B4C/Zircaloy reaction at temperatures from 1,173 to 1,953 K

With a view to obtaining the basic information on the influence of the BWR type control rod failure on the bundle degradation during a severe accident, the chemical interactions between B4C neutron absorber material and Zircaloy were investigated in the temperature range 1,173 to 1,953 K. Reaction layers were formed at the interface between two materials. Growth of the reaction layer and decrease in the Zircaloy thickness were measured as a function of temperature and time, thereby the reaction kinetics was evaluated. The overall reaction generally obeyed a parabolic rate law and the reaction rate constant at each examined temperature was determined. A sudden increase of the reaction rate was found at a temperature between 1,823 and 1,873 K. It could be caused by liquid phase formation of the zirconium-boron carbide system at the corresponding temperature, which occurred locally at the reaction front. Accordingly, the temperature dependence of the reaction rate constants was estimated separately for the two temperature ranges of 1,173 to 1,823 K and 1,873 to 1,953 K.