Merge pull request #1511 from SamPUG/develop

Added documentation for material mixing

docs/source/usersguide/materials.rst

@@ -155,6 +155,33 @@ attribute, e.g.,
              :attr:`Material.temperature` or :attr:`Cell.temperature`
              attributes, respectively.
 
+-----------------
+Material Mixtures
+-----------------
+
+In OpenMC it is possible to mix any number of materials to create a new material
+with the correct nuclide composition and density. The 
+:meth:`Material.mix_materials` method takes a list of materials and
+a list of their mixing fractions. Mixing fractions can be provided as atomic 
+fractions, weight fractions, or volume fractions. The fraction type
+can be specified by passing 'ao', 'wo', or 'vo' as the third argument, respectively. 
+For example, assuming the required materials have already been defined, a MOX 
+material with 3% plutonium oxide by weight could be created using the following:
+
+::
+
+   mox = openmc.Material.mix_materials([uo2, puo2], [0.97, 0.03], 'wo')
+
+It should be noted that, if mixing fractions are specifed as atomic or weight 
+fractions, the supplied fractions should sum to one. If the fractions are specified
+as volume fractions, and the sum of the fractions is less than one, then the remaining 
+fraction is set as void material. 
+
+.. warning:: Materials with :math:`S(\alpha,\beta)` thermal scattering data
+             cannot be used in :meth:`Material.mix_materials`. However, thermal
+             scattering data can be added to a material created by 
+             :meth:`Material.mix_materials`.
+
 --------------------
 Material Collections
 --------------------