diff --git a/openmc/geometry.py b/openmc/geometry.py
index 4bb6e23e46..02bcbe180d 100644
--- a/openmc/geometry.py
+++ b/openmc/geometry.py
@@ -63,15 +63,15 @@ class Geometry(object):
 
         """
         if volume_calc.domain_type == 'cell':
-            for cell in self.get_all_cells():
+            for cell in self.get_all_cells().values():
                 if cell.id in volume_calc.volumes:
                     cell.add_volume_information(volume_calc)
         elif volume_calc.domain_type == 'material':
-            for material in self.get_all_materials():
+            for material in self.get_all_materials().values():
                 if material.id in volume_calc.volumes:
                     material.add_volume_information(volume_calc)
         elif volume_calc.domain_type == 'universe':
-            for universe in self.get_all_universes():
+            for universe in self.get_all_universes().values():
                 if universe.id in volume_calc.volumes:
                     universe.add_volume_information(volume_calc)
 
diff --git a/openmc/material.py b/openmc/material.py
index cd19225364..92d992a6f6 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -352,7 +352,7 @@ class Material(object):
         """
         if volume_calc.domain_type == 'material':
             if self.id in volume_calc.volumes:
-                self._volume = volume_calc.volumes[self.id]
+                self._volume = volume_calc.volumes[self.id][0]
                 self._atoms = volume_calc.atoms[self.id]
             else:
                 raise ValueError('No volume information found for this material.')
diff --git a/openmc/universe.py b/openmc/universe.py
index 12bef012b9..1c631e7e07 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -184,9 +184,9 @@ class Universe(object):
             Results from a stochastic volume calculation
 
         """
-        if volume_calc.domain_type == 'cell':
+        if volume_calc.domain_type == 'universe':
             if self.id in volume_calc.volumes:
-                self._volume = volume_calc.volumes[self.id]
+                self._volume = volume_calc.volumes[self.id][0]
                 self._atoms = volume_calc.atoms[self.id]
             else:
                 raise ValueError('No volume information found for this universe.')
@@ -441,9 +441,22 @@ class Universe(object):
             (nuclide, density)
 
         """
+        nuclides = OrderedDict()
 
-        raise NotImplementedError('Determining average nuclide densities over '
-                                  'an entire universe not yet supported.')
+        if self._atoms is not None:
+            volume = self.volume
+            for name, atoms in self._atoms.items():
+                nuclide = openmc.Nuclide(name)
+                density = 1.0e-24 * atoms[0]/volume  # density in atoms/b-cm
+                nuclides[name] = (nuclide, density)
+        else:
+            raise RuntimeError(
+                'Volume information is needed to calculate microscopic cross '
+                'sections for universe {}. This can be done by running a '
+                'stochastic volume calculation via the '
+                'openmc.VolumeCalculation object'.format(self.id))
+
+        return nuclides
 
     def get_all_cells(self):
         """Return all cells that are contained within the universe
diff --git a/tests/test_volume_calc/test_volume_calc.py b/tests/test_volume_calc/test_volume_calc.py
index 52b474cb7d..003528b0df 100644
--- a/tests/test_volume_calc/test_volume_calc.py
+++ b/tests/test_volume_calc/test_volume_calc.py
@@ -42,7 +42,7 @@ class VolumeTest(PyAPITestHarness):
         geometry.export_to_xml()
 
         # Set up stochastic volume calculation
-        ll, ur = openmc.Union(*[c.region for c in root.cells.values()]).bounding_box
+        ll, ur = root.bounding_box
         vol_calcs = [
             openmc.VolumeCalculation(list(root.cells.values()), 100000),
             openmc.VolumeCalculation([water, fuel], 100000, ll, ur),