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Add k-infinity estimator for IndependentOperator - #4053

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@EdenRochmanSharabiEdenRochmanSharabi commented Aug 6, 2026•
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Summary

  • Add calculate_kinf=True parameter to IndependentOperator and from_nuclides (mutually exclusive with keff)
  • Estimate k-infinity as the ratio of nu-fission production rate to neutron loss rate, following OpenMC's keff convention: (n,xn) reactions reduce the loss term by (x - 1) rather than contributing to production
  • Add 'nu-fission' as a valid reaction in MicroXS, with a new _collapse_nu_fission() helper that integrates nu(E)*sigma_f(E) from pointwise ENDF data against the user-provided multigroup flux
  • The estimate is reported as the operator eigenvalue and flows into Results.get_keff()

Changes

  • openmc/deplete/independent_operator.py: _neutrons_emitted() parser, _estimate_k_inf() method (MPI-aware), calculate_kinf parameter with validation
  • openmc/deplete/microxs.py: 'nu-fission' in _valid_rxns, _collapse_nu_fission() for pointwise collapse, from_multigroup_flux() routing for nu-fission
  • docs/source/usersguide/depletion.rst: user guide section on calculate_kinf
  • tests/unit_tests/test_deplete_independent_operator.py: analytic k_inf (with (n,2n)), multigroup flux, decay step, _neutrons_emitted table, error paths
  • tests/unit_tests/test_deplete_microxs.py: nu-bar validation against ENDF data (U235 thermal ~2.43)

Test plan

  • Analytic k_inf matches expected value including (n,2n) contribution
  • Multigroup flux weighting produces correct result
  • Decay step (zero source rate) still reports k_inf estimate
  • _neutrons_emitted handles all reaction name patterns correctly
  • ValueError raised for missing nu-fission data and keff/calculate_kinf conflict
  • CI (17/17 passed, coverage +0.01%)

Closes#2132

@EdenRochmanSharabi
Add k-infinity estimator for IndependentOperator
72be529
Add calculate_kinf parameter to IndependentOperator that estimates the
infinite multiplication factor from one-group reaction rates, following
OpenMC's keff convention for (n,xn) reactions. Add nu-fission one-group
cross section support to MicroXS.
@GuySten

GuySten commented Aug 20, 2026

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Looks like a nice PR.
I have some structural suggestions:

  1. Calculate kinf by default when there is some fission reaction and every fission score has a corresponding nu-fission score.
    That way we do not need the calculate_kinf keyword argument.
  2. Extend chain.REACTIONS to save all emitted particles. Then use it to figure out how many neutrons are emitted for every supported reaction instead of using regex.
  3. Extending Nuclide::collapse_rate to collapse nu_total according to the fission * flux spectrum. That way we plug into the existing machinery instead of adding a special python function to handle the nu-fission case.

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Add k-infinity estimator for IndependentOperator by EdenRochmanSharabi · Pull Request #4053 · openmc-dev/openmc · GitHub
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Add k-infinity estimator for IndependentOperator - #4053
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@EdenRochmanSharabiEdenRochmanSharabi
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Summary
  • Add calculate_kinf=True parameter to IndependentOperator and from_nuclides (mutually exclusive with keff)
  • Estimate k-infinity as the ratio of nu-fission production rate to neutron loss rate, following OpenMC's keff convention: (n,xn) reactions reduce the loss term by (x - 1) rather than contributing to production
  • Add 'nu-fission' as a valid reaction in MicroXS, with a new _collapse_nu_fission() helper that integrates nu(E)*sigma_f(E) from pointwise ENDF data against the user-provided multigroup flux
  • The estimate is reported as the operator eigenvalue and flows into Results.get_keff()
Changes
  • openmc/deplete/independent_operator.py: _neutrons_emitted() parser, _estimate_k_inf() method (MPI-aware), calculate_kinf parameter with validation
  • openmc/deplete/microxs.py: 'nu-fission' in _valid_rxns, _collapse_nu_fission() for pointwise collapse, from_multigroup_flux() routing for nu-fission
  • docs/source/usersguide/depletion.rst: user guide section on calculate_kinf
  • tests/unit_tests/test_deplete_independent_operator.py: analytic k_inf (with (n,2n)), multigroup flux, decay step, _neutrons_emitted table, error paths
  • tests/unit_tests/test_deplete_microxs.py: nu-bar validation against ENDF data (U235 thermal ~2.43)
Test plan
  •  Analytic k_inf matches expected value including (n,2n) contribution
  •  Multigroup flux weighting produces correct result
  •  Decay step (zero source rate) still reports k_inf estimate
  • _neutrons_emitted handles all reaction name patterns correctly
  •  ValueError raised for missing nu-fission data and keff/calculate_kinf conflict
  •  CI (17/17 passed, coverage +0.01%)
Closes#2132
@EdenRochmanSharabi
Add k-infinity estimator for IndependentOperator
72be529
Add calculate_kinf parameter to IndependentOperator that estimates the
infinite multiplication factor from one-group reaction rates, following
OpenMC's keff convention for (n,xn) reactions. Add nu-fission one-group
cross section support to MicroXS.
@GuySten
GuySten
commented
Aug 20, 2026

