3459)
Adding checks to geometry.plot to avoid material name overlaps (#3458)
Fixing crash when calling Geometry.plot when DAGMCUniverse in geometry (#3455)
fixing expansion of elemental Ta bug (#3443)
Prevent Adjoint Sources from Trending towards Infinity (#3449)
adding plot function to DAGMCUnvierse (#3451)
Allow specifying number of equiprobable angles for thermal scattering data generation (#3346)
Change Dockerfile from debian:bookworm-slim to ubuntu:24.04 (#3442)
Fix Resetting of Auto IDs When Generating MGXS (#3437)
Allowing chain_file to be chain object to save reloading time (#3436)
update units for flux (#3441)
Fix raytrace infinite loop (#3423)
Apply Max Number of Events Check to Random Rays (#3438)
Add user setting for source rejection fraction (#3433)
Adding fix and tests for spherical mesh as spatial distribution (#3428)
Random Ray Missed Cell Policy Change for Adjoint Mode (#3434)
Random Ray External Source Plotting Fix (#3430)
Avoid negative heating values during pair production and bremsstrahlung (#3426)
Fix no serialization of periodic_surface_id bug (#3421)
Update _get_start_data to always grab the beginning of timestep time (#3414)
Fixed a bug in charged particle energy deposition (#3416)
Fix bug where the same mesh is written multiple times to settings.xml (#3418)
small typo - spelling of Debian (#3411)
added test for dagmc geometry plot (#3375)
Random Ray Misc Memory Error Fixes (#3405)
added type hints to model file (#3399)
Apply resolve paths to path values in config (#3400)
Fixing an incorrect computation of CDF of bremsstrahlung photons (#3396)
Fix weight modification for uniform source sampling (#3395)
Updates to VTK data checks (#3371)
Map Compton subshell data to atomic relaxation data (#3392)
Skip atomic relaxation if binding energy is larger than photon energy (#3391)
Fix extremely large yields from Bremsstrahlung (#3386)
corrected tally name in D1S example (#3383)
Install MCPL using same build type as OpenMC in CI (#3388)
using reduce chain level to remove need for reduce chain (#3377)
Fix negative distances from bins_crossed for CylindricalMesh (#3370)
Add check for equal value bins in an EnergyFilter (#3372)
Fix for Issue Loading MGXS Data Files with LLVM 20 or Newer (#3368)
Report plot ID instead of index for unsupported plot types in random ray mode (#3361)
Handle Missing Tags in Versioning by Setting Default to 0 (#3359)
added kg units to doc string in results class (#3358)

Compatibility Notes and Deprecations

The openmc.mgxs.Library.add_to_tallies_file method has been renamed to openmc.mgxs.Library.add_to_tallies.

New Features

A new collision tracking feature enables detailed tracking of particle interactions (#3417)

Added openmc.Model.keff_search method for automated criticality searches (#3569)

Introduced automated workflow for mesh- or cell-based R2S calculations (#3508)

Ability to source electron/positrons directly for charged particle simulations (#3404)

Multi-group capability for kinetics parameter calculations with Iterated Fission Probability (#3425)

Introduced a new openmc.MeshMaterialFilter class (#3406)

Added support for distributed cell densities (#3546)

Implemented a openmc.WeightWindowsList class that enables export to HDF5 (#3456)

Added openmc.Material.mean_free_path method (#3469)

Introduced openmc.lib.TemporarySession context manager (#3475)

Added material depletion function for tracking individual material depletion (#3420)

Added methods on openmc.Material class for waste disposal rating / classification (#3366, #3376)

Support for thermochemical redox control transfer rates in depletion (#2783)

Support for external transfer rates source term in depletion (#3088)

Added combing capability for fission site sampling and delayed neutron emission time (#2992)

Ability to specify reference direction for azimuthal angle in openmc.stats.PolarAzimuthal distribution (#3582)

Allow spatial constraints on element sources within openmc.MeshSource (#3431)

Added VTK HDF (.vtkhdf) format support for writing VTK data (#3252)

Implemented filter weight capability (#3345)

Optionally collect higher moments for tallies (#3363)

Several random ray solver enhancements:

