From 98c287b83a692beac38eb1617f3f7315a6fa8a20 Mon Sep 17 00:00:00 2001
From: Jonathan Shimwell <mail@jshimwell.com>
Date: Tue, 2 Aug 2022 17:24:48 +0100
Subject: [PATCH] volume not needed when finding specific activity

---
 openmc/material.py                |  4 ++--
 tests/unit_tests/test_material.py | 21 ++++++++-------------
 2 files changed, 10 insertions(+), 15 deletions(-)

diff --git a/openmc/material.py b/openmc/material.py
index 61015f5b0..57f9a6f86 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -945,9 +945,9 @@ class Material(IDManagerMixin):
             activity in [Bq/g].
         """
         activity = {}
-        for nuclide, atoms in self.get_nuclide_atoms().items():
+        for nuclide, atoms in self.get_nuclide_atom_densities().items():
             inv_seconds = openmc.data.decay_constant(nuclide)
-            activity[nuclide] = (inv_seconds * atoms) / self.get_mass()
+            activity[nuclide] = (inv_seconds * atoms * 1.0e24) / self.density
         return activity
 
     def get_nuclide_atoms(self):
diff --git a/tests/unit_tests/test_material.py b/tests/unit_tests/test_material.py
index d70161f48..c959ba276 100644
--- a/tests/unit_tests/test_material.py
+++ b/tests/unit_tests/test_material.py
@@ -481,12 +481,16 @@ def test_activity_of_stable():
 
 
 def test_activity_of_tritium():
-    """Checks that 1g of tritium has the correct activity"""
+    """Checks that 1g of tritium has the correct activity activity scaling"""
     m1 = openmc.Material()
     m1.add_nuclide("H3", 1)
     m1.set_density('g/cm3', 1)
     m1.volume = 1
     assert pytest.approx(m1.activity) == 3.559778e14
+    m1.set_density('g/cm3', 2)
+    assert pytest.approx(m1.activity) == 3.559778e14*2
+    m1.volume = 3
+    assert pytest.approx(m1.activity) == 3.559778e14*2*3
 
 
 def test_activity_of_metastable():
@@ -499,18 +503,9 @@ def test_activity_of_metastable():
 
 
 def test_specific_activity_of_tritium():
-    """Checks that specific activity stays the same for all volumes and
-    densities while activity changes proportionally to mass"""
+    """Checks that specific activity of tritium is correct"""
     m1 = openmc.Material()
     m1.add_nuclide("H3", 1)
     m1.set_density('g/cm3', 1)
-    m1.volume = 1
-    # activity and specific_activity are initially the same as we have 1g
-    assert pytest.approx(m1.specific_activity) == 3.559778e14
-    assert pytest.approx(m1.activity) == 3.559778e14
-    m1.set_density('g/cm3', 2)
-    assert pytest.approx(m1.specific_activity) == 3.559778e14
-    assert pytest.approx(m1.activity) == 3.559778e14*2
-    m1.volume = 3
-    assert pytest.approx(m1.specific_activity) == 3.559778e14
-    assert pytest.approx(m1.activity) == 3.559778e14*2*3
+    assert m1.specific_activity == 355978108155965.9
+    assert m1.get_nuclide_specific_activity() == {"H3": 355978108155965.9}
\ No newline at end of file