ABSTRACT
This chapter discusses features of the changes in chemical constitution which occur in fuel and fuel rods for water reactors during operation and in fault conditions. The fuel for water reactors consists of pellets of urania clad in Zircaloy. A knowledge of the nature of the defects in the urania lattice is an essential step for the prediction of diffusion behaviour in this fluorite structured lattice and that containing fission product atoms and ions. Studies of kinetic and thermodynamic aspects of fission product oxide fuel systems have enabled considerable understanding of the development of and changes in chemical constitution of fuel during operation and failure. The fate of the released fission product species in steam has to be considered in the modelling of their chemistry in other regions of the reactor system. In a pressurized water reactor fuel rod, which operates at relatively low temperatures little diffusion of fission product elements to the gap would occur.
