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

  <surface id="1"  type="sphere" coeffs="0. 0. 0. 1.4" />
  <surface id="2"  type="sphere" coeffs="0. 0. 0. 5.35" boundary="vacuum" />

  <cell id="1"  material="void" region="-1" />      <!-- cavity -->
  <cell id="2"  material="1"    region=" 1  -2" />  <!-- Pu Core -->

</geometry>
