From f805fddb6b2efd4ff44b3c491448ddccd57294ad Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Thu, 9 Mar 2017 15:48:23 -0600
Subject: [PATCH] Handle unbounded universe

---
 openmc/universe.py | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/openmc/universe.py b/openmc/universe.py
index 71bb2ab624..0d5b8395a5 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -113,8 +113,13 @@ class Universe(object):
 
     @property
     def bounding_box(self):
-        regions = [c.region for c in self.cells.values()]
-        return openmc.Union(*regions).bounding_box
+        regions = [c.region for c in self.cells.values()
+                   if c.region is not None]
+        if regions:
+            return openmc.Union(*regions).bounding_box
+        else:
+            # Infinite bounding box
+            return openmc.Intersection().bounding_box
 
     @id.setter
     def id(self, universe_id):