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Looks like a nice PR.

I have some structural suggestions:
  1. Calculate kinf by default when there is some fission reaction and every fission score has a corresponding nu-fission score.
    
That way we do not need the calculate_kinf keyword argument.
  2. Extend chain.REACTIONS to save all emitted particles. Then use it to figure out how many neutrons are emitted for every supported reaction instead of using regex.
  3. Extending Nuclide::collapse_rate to collapse nu_total according to the fission * flux spectrum. That way we plug into the existing machinery instead of adding a special python function to handle the nu-fission case.
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k_inf estimator for IndependentOperator

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Add k-infinity estimator for IndependentOperator by EdenRochmanSharabi · Pull Request #4053 · openmc-dev/openmc · GitHub
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Add k-infinity estimator for IndependentOperator - #4053
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EdenRochmanSharabi wants to merge 1 commit into
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EdenRochmanSharabi:feature/kinf-estimator
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Add k-infinity estimator for IndependentOperator#4053
EdenRochmanSharabi wants to merge 1 commit into
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EdenRochmanSharabi:feature/kinf-estimator
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@EdenRochmanSharabiEdenRochmanSharabi
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Aug 6, 2026•

edited

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Summary
  • Add calculate_kinf=True parameter to IndependentOperator and from_nuclides (mutually exclusive with keff)
  • Estimate k-infinity as the ratio of nu-fission production rate to neutron loss rate, following OpenMC's keff convention: (n,xn) reactions reduce the loss term by (x - 1) rather than contributing to production
  • Add 'nu-fission' as a valid reaction in MicroXS, with a new _collapse_nu_fission() helper that integrates nu(E)*sigma_f(E) from pointwise ENDF data against the user-provided multigroup flux
  • The estimate is reported as the operator eigenvalue and flows into Results.get_keff()
Changes
  • openmc/deplete/independent_operator.py: _neutrons_emitted() parser, _estimate_k_inf() method (MPI-aware), calculate_kinf parameter with validation
  • openmc/deplete/microxs.py: 'nu-fission' in _valid_rxns, _collapse_nu_fission() for pointwise collapse, from_multigroup_flux() routing for nu-fission
  • docs/source/usersguide/depletion.rst: user guide section on calculate_kinf
  • tests/unit_tests/test_deplete_independent_operator.py: analytic k_inf (with (n,2n)), multigroup flux, decay step, _neutrons_emitted table, error paths
  • tests/unit_tests/test_deplete_microxs.py: nu-bar validation against ENDF data (U235 thermal ~2.43)
Test plan
  •  Analytic k_inf matches expected value including (n,2n) contribution
  •  Multigroup flux weighting produces correct result
  •  Decay step (zero source rate) still reports k_inf estimate
  • _neutrons_emitted handles all reaction name patterns correctly
  •  ValueError raised for missing nu-fission data and keff/calculate_kinf conflict
  •  CI (17/17 passed, coverage +0.01%)
Closes#2132
@EdenRochmanSharabi
Add k-infinity estimator for IndependentOperator
72be529
Add calculate_kinf parameter to IndependentOperator that estimates the
infinite multiplication factor from one-group reaction rates, following
OpenMC's keff convention for (n,xn) reactions. Add nu-fission one-group
cross section support to MicroXS.
@GuySten
GuySten
commented
Aug 20, 2026

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Looks like a nice PR.

