Merging with upstream

docs/source/io_formats/statepoint.rst

@@ -162,6 +162,18 @@ if run_mode == 'k-eigenvalue':
 
     Width of each mesh cell in each dimension.
 
+**/tallies/derivatives/derivative <id>/independent variable** (*char[]*)
+
+    Independent variable of tally derivative
+
+**/tallies/derivatives/derivative <id>/material** (*int*)
+
+    ID of the perturbed material
+
+**/tallies/derivatives/derivative <id>/nuclide** (*char[]*)
+
+    Alias of the perturbed nuclide
+
 **/tallies/n_tallies** (*int*)
 
     Number of user-defined tallies.
@@ -204,6 +216,10 @@ if run_mode == 'k-eigenvalue':
     Array of nuclides to tally. Note that if no nuclide is specified in the user
     input, a single 'total' nuclide appears here.
 
+**/tallies/tally <uid>/derivative** (*int*)
+
+    ID of the derivative applied to the tally.
+
 **/tallies/tally <uid>/n_score_bins** (*int*)
 
     Number of scoring bins for a single nuclide. In general, this can be greater
@@ -224,12 +240,12 @@ if run_mode == 'k-eigenvalue':
     Tallying moment orders for Legendre and spherical harmonic tally expansions
     (*e.g.*, 'P2', 'Y1,2', etc.).
 
-**/tallies/tally <uid>/results** (Compound type)
+**/tallies/tally <uid>/results** (*double[][][2]*)
 
-    Accumulated sum and sum-of-squares for each bin of the i-th tally. This is a
-    two-dimensional array, the first dimension of which represents combinations
-    of filter bins and the second dimensions of which represents scoring
-    bins. Each element of the array has fields 'sum' and 'sum_sq'.
+    Accumulated sum and sum-of-squares for each bin of the i-th tally. The first
+    dimension represents combinations of filter bins, the second dimensions
+    represents scoring bins, and the third dimension has two entries for the sum
+    and the sum-of-squares.
 
 **/source_present** (*int*)
 

docs/source/pythonapi/index.rst

@@ -293,6 +293,7 @@ Multi-group Cross Sections
     openmc.mgxs.NuScatterXS
     openmc.mgxs.NuScatterMatrixXS
     openmc.mgxs.PromptNuFissionXS
+    openmc.mgxs.PromptNuFissionMatrixXS
     openmc.mgxs.ScatterXS
     openmc.mgxs.ScatterMatrixXS
     openmc.mgxs.TotalXS
@@ -309,6 +310,7 @@ Multi-delayed-group Cross Sections
     openmc.mgxs.MDGXS
     openmc.mgxs.ChiDelayed
     openmc.mgxs.DelayedNuFissionXS
+    openmc.mgxs.DelayedNuFissionMatrixXS
     openmc.mgxs.Beta
     openmc.mgxs.DecayRate
 

docs/source/usersguide/input.rst

@@ -1916,6 +1916,13 @@ The ``<tally>`` element accepts the following sub-elements:
 
      *Default*: "all"
 
+  :derivative:
+    The id of a ``derivative`` element. This derivative will be applied to all
+    scores in the tally. Differential tallies are currently only implemented
+    for collision and analog estimators.
+
+     *Default*: None
+
 ``<mesh>`` Element
 ------------------
 
@@ -1944,6 +1951,39 @@ attributes/sub-elements:
       One of ``<upper_right>`` or ``<width>`` must be specified, but not both
       (even if they are consistent with one another).
 
+``<derivative>`` Element
+------------------------
+
+OpenMC can take the first-order derivative of many tallies with respect to
+material perturbations. It works by propagating a derivative through the
+transport equation. Essentially, OpenMC keeps track of how each particle's
+weight would change as materials are perturbed, and then accounts for that
+weight change in the tallies. Note that this assumes material perturbations are
+small enough not to change the distribution of fission sites. This element has
+the following attributes/sub-elements:
+
+  :id:
+    A unique integer that can be used to identify the derivative.
+
+  :variable:
+    The independent variable of the derivative. Accepted options are "density",
+    "nuclide_density", and "temperature". A "density" derivative will give the
+    derivative with respect to the density of the material in [g / cm^3]. A
+    "nuclide_density" derivative will give the derivative with respect to the
+    density of a particular nuclide in units of [atom / b / cm].  A
+    "temperature" derivative is with respect to a material temperature in units
+    of [K].  The temperature derivative requires windowed multipole to be
+    turned on.  Note also that the temperature derivative only accounts for
+    resolved resonance Doppler broadening.  It does not account for thermal
+    expansion, S(a, b) scattering, resonance scattering, or unresolved Doppler
+    broadening.
+
+  :material:
+    The perturbed material. (Necessary for all derivative types)
+
+  :nuclide:
+    The perturbed nuclide. (Necessary only for "nuclide_density")
+
 ``<assume_separate>`` Element
 -----------------------------