diff --git a/openmc/clean_xml.py b/openmc/clean_xml.py
index 564281a5cc..6aaf64c661 100644
--- a/openmc/clean_xml.py
+++ b/openmc/clean_xml.py
@@ -65,25 +65,25 @@ def sort_xml_elements(tree):
     tree.extend(sorted_elements)
 
 
-def clean_xml_indentation(element, level=0):
+def clean_xml_indentation(element, level=0, spaces_per_level=4):
     """
     copy and paste from http://effbot.org/zone/elementent-lib.htm#prettyprint
     it basically walks your tree and adds spaces and newlines so the tree is
     printed in a nice way
     """
 
-    i = "\n" + level*"    "
+    i = "\n" + level*spaces_per_level*" "
 
     if len(element):
 
         if not element.text or not element.text.strip():
-            element.text = i + "    "
+            element.text = i + spaces_per_level*" "
 
         if not element.tail or not element.tail.strip():
             element.tail = i
 
         for sub_element in element:
-            clean_xml_indentation(sub_element, level+1)
+            clean_xml_indentation(sub_element, level+1, spaces_per_level)
 
         if not sub_element.tail or not sub_element.tail.strip():
             sub_element.tail = i