I have some structural suggestions:
  1. Calculate kinf by default when there is some fission reaction and every fission score has a corresponding nu-fission score.
    
That way we do not need the calculate_kinf keyword argument.
  2. Extend chain.REACTIONS to save all emitted particles. Then use it to figure out how many neutrons are emitted for every supported reaction instead of using regex.
  3. Extending Nuclide::collapse_rate to collapse nu_total according to the fission * flux spectrum. That way we plug into the existing machinery instead of adding a special python function to handle the nu-fission case.
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Add k-infinity estimator for IndependentOperator by EdenRochmanSharabi · Pull Request #4053 · openmc-dev/openmc · GitHub
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Add k-infinity estimator for IndependentOperator - #4053
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@EdenRochmanSharabiEdenRochmanSharabi
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Summary
  • Add calculate_kinf=True parameter to IndependentOperator and from_nuclides (mutually exclusive with keff)
  • Estimate k-infinity as the ratio of nu-fission production rate to neutron loss rate, following OpenMC's keff convention: (n,xn) reactions reduce the loss term by (x - 1) rather than contributing to production
  • Add 'nu-fission' as a valid reaction in MicroXS, with a new _collapse_nu_fission() helper that integrates nu(E)*sigma_f(E) from pointwise ENDF data against the user-provided multigroup flux
  • The estimate is reported as the operator eigenvalue and flows into Results.get_keff()
Changes
  • openmc/deplete/independent_operator.py: _neutrons_emitted() parser, _estimate_k_inf() method (MPI-aware), calculate_kinf parameter with validation
  • openmc/deplete/microxs.py: 'nu-fission' in _valid_rxns, _collapse_nu_fission() for pointwise collapse, from_multigroup_flux() routing for nu-fission
  • docs/source/usersguide/depletion.rst: user guide section on calculate_kinf
  • tests/unit_tests/test_deplete_independent_operator.py: analytic k_inf (with (n,2n)), multigroup flux, decay step, _neutrons_emitted table, error paths
  • tests/unit_tests/test_deplete_microxs.py: nu-bar validation against ENDF data (U235 thermal ~2.43)
Test plan
  •  Analytic k_inf matches expected value including (n,2n) contribution
  •  Multigroup flux weighting produces correct result
  •  Decay step (zero source rate) still reports k_inf estimate
  • _neutrons_emitted handles all reaction name patterns correctly
  •  ValueError raised for missing nu-fission data and keff/calculate_kinf conflict
  •  CI (17/17 passed, coverage +0.01%)
Closes#2132
@EdenRochmanSharabi
Add k-infinity estimator for IndependentOperator
72be529
Add calculate_kinf parameter to IndependentOperator that estimates the
infinite multiplication factor from one-group reaction rates, following
OpenMC's keff convention for (n,xn) reactions. Add nu-fission one-group
cross section support to MicroXS.
@GuySten
GuySten
commented
Aug 20, 2026

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Looks like a nice PR.

I have some structural suggestions:
  1. Calculate kinf by default when there is some fission reaction and every fission score has a corresponding nu-fission score.
    
