Chemical interactions that may occur in a light water reactor fuel rod bundle containing Ag-In-Cd absorber rods or Al203/B4C burnable poison rods with increasing temperature up to the complete melting of the components and the reaction products formed are described. The kinetics of the most important chemical interactions is investigated and the results are described. In most cases, the reaction products have lower melting points or ranges than the original components. This results in a relocation of liquefied components, often far below their melting points. Three distinct temperature regimes exist in which liquid phases can form in the core in different large quantities and are described in detail. The phase relations in the important ternary U-Zr-0 system are extensively studied. The effect of steel constituents on the phase relations is also given. These considerations focus on pressurized water reactor conditions only.