Random Ray AutoMagic Setup for automatic configuration (#3351)
Point source locator for random ray mode (#3360)
Support for DAGMC geometries (#3374)
Optimized mapping of source regions to tallies (#3465)
Base source region refactor (#3576)

Bug Fixes and Small Changes

Add two MPI barriers in R2S workflow (#3646)

Fix a few warnings, rename add_to_tallies_file (#3639)

Fix typo in DAGMC lost particle test (#3634)

Avoid multiprocessing Pool when running depletion tests with MPI (#3633)

Support MPI parallelism in R2SManager (#3632)

Update documentation for particle tracks (#3627)

Adding variance of variance and normality tests for tally statistics (#3454)

Avoid divide-by-zero in from_multigroup_flux when flux is zero (#3624)

Write particle states as separate lines in track VTK files (#3628)

Reset DAGMC history when reviving from source (#3601)

Add energy group structure: SCALE-999 (#3564)

Fix bug in normalization of tally results with no_reduce (#3619)

Enable nuclide filters with get_decay_photon_energy (#3614)

Update check_type calls to accept both str and os.PathLike objects (#3618)

Speed up apply_time_correction by reducing file I/O and deepcopies (#3617)

FW-CADIS Disregard Max Realizations Setting (#3616)

Random Ray Geometry Debug Mode Fix (#3615)

Don't write reaction rates in depletion results by default (#3609)

Allow Path objects in MGXSLibrary.export_to_hdf5 (#3608)

Clip mixture distributions based on mean times integral (#3603)

Allow V0 in atomic_mass function (for ENDF/B-VII.0 data) (#3607)

Re-run flaky tests when needed (#3604)

Ability to load mesh objects from weight_windows.h5 file (#3598)

Switch to using coveralls github action for reporting (#3594)

Add user setting for free gas threshold (#3593)

Speed up time correction factors (#3592)

Fix caching issue when using NCrystal materials (#3538)

Fix random ray source region mesh export when using model.export_to_xml() (#3579)

Ensure weight_windows_file information is read from XML (#3587)

Add missing documentation on <source> in depletion chain file format (#3590)

Adding tally filter type option to statepoint get_tally (#3584)

Optional separation of mesh-material-volume calc from get_homogenized_materials (#3581)

Fix IFP implementation (#3580)

Remove several TODOs related to C++17 support (#3574)

Fix performance regression in libMesh unstructured mesh tallies (#3577)

Update find_package calls in OpenMCConfig.cmake (#3572)

Ensure n_dimension_ attribute is set for unstructured meshes (#3575)

Allow newer Sphinx version and fix docbuild warnings (#3571)

Fixed a bug when combining TimeFilter, MeshFilter, and tracklength estimator (#3525)

PowerLaw raises an error if sampling interval contains negative values (#3542)

depletion: fix performance of chain matrix construction (#3567)

Do not apply boundary conditions when initialized in volume calculation mode (#3562)

Bump up tolerance for flaky activation test (#3560)

Fixed a bug in plotting cross sections with S(a,b) data (#3558)

Change test order to run unit tests first (#3533)

adding ecco 33 (#3556)

Refactor endf_data to be a fixture (#3539)

Revert "fix broken CI" (#3554)

fix broken CI (#3551)

Leverage particle.move_distance in event advance (#3544)

fix tests that accidentaly got broken (#3543)

not printing nuclides with 0 percent to terminal (option 2 ) (#3448)

Fix a bug in time cutoff behavior (#3526)

Avoid duplicate materials written to XML (#3536)

Use cached property for openmc.data.Decay.sources (#3535)

more helpful error message for dose_coefficients (#3534)

Adding 616 group structure (#3531)

Remove unused special accessors for tallies (#3527)

Consistent XML parsing using functions from _xml module (#3517)

Add stat:sum field to MCPL files for proper weight normalization (#3522)

Remove reorder_attributes from openmc._xml (#3519)

fixed a bug in MeshMaterialFilter.from_volumes (#3520)

Fixed a bug in distribcell offsets logic (#3424)

Add test for FW-CADIS based WW generation on a DAGMC model (#3504)

