Microstructure and fission gas bubbles in irradiated mixed carbide fuels at 2 to 11 a/o burnup

An analysis of the defect structure and of small fission gas bubbles has been performed on mixed carbide fuels with burn-ups between 1.8 and 11 a/o by transmission electron microscopy (TEM). A complex defect structure consisting of dislocations, loops and at least 3 types of solid fission product precipitates was observed. Na-bonded carbides develop predominantly a dislocation network increasing in density with burn-up whereas He-bonded carbides showed mainly a corresponding network of crystallographic needle precipitates. Locally the nucleation and growth of small fission gas bubbles with 1 to 20 nm diameters (bubble population P1) is closely related to their dislocation or needle environment, larger bubbles with diameters 30 to 50 nm appear to be mostly associated with platelike precipitates or dislocation boundaries. The local swelling contribution μ1 of bubble population P1 is < or approx. 0.5% of the total amount of gas created over the whole burn-up range investigated. (orig.)