added chain get daughters

openmc/deplete/chain.py

@@ -1237,6 +1237,49 @@ class Chain:
 
         return new_chain
 
+    def get_decay_daughters(self, nuclide):
+        """Return all radioactive nuclides reachable via decay.
+
+        Unlike :meth:`reduce`, this follows only decay paths (not
+        reactions such as neutron capture) and returns a set of nuclide
+        names rather than a new :class:`Chain`. Stable nuclides are not
+        included in the result.
+
+        Parameters
+        ----------
+        nuclide : str
+            Name of the parent nuclide.
+
+        Returns
+        -------
+        set of str
+            Names of all radioactive decay daughters reachable from the
+            parent.
+
+        """
+        daughters = set()
+        stack = [nuclide]
+        visited = set()
+        while stack:
+            name = stack.pop()
+            if name in visited:
+                continue
+            visited.add(name)
+            if name not in self.nuclide_dict:
+                continue
+            for mode in self[name].decay_modes:
+                target = mode.target
+                if target is None or target in visited:
+                    continue
+                if target not in self.nuclide_dict:
+                    continue
+                target_nuc = self[target]
+                if target_nuc.half_life is None or target_nuc.half_life <= 0:
+                    continue
+                daughters.add(target)
+                stack.append(target)
+        return daughters
+
     def _follow(self, isotopes, level):
         """Return all isotopes present up to depth level"""
         found = isotopes.copy()

tests/unit_tests/test_deplete_chain.py

@@ -587,3 +587,35 @@ def test_reduce(gnd_simple_chain, endf_chain):
     reduced_chain = endf_chain.reduce(['U235'])
     assert 'H1' in reduced_chain
     assert 'H2' in reduced_chain
+
+
+def test_get_decay_daughters(gnd_simple_chain):
+    """Test get_decay_daughters follows only decay paths."""
+    # I135 decays to Xe135 (radioactive), which decays to Cs135 (stable)
+    daughters = gnd_simple_chain.get_decay_daughters("I135")
+    assert daughters == {"Xe135"}
+
+    # Xe135 decays to Cs135 (stable) — no radioactive daughters
+    daughters = gnd_simple_chain.get_decay_daughters("Xe135")
+    assert daughters == set()
+
+    # Reaction product Xe136 should NOT appear (it comes from n,gamma)
+    daughters = gnd_simple_chain.get_decay_daughters("I135")
+    assert "Xe136" not in daughters
+
+    # Stable nuclide returns empty set
+    daughters = gnd_simple_chain.get_decay_daughters("Cs135")
+    assert daughters == set()
+
+    # Nuclide not in chain returns empty set
+    daughters = gnd_simple_chain.get_decay_daughters("Xx999")
+    assert daughters == set()
+
+
+def test_get_decay_daughters_cyclic(simple_chain):
+    """Test get_decay_daughters handles cyclic decay chains."""
+    daughters = simple_chain.get_decay_daughters("A")
+    assert daughters == {"B"}
+
+    daughters = simple_chain.get_decay_daughters("B")
+    assert daughters == {"A"}