addressed PR comments

openmc/data/data.py

@@ -1,5 +1,6 @@
 import itertools
 import os
+import re
 
 
 # Isotopic abundances from M. Berglund and M. E. Wieser, "Isotopic compositions
@@ -150,17 +151,6 @@ def atomic_mass(isotope):
     """
     if not _ATOMIC_MASS:
 
-        # For the isotopes representing all natural isotopes of their element
-        # (e.g. C0), set atomic mass manually using the values from Atomic
-        # weights of the elements 2013 (IUPAC Technical Report)
-        # (doi:10.1515/pac-2015-0305). In cases where an atomic mass range is
-        # given (e.g. C), the average value is used.
-        _ATOMIC_MASS['c0']  = 12.0106
-        _ATOMIC_MASS['zn0'] = 65.38
-        _ATOMIC_MASS['pt0'] = 195.084
-        _ATOMIC_MASS['os0'] = 190.23
-        _ATOMIC_MASS['tl0'] = 204.3835
-
         # Load data from AME2012 file
         mass_file = os.path.join(os.path.dirname(__file__), 'mass.mas12')
         with open(mass_file, 'r') as ame:
@@ -171,6 +161,18 @@ def atomic_mass(isotope):
                     line[100:106] + '.' + line[107:112])
                 _ATOMIC_MASS[name.lower()] = mass
 
+        # For isotopes found in some libraries that represent all natural
+        # isotopes of their element (e.g. C0), calculate the atomic mass as
+        # the sum of the atomic mass times the natural abudance of the isotopes
+        # that make up the element.
+        for element in ['C', 'Zn', 'Pt', 'Os', 'Tl']:
+            isotope_zero = element.lower() + '0'
+            _ATOMIC_MASS[isotope_zero] = 0.
+            for iso, abundance in NATURAL_ABUNDANCE.items():
+                if re.match(r'{}\d+'.format(element), iso):
+                    _ATOMIC_MASS[isotope_zero] += abundance * \
+                                                  _ATOMIC_MASS[iso.lower()]
+
     # Get rid of metastable information
     if '_' in isotope:
         isotope = isotope[:isotope.find('_')]

openmc/material.py

@@ -64,6 +64,12 @@ class Material(object):
         List in which each item is a 3-tuple consisting of an
         :class:`openmc.Nuclide` instance, the percent density, and the percent
         type ('ao' or 'wo').
+    molar_mass : float
+        The molar mass of the material computed in units of grams per mole of
+        nuclides in a material. This entails that the molar mass does not depend
+        on the magnitude of the sum of atomic amounts of elements and nuclides
+        in the material. For instance, the molar mass of UO2 would be
+        ~90 g/mol.
 
     """
 
@@ -194,6 +200,27 @@ class Material(object):
     def distrib_otf_file(self):
         return self._distrib_otf_file
 
+    @property
+    def molar_mass(self):
+
+        # Get a list of all the nuclides, with elements expanded
+        nuclide_densities = self.get_nuclide_densities()
+
+        # Using the sum of specified atomic or weight amounts as a basis, sum
+        # the mass and moles of the material
+        mass = 0.
+        moles = 0.
+        for nuc, vals in nuclide_densities.items():
+            if vals[2] == 'ao':
+                mass += vals[1] * openmc.data.atomic_mass(nuc)
+                moles += vals[1]
+            else:
+                moles += vals[1] / openmc.data.atomic_mass(nuc)
+                mass += vals[1]
+
+        # Compute and return the molar mass
+        return mass / moles
+
     @id.setter
     def id(self, material_id):
 
@@ -581,35 +608,6 @@ class Material(object):
 
         return nuclides
 
-    def get_molar_mass(self):
-        """Returns the molar mass of the material
-
-        Returns
-        -------
-        molar_mass : float
-            The molar mass of the material
-
-        """
-
-        # Get a list of all the nuclides, with elements expanded
-        nuclide_densities = self.get_nuclide_densities()
-
-        # Using the sum of specified atomic or weight amounts as a basis, sum
-        # the mass and moles of the material
-        mass = 0.
-        moles = 0.
-        for nuc,vals in nuclide_densities.items():
-            if vals[2] == 'ao':
-                mass += vals[1] * openmc.data.atomic_mass(nuc)
-                moles += vals[1]
-            else:
-                moles += vals[1] / openmc.data.atomic_mass(nuc)
-                mass += vals[1]
-
-        # Compute and return the molar mass
-        molar_mass = mass / moles
-        return molar_mass
-
     def _get_nuclide_xml(self, nuclide, distrib=False):
         xml_element = ET.Element("nuclide")
         xml_element.set("name", nuclide[0].name)