<?xml version="1.0"?>
<materials>
  <material id="1">
    <density value="10.0" units="g/cm3" />
    <nuclide name="U235" ao="1.0" />
  </material>
  <material id="2">
    <density value="0.01" units="g/cm3" />
    <nuclide name="He4" ao="1.0" />
  </material>
  <material id="3">
    <density value="5.77" units="g/cm3" />
    <nuclide name="Zr90" ao="1.0" />
  </material>
</materials>
