The results of a series of irradiations of refractory fuels at cladding surface temperatures from 1000 to 1900°C were examined to determine which factors have the greatest influence on swelling rates at high temperatures. The effect of temperature on swelling rates is so large that it was found desirable to plot the data as Arrhenius functions of temperature in order to assess the relatively smaller effects of cladding restraint, stoichiometry, burnup, distributed voidage, cracks, and other factors.