The microstructural response of fast breeder reactor fuel to accident transients has been analyzed. Based on experimental results, fuel response can be classified as either basically brittle or basically ductile in nature. In the analysis, the type of response is assumed to be determined by the behavior of grain boundary fission gas. The transient variables taken into consideration are the temperature, heating rate, the mean gas content per bubble, mean bubble spacing in the grain boundary, and the stresses resolved normal to grain boundaries containing gas bubbles. By calculating the rate at which a grain boundary bubble grows as a sharp crack and comparing it to the rate of bubble growth by mass transport, a criterion is established to predict the characteristic response of a fuel sample to a specified thermal transient. A swelling threshold time is also determined for the case of ductile fuel behavior. Tensile stresses applied to the grain boundary are shown to enhance brittle behavior, and compressive stresses are shown to enhance ductile behavior. When average values of the relevant variables are extracted from a number of fission gas release and direct electric heating experiments and are used in the above calculation, fuel behavior predictions for these tests are found to correspond well with the experimental results.