That way we do not need the calculate_kinf keyword argument.
  2. Extend chain.REACTIONS to save all emitted particles. Then use it to figure out how many neutrons are emitted for every supported reaction instead of using regex.
  3. Extending Nuclide::collapse_rate to collapse nu_total according to the fission * flux spectrum. That way we plug into the existing machinery instead of adding a special python function to handle the nu-fission case.
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Add k-infinity estimator for IndependentOperator by EdenRochmanSharabi · Pull Request #4053 · openmc-dev/openmc · GitHub
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Add k-infinity estimator for IndependentOperator - #4053
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@EdenRochmanSharabiEdenRochmanSharabi
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Summary
  • Add calculate_kinf=True parameter to IndependentOperator and from_nuclides (mutually exclusive with keff)
  • Estimate k-infinity as the ratio of nu-fission production rate to neutron loss rate, following OpenMC's keff convention: (n,xn) reactions reduce the loss term by (x - 1) rather than contributing to production
  • Add 'nu-fission' as a valid reaction in MicroXS, with a new _collapse_nu_fission() helper that integrates nu(E)*sigma_f(E) from pointwise ENDF data against the user-provided multigroup flux
  • The estimate is reported as the operator eigenvalue and flows into Results.get_keff()
Changes
  • openmc/deplete/independent_operator.py: _neutrons_emitted() parser, _estimate_k_inf() method (MPI-aware), calculate_kinf parameter with validation
  • openmc/deplete/microxs.py: 'nu-fission' in _valid_rxns, _collapse_nu_fission() for pointwise collapse, from_multigroup_flux() routing for nu-fission
  • docs/source/usersguide/depletion.rst: user guide section on calculate_kinf
  • tests/unit_tests/test_deplete_independent_operator.py: analytic k_inf (with (n,2n)), multigroup flux, decay step, _neutrons_emitted table, error paths
  • tests/unit_tests/test_deplete_microxs.py: nu-bar validation against ENDF data (U235 thermal ~2.43)
Test plan
  •  Analytic k_inf matches expected value including (n,2n) contribution
  •  Multigroup flux weighting produces correct result
  •  Decay step (zero source rate) still reports k_inf estimate
  • _neutrons_emitted handles all reaction name patterns correctly
  •  ValueError raised for missing nu-fission data and keff/calculate_kinf conflict
  •  CI (17/17 passed, coverage +0.01%)
Closes#2132
@EdenRochmanSharabi
Add k-infinity estimator for IndependentOperator
72be529
Add calculate_kinf parameter to IndependentOperator that estimates the
infinite multiplication factor from one-group reaction rates, following
OpenMC's keff convention for (n,xn) reactions. Add nu-fission one-group
cross section support to MicroXS.
@GuySten
GuySten
commented
Aug 20, 2026

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Contributor
Looks like a nice PR.

I have some structural suggestions:
  1. Calculate kinf by default when there is some fission reaction and every fission score has a corresponding nu-fission score.
    
That way we do not need the calculate_kinf keyword argument.
  2. Extend chain.REACTIONS to save all emitted particles. Then use it to figure out how many neutrons are emitted for every supported reaction instead of using regex.
  3. Extending Nuclide::collapse_rate to collapse nu_total according to the fission * flux spectrum. That way we plug into the existing machinery instead of adding a special python function to handle the nu-fission case.
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k_inf estimator for IndependentOperator

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@EdenRochmanSharabi@GuySten

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Add k-infinity estimator for IndependentOperator by EdenRochmanSharabi · Pull Request #4053 · openmc-dev/openmc · GitHub
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Add k-infinity estimator for IndependentOperator - #4053
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Add k-infinity estimator for IndependentOperator#4053
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Aug 6, 2026•

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Summary
  • Add calculate_kinf=True parameter to IndependentOperator and from_nuclides (mutually exclusive with keff)
  • Estimate k-infinity as the ratio of nu-fission production rate to neutron loss rate, following OpenMC's keff convention: (n,xn) reactions reduce the loss term by (x - 1) rather than contributing to production
  • Add 'nu-fission' as a valid reaction in MicroXS, with a new _collapse_nu_fission() helper that integrates nu(E)*sigma_f(E) from pointwise ENDF data against the user-provided multigroup flux
  • The estimate is reported as the operator eigenvalue and flows into Results.get_keff()
Changes
  • openmc/deplete/independent_operator.py: _neutrons_emitted() parser, _estimate_k_inf() method (MPI-aware), calculate_kinf parameter with validation
  • openmc/deplete/microxs.py: 'nu-fission' in _valid_rxns, _collapse_nu_fission() for pointwise collapse, from_multigroup_flux() routing for nu-fission
  • docs/source/usersguide/depletion.rst: user guide section on calculate_kinf
  • tests/unit_tests/test_deplete_independent_operator.py: analytic k_inf (with (n,2n)), multigroup flux, decay step, _neutrons_emitted table, error paths
  • tests/unit_tests/test_deplete_microxs.py: nu-bar validation against ENDF data (U235 thermal ~2.43)
Test plan
  •  Analytic k_inf matches expected value including (n,2n) contribution
  •  Multigroup flux weighting produces correct result
  •  Decay step (zero source rate) still reports k_inf estimate
  • _neutrons_emitted handles all reaction name patterns correctly
  •  ValueError raised for missing nu-fission data and keff/calculate_kinf conflict
  •  CI (17/17 passed, coverage +0.01%)
Closes#2132
@EdenRochmanSharabi
Add k-infinity estimator for IndependentOperator
72be529
Add calculate_kinf parameter to IndependentOperator that estimates the
infinite multiplication factor from one-group reaction rates, following
OpenMC's keff convention for (n,xn) reactions. Add nu-fission one-group
cross section support to MicroXS.
@GuySten
GuySten
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Aug 20, 2026

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Looks like a nice PR.

