text: The heating of pellets can result in some of the fission products being lost from the core of the pellet. If the xenon can rapidly leave the pellet then the amount of Cs and Cs which is present in the gap between the cladding and the fuel will increase. As a result, if the zircaloy tubes holding the pellet are broken then a greater release of radioactive caesium from the fuel will occur. It is important to understand that the Cs and Cs are formed in different ways, and hence as a result the two caesium isotopes can be found at different parts of a fuel pin.
  It is clear that the volatile iodine and xenon isotopes have minutes in which they can diffuse out of the pellet and into the gap between the fuel and the cladding. Here the xenon can decay to the long lived caesium isotope.