Fix the bug in the Material.from_xml_element function (openmc-dev#3278)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

openmc/material.py

@@ -1582,7 +1582,6 @@ def from_xml_element(cls, elem: ET.Element) -> Material:
 
         if 'volume' in elem.attrib:
             mat.volume = float(elem.get('volume'))
-        mat.depletable = bool(elem.get('depletable'))
 
         # Get each nuclide
         for nuclide in elem.findall('nuclide'):
@@ -1592,6 +1591,9 @@ def from_xml_element(cls, elem: ET.Element) -> Material:
             elif 'wo' in nuclide.attrib:
                 mat.add_nuclide(name, float(nuclide.attrib['wo']), 'wo')
 
+        # Get depletable attribute
+        mat.depletable = elem.get('depletable') in ('true', '1')
+
         # Get each S(a,b) table
         for sab in elem.findall('sab'):
             fraction = float(sab.get('fraction', 1.0))