I have some structural suggestions:
  1. Calculate kinf by default when there is some fission reaction and every fission score has a corresponding nu-fission score.
    
That way we do not need the calculate_kinf keyword argument.
  2. Extend chain.REACTIONS to save all emitted particles. Then use it to figure out how many neutrons are emitted for every supported reaction instead of using regex.
  3. Extending Nuclide::collapse_rate to collapse nu_total according to the fission * flux spectrum. That way we plug into the existing machinery instead of adding a special python function to handle the nu-fission case.
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Add k-infinity estimator for IndependentOperator by EdenRochmanSharabi · Pull Request #4053 · openmc-dev/openmc · GitHub
Skip to content
Add k-infinity estimator for IndependentOperator - #4053
Open
EdenRochmanSharabi wants to merge 1 commit into
openmc-dev:developfrom 
EdenRochmanSharabi:feature/kinf-estimator
Open
Add k-infinity estimator for IndependentOperator#4053
EdenRochmanSharabi wants to merge 1 commit into
openmc-dev:developfrom 
EdenRochmanSharabi:feature/kinf-estimator
Conversation
@EdenRochmanSharabi
@EdenRochmanSharabiEdenRochmanSharabi
commented
Aug 6, 2026•

edited

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Summary
  • Add calculate_kinf=True parameter to IndependentOperator and from_nuclides (mutually exclusive with keff)
  • Estimate k-infinity as the ratio of nu-fission production rate to neutron loss rate, following OpenMC's keff convention: (n,xn) reactions reduce the loss term by (x - 1) rather than contributing to production
  • Add 'nu-fission' as a valid reaction in MicroXS, with a new _collapse_nu_fission() helper that integrates nu(E)*sigma_f(E) from pointwise ENDF data against the user-provided multigroup flux
  • The estimate is reported as the operator eigenvalue and flows into Results.get_keff()
Changes
  • openmc/deplete/independent_operator.py: _neutrons_emitted() parser, _estimate_k_inf() method (MPI-aware), calculate_kinf parameter with validation
  • openmc/deplete/microxs.py: 'nu-fission' in _valid_rxns, _collapse_nu_fission() for pointwise collapse, from_multigroup_flux() routing for nu-fission
  • docs/source/usersguide/depletion.rst: user guide section on calculate_kinf
  • tests/unit_tests/test_deplete_independent_operator.py: analytic k_inf (with (n,2n)), multigroup flux, decay step, _neutrons_emitted table, error paths
  • tests/unit_tests/test_deplete_microxs.py: nu-bar validation against ENDF data (U235 thermal ~2.43)
Test plan
  •  Analytic k_inf matches expected value including (n,2n) contribution
  •  Multigroup flux weighting produces correct result
  •  Decay step (zero source rate) still reports k_inf estimate
  • _neutrons_emitted handles all reaction name patterns correctly
  •  ValueError raised for missing nu-fission data and keff/calculate_kinf conflict
  •  CI (17/17 passed, coverage +0.01%)
Closes#2132
@EdenRochmanSharabi
Add k-infinity estimator for IndependentOperator
72be529
Add calculate_kinf parameter to IndependentOperator that estimates the
infinite multiplication factor from one-group reaction rates, following
OpenMC's keff convention for (n,xn) reactions. Add nu-fission one-group
cross section support to MicroXS.
@GuySten
GuySten
commented
Aug 20, 2026

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Contributor
Looks like a nice PR.

I have some structural suggestions:
  1. Calculate kinf by default when there is some fission reaction and every fission score has a corresponding nu-fission score.
    
