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Implement surface flux tallies (#3742)

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Co-authored-by: Paul Romano <paul.k.romano@gmail.com>
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GuySten 2026-03-04 14:17:48 +02:00 • committed by GitHub
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17
docs/source/io_formats/settings.rst
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@ -1234,6 +1234,23 @@ attributes/sub-elements:
are not eligible to store any particles when using ``cell``, ``cellfrom``
or ``cellto`` attributes. It is recommended to use surface IDs instead.
------------------------------------
``<surface_grazing_cutoff>`` Element
------------------------------------
The ``<surface_grazing_cutoff>`` element specifies the surface flux cosine cutoff.
*Default*: 0.001
-----------------------------------
``<surface_grazing_ratio>`` Element
-----------------------------------
The ``<surface_grazing_ratio>`` element specifies the surface flux cosine
substitution ratio.
*Default*: 0.5
------------------------------
``<survival_biasing>`` Element
------------------------------

66
docs/source/methods/tallies.rst
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@ -205,7 +205,71 @@ had a collision at every event. Thus, for tallies with outgoing-energy filters
or for tallies of scattering moments (which require the scattering cosine of
the change-in-angle), we must use an analog estimator.
.. TODO: Add description of surface current tallies
-----------------------------------
Surface-Integrated Flux and Current
-----------------------------------
Surface tallies allow you to measure particle behavior as they cross specific
boundaries in your geometry. Unlike volume tallies, which integrate over a
volumetric region, surface tallies capture the current or flux passing through a
surface. Surface tallies are estimated using an analog estimator.
Current Score
-------------
When tallying the current across a surface, we simply count the weight of
particles that cross the surface of interest:
.. math::
:label: analog-current-estimator
J = \frac{1}{W} \sum_{i \in S} w_i.
where :math:`J` is the area-integrated current passing through surface
:math:`S`, :math:`W` is the total starting weight of the particles, and
:math:`w_i` is the weight of the particle as it crosses the surface :math:`S`.
Flux Score
----------
When tallying flux over a surface, we use the relationship between current and
flux:
.. math::
:label: surface-flux-estimator
\phi_S = \frac{1}{W} \sum_{i \in S} \frac{w_i}{|\mu|}.
where :math:`\phi_S` is the area-integrated flux over surface :math:`S`,
:math:`W` is the total starting weight of the particles, :math:`w_i` is the
weight of the particle as it crosses the surface :math:`S` and :math:`\mu` is
the cosine of angle between the particle direction and the surface normal.
This equation diverges when the particle crossing the surface is nearly parallel
to it (that is, as :math:`\mu` approaches zero). To remove this divergence,
OpenMC scores:
.. math::
:label: modified-surface-flux-estimator
\phi_S = \frac{1}{W} \sum_{i \in S} w_i f(\mu).
and the function :math:`f` is defined by:
.. math::
f(\mu) = \begin{cases}
\frac{1}{|\mu|} & |\mu| > \mu_\text{cut} \\
\frac{1}{c\mu_\text{cut}} & |\mu| \le \mu_\text{cut}
\end{cases}
where :math:`\mu_\text{cut}` is the grazing cosine cutoff and :math:`c` is the
cosine substitution ratio. The parameters :math:`\mu_\text{cut}` and :math:`c`
can be set by the user via the :attr:`openmc.Settings.surface_grazing_cutoff`
and :attr:`openmc.Settings.surface_grazing_ratio` attributes, respectively. The
default values for these parameters are 0.001 and 0.5 as recommended by
`Favorite, Thomas, and Booth <https://doi.org/10.13182/NSE09-72>`_.
.. _tallies_statistics:

13
docs/source/usersguide/tallies.rst
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@ -261,18 +261,21 @@ The following tables show all valid scores:
+----------------------+---------------------------------------------------+
|Score | Description |
+======================+===================================================+
|current |Used in combination with a meshsurface filter: |
|current |It may not be used in conjunction with any other |
| |score except flux. |
| | |
| |When used in combination with a meshsurface filter:|
| |Partial currents on the boundaries of each cell in |
| |a mesh. It may not be used in conjunction with any |
| |other score. Only energy and mesh filters may be |
| |used. |
| |Used in combination with a surface filter: |
| |a mesh. |
| | |
| |When used in combination with a surface filter: |
| |Net currents on any surface previously defined in |
| |the geometry. It may be used along with any other |
| |filter, except meshsurface filters. |
| |Surfaces can alternatively be defined with cell |
| |from and cell filters thereby resulting in tallying|
| |partial currents. |
| | |
| |Units are particles per source particle. |
+----------------------+---------------------------------------------------+
|events |Number of scoring events. Units are events per |