CHEMICAL INTERACTIONS IN WATER-COOLED NUCLEAR-FUEL - A THERMOCHEMICAL APPROACH

The complex equilibria in irradiated water-cooled nuclear fuel are discussed. Thermochemical calculations have been performed to predict the chemical state of the fission products I, Cs, Te, Ba, Sr, Mo, Ru, Zr and the rare earths in the fuel as a function of oxygen potential and temperature. Under normal operating conditions the compounds CsI, Cs//2Te, and Cs//2MoO//4 are stable. During reactor accidents, as the temperature rises, these phases will all nucleate and will determine the release of fission products from defected fuel elements, for instance, by the interaction of the fuel with the cooling water. It has been demonstrated that large gaps exist in our knowledge of the thermochemistry of volatile compounds. Consequently, it is still difficult to give an accurate analysis of the observed phenomena, and the possible reactions, based on thermochemical calculations.