From 064426a05fdac019917d613eb2805fb80f5fb5ad Mon Sep 17 00:00:00 2001
From: Jonathan Shimwell <mail@jshimwell.com>
Date: Tue, 2 Aug 2022 13:17:33 +0100
Subject: [PATCH] added specific_activity

---
 openmc/material.py                | 25 +++++++++++++++++++++++++
 tests/unit_tests/test_material.py | 18 ++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/openmc/material.py b/openmc/material.py
index 6c2641680..61015f5b0 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -94,6 +94,9 @@ class Material(IDManagerMixin):
     activity : float
         Activity of the material in [Bq]. Requires that the :attr:`volume`
         attribute is set.
+    specific_activity : float
+        Activity of the material per unit mass in [Bq/kg]. Requires that the
+        :attr:`volume` and :attr:`density` attributes are set.
 
     """
 
@@ -155,6 +158,11 @@ class Material(IDManagerMixin):
         """Returns the total activity of the material in Becquerels."""
         return sum(self.get_nuclide_activity().values())
 
+    @property
+    def specific_activity(self):
+        """Returns the total specific activity of the material in Becquerels per gram."""
+        return sum(self.get_nuclide_specific_activity().values())
+
     @property
     def name(self):
         return self._name
@@ -925,6 +933,23 @@ class Material(IDManagerMixin):
             activity[nuclide] = inv_seconds * atoms
         return activity
 
+    def get_nuclide_specific_activity(self):
+        """Return specific activity in [Bq/g] for each nuclide in the material
+
+        .. versionadded:: 0.13.1
+
+        Returns
+        -------
+        dict
+            Dictionary whose keys are nuclide names and values are specific
+            activity in [Bq/g].
+        """
+        activity = {}
+        for nuclide, atoms in self.get_nuclide_atoms().items():
+            inv_seconds = openmc.data.decay_constant(nuclide)
+            activity[nuclide] = (inv_seconds * atoms) / self.get_mass()
+        return activity
+
     def get_nuclide_atoms(self):
         """Return number of atoms of each nuclide in the material
 
diff --git a/tests/unit_tests/test_material.py b/tests/unit_tests/test_material.py
index 2c5cc2874..d70161f48 100644
--- a/tests/unit_tests/test_material.py
+++ b/tests/unit_tests/test_material.py
@@ -496,3 +496,21 @@ def test_activity_of_metastable():
     m1.set_density('g/cm3', 1)
     m1.volume = 98.9
     assert pytest.approx(m1.activity, rel=0.001) == 1.93e19
+
+
+def test_specific_activity_of_tritium():
+    """Checks that specific activity stays the same for all volumes and
+    densities while activity changes proportionally to mass"""
+    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