OpenMC/examples/xml/boxes/materials.xml

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

  <material id="1">
    <density value="4.5" units="g/cc" />
    <nuclide name="U235" ao="1.0" />
  </material>

  <material id="2">
    <density value="4.5" units="g/cc" />
    <nuclide name="U238" ao="1.0" />
  </material>

  <material id="3">
    <density value="1.0" units="g/cc" />
    <nuclide name="O16" ao="1.0" />
    <nuclide name="H1" ao="2.0" />
    <sab name="c_H_in_H2O" />
  </material>

</materials>