Print warning if no natural isotopes when using add_element and wrote…

… unit test (openmc-dev#2938)

Co-authored-by: Catherine Yu <[email protected]>
Co-authored-by: Paul Romano <[email protected]>

openmc/element.py

@@ -1,4 +1,5 @@
 import re
+import warnings
 
 import lxml.etree as ET
 
@@ -123,6 +124,10 @@ def expand(self, percent, percent_type, enrichment=None,
         # Get the nuclides present in nature
         natural_nuclides = {name for name, abundance in natural_isotopes(self)}
 
+        # Issue warning if no existing nuclides
+        if len(natural_nuclides) == 0:
+            warnings.warn(f"No naturally occurring isotopes found for {self}.")
+
         # Create dict to store the expanded nuclides and abundances
         abundances = {}
 

tests/unit_tests/test_element.py

@@ -1,5 +1,5 @@
 import openmc
-from pytest import approx, raises
+from pytest import approx, raises, warns
 
 from openmc.data import NATURAL_ABUNDANCE, atomic_mass
 
@@ -37,6 +37,13 @@ def test_expand_enrichment():
         assert isotope[1] == approx(ref[isotope[0]])
 
 
+def test_expand_no_isotopes():
+    """Test that correct warning is raised for elements with no isotopes"""
+    with warns(UserWarning, match='No naturally occurring'):
+        element = openmc.Element('Tc')
+        element.expand(100.0, 'ao')
+
+
 def test_expand_exceptions():
     """ Test that correct exceptions are raised for invalid input """