That way we do not need the calculate_kinf keyword argument.
  2. Extend chain.REACTIONS to save all emitted particles. Then use it to figure out how many neutrons are emitted for every supported reaction instead of using regex.
  3. Extending Nuclide::collapse_rate to collapse nu_total according to the fission * flux spectrum. That way we plug into the existing machinery instead of adding a special python function to handle the nu-fission case.
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@paulromanopaulromanoAwaiting requested review from paulromanopaulromano is a code owner

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k_inf estimator for IndependentOperator

2 participants

@EdenRochmanSharabi@GuySten

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Add k-infinity estimator for IndependentOperator by EdenRochmanSharabi · Pull Request #4053 · openmc-dev/openmc · GitHub
Skip to content
Add k-infinity estimator for IndependentOperator - #4053
Open
EdenRochmanSharabi wants to merge 1 commit into
openmc-dev:developfrom 
EdenRochmanSharabi:feature/kinf-estimator
Open
Add k-infinity estimator for IndependentOperator#4053
EdenRochmanSharabi wants to merge 1 commit into
openmc-dev:developfrom 
EdenRochmanSharabi:feature/kinf-estimator
Conversation
@EdenRochmanSharabi
@EdenRochmanSharabiEdenRochmanSharabi
commented
Aug 6, 2026•

edited

Loading

Copy link


Copy Markdown

Contributor
Summary
  • Add calculate_kinf=True parameter to IndependentOperator and from_nuclides (mutually exclusive with keff)
  • Estimate k-infinity as the ratio of nu-fission production rate to neutron loss rate, following OpenMC's keff convention: (n,xn) reactions reduce the loss term by (x - 1) rather than contributing to production
  • Add 'nu-fission' as a valid reaction in MicroXS, with a new _collapse_nu_fission() helper that integrates nu(E)*sigma_f(E) from pointwise ENDF data against the user-provided multigroup flux
  • The estimate is reported as the operator eigenvalue and flows into Results.get_keff()
Changes
  • openmc/deplete/independent_operator.py: _neutrons_emitted() parser, _estimate_k_inf() method (MPI-aware), calculate_kinf parameter with validation
  • openmc/deplete/microxs.py: 'nu-fission' in _valid_rxns, _collapse_nu_fission() for pointwise collapse, from_multigroup_flux() routing for nu-fission
  • docs/source/usersguide/depletion.rst: user guide section on calculate_kinf
  • tests/unit_tests/test_deplete_independent_operator.py: analytic k_inf (with (n,2n)), multigroup flux, decay step, _neutrons_emitted table, error paths
  • tests/unit_tests/test_deplete_microxs.py: nu-bar validation against ENDF data (U235 thermal ~2.43)
Test plan
  •  Analytic k_inf matches expected value including (n,2n) contribution
  •  Multigroup flux weighting produces correct result
  •  Decay step (zero source rate) still reports k_inf estimate
  • _neutrons_emitted handles all reaction name patterns correctly
  •  ValueError raised for missing nu-fission data and keff/calculate_kinf conflict
  •  CI (17/17 passed, coverage +0.01%)
Closes#2132
@EdenRochmanSharabi
Add k-infinity estimator for IndependentOperator
72be529
Add calculate_kinf parameter to IndependentOperator that estimates the
infinite multiplication factor from one-group reaction rates, following
OpenMC's keff convention for (n,xn) reactions. Add nu-fission one-group
cross section support to MicroXS.
@GuySten
GuySten
commented
Aug 20, 2026

Copy link


Copy Markdown

Contributor
Looks like a nice PR.

I have some structural suggestions:
  1. Calculate kinf by default when there is some fission reaction and every fission score has a corresponding nu-fission score.
    
That way we do not need the calculate_kinf keyword argument.
  2. Extend chain.REACTIONS to save all emitted particles. Then use it to figure out how many neutrons are emitted for every supported reaction instead of using regex.
  3. Extending Nuclide::collapse_rate to collapse nu_total according to the fission * flux spectrum. That way we plug into the existing machinery instead of adding a special python function to handle the nu-fission case.
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@paulromanopaulromanoAwaiting requested review from paulromanopaulromano is a code owner

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Development

Successfully merging this pull request may close these issues.
k_inf estimator for IndependentOperator

2 participants

@EdenRochmanSharabi@GuySten