Updating volume documentation.

docs/source/io_formats/volume.rst

@@ -23,6 +23,7 @@ The current version of the volume file format is 1.0.
                bounding box
              - **upper_right** (*double[3]*) -- Upper-right coordinates of
                bounding box
+             - **threshold** (*double*) -- Threshold used for volume uncertainty
 
 **/domain_<id>/**
 

docs/source/usersguide/volume.rst

@@ -37,6 +37,16 @@ arguments are not necessary. For example,
 Of course, the volumes that you *need* this capability for are often the ones
 with complex definitions.
 
+A threshold for the uncertainty in volume estimates can be specified using
+::attr::`openmc.VolumeCalculation.threshold` ::
+
+    vol_calc.threshold = 1E-05
+
+If a threshold is provided, calculations will be performed iteratively using the 
+number of samples specified on the calculation until all volume estimates have a 
+standard deviation lower than this value. If no threshold is provided, the 
+calculation will run the number of samples specified once and return the result.
+
 Once you have one or more :class:`openmc.VolumeCalculation` objects created, you
 can then assign then to :attr:`Settings.volume_calculations`::
 
@@ -66,4 +76,4 @@ After the results are loaded, volume estimates will be stored in
 :attr:`VolumeCalculation.volumes`. There is also a
 :attr:`VolumeCalculation.atoms_dataframe` attribute that shows stochastic
 estimates of the number of atoms of each type of nuclide within the specified
-domains along with their uncertainties.
+domains along with their uncertainties.