From 1c5aa6559be235abeb1c7a4c5d596d5ec617947d Mon Sep 17 00:00:00 2001
From: GuySten <62616591+GuySten@users.noreply.github.com>
Date: Wed, 18 Jun 2025 18:10:03 +0300
Subject: [PATCH] fixing expansion of elemental Ta bug (#3443)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>
---
 openmc/data/data.py              | 2 +-
 openmc/element.py                | 9 +++++----
 tests/unit_tests/test_element.py | 7 +++++++
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/openmc/data/data.py b/openmc/data/data.py
index 408adf2e42..8ccc8d2635 100644
--- a/openmc/data/data.py
+++ b/openmc/data/data.py
@@ -94,7 +94,7 @@ NATURAL_ABUNDANCE = {
     'Yb174': 0.32025, 'Yb176': 0.12995, 'Lu175': 0.97401,
     'Lu176': 0.02599, 'Hf174': 0.0016, 'Hf176': 0.0526,
     'Hf177': 0.186, 'Hf178': 0.2728, 'Hf179': 0.1362,
-    'Hf180': 0.3508, 'Ta180': 0.0001201, 'Ta181': 0.9998799,
+    'Hf180': 0.3508, 'Ta180_m1': 0.0001201, 'Ta181': 0.9998799,
     'W180': 0.0012, 'W182': 0.265, 'W183': 0.1431,
     'W184': 0.3064, 'W186': 0.2843, 'Re185': 0.374,
     'Re187': 0.626, 'Os184': 0.0002, 'Os186': 0.0159,
diff --git a/openmc/element.py b/openmc/element.py
index 082bee5226..f1d8f5f24d 100644
--- a/openmc/element.py
+++ b/openmc/element.py
@@ -144,8 +144,7 @@ class Element(str):
             root = tree.getroot()
             for child in root.findall('library'):
                 nuclide = child.attrib['materials']
-                if re.match(r'{}\d+'.format(self), nuclide) and \
-                   '_m' not in nuclide:
+                if re.match(r'{}\d+'.format(self), nuclide):
                     library_nuclides.add(nuclide)
 
             # Get a set of the mutual and absent nuclides. Convert to lists
@@ -185,8 +184,10 @@ class Element(str):
                 for nuclide in absent_nuclides:
                     if nuclide in ['O17', 'O18'] and 'O16' in mutual_nuclides:
                         abundances['O16'] += NATURAL_ABUNDANCE[nuclide]
-                    elif nuclide == 'Ta180' and 'Ta181' in mutual_nuclides:
-                        abundances['Ta181'] += NATURAL_ABUNDANCE[nuclide]
+                    elif nuclide == 'Ta180_m1' and 'Ta180' in library_nuclides:
+                            abundances['Ta180'] = NATURAL_ABUNDANCE[nuclide]
+                    elif nuclide == 'Ta180_m1' and 'Ta181' in mutual_nuclides:
+                            abundances['Ta181'] += NATURAL_ABUNDANCE[nuclide]
                     elif nuclide == 'W180' and 'W182' in mutual_nuclides:
                         abundances['W182'] += NATURAL_ABUNDANCE[nuclide]
                     else:
diff --git a/tests/unit_tests/test_element.py b/tests/unit_tests/test_element.py
index d3555701e2..d91cfaf6e2 100644
--- a/tests/unit_tests/test_element.py
+++ b/tests/unit_tests/test_element.py
@@ -44,6 +44,13 @@ def test_expand_no_isotopes():
         element.expand(100.0, 'ao')
 
 
+def test_expand_ta():
+    ref = {'Ta180': 0.01201, 'Ta181': 99.98799}
+    element = openmc.Element('Ta')
+    for isotope in element.expand(100.0, 'ao'):
+        assert isotope[1] == approx(ref[isotope[0]])
+    
+
 def test_expand_exceptions():
     """ Test that correct exceptions are raised for invalid input """