Fix for Weight Window Scaling Bug (#3511)

Fix: materials, plots, and tallies cannot be passed as lists (#3513)

Allow already-initialized openmc.lib in TemporarySession (#3505)

Update DAGMC and libMesh precompiler definitions (#3510)

Avoid adding ParentNuclideFilter twice when calling prepare_tallies (#3506)

Enabling MCPL source files to be read when using surf_source_read (#3472)

Boundary info accessors (#3496)

automatically finding appropriate dimension when making regular mesh from domain (#3468)

Add accessor methods for LocalCoord (#3494)

Make MCPL a Runtime Optional Dependency (#3429)

Use auto-chunking for StepResult HDF5 writing (#3498)

Provide a way to get ID maps from plot parameters on the Model class (#3481)

Update OSX install instructions to point to x64 platform (#3501)

Update conda install instructions for macOS Apple silicon (#3488)

Only show warning if in restart mode (#3478)

Add flag to CMakeLists to use submodules instead of searching (#3480)

Added citation metadata file (#3409)

fix zam parsing (#3484)

Support flux collapse method in get_microxs_and_flux (#3466)

Stabilize Adjoint Source (#3476)

Refactor and Harden Configuration Management (#3461)

Updated Docs to Not Give Specific Python Version Requirement (#3473)

Parallelization of Weight Window Update (#3467)

Limit Random Ray Weight Window Generation to Final Batch (#3464)

Fix Dockerfile DAGMC build (#3463)

Fix Weight Window Infinite Loop Bug (#3457)

Weight Window Birth Scaling (#3459)

Adding checks to geometry.plot to avoid material name overlaps (#3458)

Fixing crash when calling Geometry.plot when DAGMCUniverse in geometry (#3455)

fixing expansion of elemental Ta bug (#3443)

Prevent Adjoint Sources from Trending towards Infinity (#3449)

adding plot function to DAGMCUnvierse (#3451)

Allow specifying number of equiprobable angles for thermal scattering data generation (#3346)

Change Dockerfile from debian:bookworm-slim to ubuntu:24.04 (#3442)

Fix Resetting of Auto IDs When Generating MGXS (#3437)

Allowing chain_file to be chain object to save reloading time (#3436)

update units for flux (#3441)

Fix raytrace infinite loop (#3423)

Apply Max Number of Events Check to Random Rays (#3438)

Add user setting for source rejection fraction (#3433)

Adding fix and tests for spherical mesh as spatial distribution (#3428)

Random Ray Missed Cell Policy Change for Adjoint Mode (#3434)

Random Ray External Source Plotting Fix (#3430)

Avoid negative heating values during pair production and bremsstrahlung (#3426)

Fix no serialization of periodic_surface_id bug (#3421)

Update _get_start_data to always grab the beginning of timestep time (#3414)

Fixed a bug in charged particle energy deposition (#3416)

Fix bug where the same mesh is written multiple times to settings.xml (#3418)

small typo - spelling of Debian (#3411)

added test for dagmc geometry plot (#3375)

Random Ray Misc Memory Error Fixes (#3405)

added type hints to model file (#3399)

Apply resolve paths to path values in config (#3400)

Fixing an incorrect computation of CDF of bremsstrahlung photons (#3396)

Fix weight modification for uniform source sampling (#3395)

Updates to VTK data checks (#3371)

Map Compton subshell data to atomic relaxation data (#3392)

Skip atomic relaxation if binding energy is larger than photon energy (#3391)

Fix extremely large yields from Bremsstrahlung (#3386)

corrected tally name in D1S example (#3383)

Install MCPL using same build type as OpenMC in CI (#3388)

using reduce chain level to remove need for reduce chain (#3377)

Fix negative distances from bins_crossed for CylindricalMesh (#3370)

Add check for equal value bins in an EnergyFilter (#3372)

Fix for Issue Loading MGXS Data Files with LLVM 20 or Newer (#3368)

Report plot ID instead of index for unsupported plot types in random ray mode (#3361)

Handle Missing Tags in Versioning by Setting Default to 0 (#3359)

added kg units to doc string in results class (#3358)

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OpenMC 0.15.3

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