OpenMC/examples/lattice/nested/materials.xml

<?xml version="1.0"?>
<materials>

  <default_xs>70c</default_xs>

  <!-- Definition of materials -->
  <material id="1">
    <density value="4.5" units="g/cc" />
    <nuclide name="U-235" ao="1.0" />
  </material>

  <material id="2">
    <density value="1.0" units="g/cc" />
    <nuclide name="H-1"  ao="2.0" />
    <nuclide name="O-16" ao="1.0" />
    <sab name="lwtr" xs="10t" />
  </material>

</materials>