Address comments by @simondrichards

Also remove spaces between Raises entries in docstring of Element.expand

docs/source/usersguide/materials.rst

@@ -55,6 +55,20 @@ following would add 3.2% enriched uranium to a material::
 In addition to U235 and U238, concentrations of U234 and U236 will be present
 and are determined through a correlation based on measured data.
 
+It is also possible to perform enrichment of any element that is composed
+of two naturally-occurring isotopes (e.g. Li, B) in terms of atomic percent.
+To invoke it, provide additional arguments `enrichment_target` to
+:meth:`Material.add_element`. For example the following would enrich B10
+to 30ao%::
+
+   mat.add_element('B', 1.0, enrichment=30.0, enrichment_target='B10')
+
+In order to enrich an isotope in terms of mass percent (wo%). Provide extra
+argument `enrichment_type`. For example the following would enrich Li6 to 15wo%::
+
+   mat.add_element('Li', 1.0, enrichment=15.0, enrichment_target='Li6',
+                   enrichment_type='wo')
+
 Often, cross section libraries don't actually have all naturally-occurring
 isotopes for a given element. For example, in ENDF/B-VII.1, cross section
 evaluations are given for O16 and O17 but not for O18. If OpenMC is aware of

openmc/element.py

@@ -78,25 +78,19 @@ class Element(str):
         ------
         ValueError
             No data is available for any of natural isotopes of the element
-
         ValueError
             If only some natural isotopes are avaiable in cross-sections data
             library and element is not O, W or Ta
-
         ValueError
             Enrichment of isotope which is not present in natural composition
             of the element is requested
-
         ValueError
             Enrichment is requested of the element that is not composed of
             two isotopes.
-
         ValueError
             If `enrichment_type` is not 'ao' or 'wo'
-
         ValueError
             If `enrichment` is outside <0;100> range
-
         ValueError
             If enrichment procedure for Uranium is used when element is not
             Uranium.
@@ -112,7 +106,7 @@ class Element(str):
 
         When the `enrichment` argument is specified with `enrichment_target` a
         general enrichment procedure is used. It will raise exception unless
-        element is composed of excatly 2 isotopes. `enrichment` is interpreted
+        element is composed of exactly 2 isotopes. `enrichment` is interpreted
         as percent. By default it is atomic. Can be controlled by variable
         `enrichment_type`.
 
@@ -241,11 +235,21 @@ class Element(str):
         # Interpret required enrichment as weight
         elif enrichment is not None and enrichment_target is not None:
 
-            # Check if is a single isotope mixture
+            # Provide more informative error message for U235
+            if enrichment_target == 'U235':
+                msg = "There is a special procedure for enrichment of U235 "\
+                      "in U. To invoke it do not specify neither "\
+                      "'enrichment_target' nor 'enrichment_type'. Provide "\
+                      "only enrichment as 'wo%'. See User Guide for more "\
+                      "details"
+                raise ValueError(msg)
+
+            # Check if it is two-isotope mixture
             if len(abundances) != 2:
-                msg = 'Element {0} does not consist of 2 isotopes. Thus it '\
-                      'cannot be enriched with in-build procedure. Please '\
-                      'enter isotopic abundances manually. '.format(self)
+                msg = 'Element {0} does not consist of 2 naturally-occurring '\
+                      'isotopes. Therefore it cannot be enriched with the '\
+                      'in-build procedure. Please enter isotopic abundances '\
+                      'manually.'.format(self)
                 raise ValueError(msg)
 
             # Check if the target nuclide is present in the mixture

openmc/material.py

@@ -529,7 +529,7 @@ class Material(IDManagerMixin):
         Notes
         -----
         General enrichment procedure is allowed only for elements composed of
-        two isotopes. If `enrichment_target` is given withou `enrichment`
+        two isotopes. If `enrichment_target` is given without `enrichment`
         natural composition is added to the material.
 
         """

tests/unit_tests/test_element.py

@@ -7,7 +7,7 @@ from openmc.data import NATURAL_ABUNDANCE, atomic_mass
 TOL = 1e-9
 
 
-def test_expand_no_ernichment():
+def test_expand_no_enrichment():
     """ Expand Li in natural compositions"""
     lithium = openmc.element.Element('Li')
 
@@ -15,7 +15,7 @@ def test_expand_no_ernichment():
     for isotope in lithium.expand(100.0, 'ao'):
         assert isotope[1] == pt.approx(NATURAL_ABUNDANCE[isotope[0]] * 100.0, rel=TOL)
 
-    # Verify the expanction into WEIGHT fraction against natural composition
+    # Verify the expansion into WEIGHT fraction against natural composition
     natural = {'Li6': NATURAL_ABUNDANCE['Li6'] * atomic_mass('Li6'),
                'Li7': NATURAL_ABUNDANCE['Li7'] * atomic_mass('Li7')}
     li_am = sum(natural.values())
@@ -41,7 +41,7 @@ def test_expand_enrichment():
 
 
 def test_expand_exceptions():
-    """ Test that correct exceptions are raiused for invalid input """
+    """ Test that correct exceptions are raised for invalid input """
 
     # 1 Isotope Element
     with pt.raises(ValueError):
@@ -71,5 +71,9 @@ def test_expand_exceptions():
     # Invalid Enrichment Type Entry
     with pt.raises(ValueError):
         element = openmc.element.Element('Li')
-        fail = element.expand(70.0, 'ao', 4.0, 'Li7','Grand Moff Tarkin')
+        fail = element.expand(70.0, 'ao', 4.0, 'Li7', 'Grand Moff Tarkin')
 
+    # Trying to enrich Uranium
+    with pt.raises(ValueError):
+        element = openmc.element.Element('U')
+        fail = element.expand(70.0, 'ao', 4.0, 'U235', 'wo')