[en] Two Zircaloy-2 clad specimens containing stoichiometric UO₂ pellets were irradiated in a pressurized water loop for 379 hours at heat ratings sufficient to cause central melting of the UO₂. There was no appearance of localized overheating or accelerated corrosion of the sheath, but the diametral increases were considerably larger than those observed in loop specimens irradiated at lower heat ratings. The length increases, however, were approximately the same as those measured for specimens at lower ratings. There was a clearly visible demarcation between UO₂ that had been molten and that which had not. The value of ∫_500^oC^Tm kdθ = 74 ± W/cm was essentially the same as that obtained from the short-duration tests in the Hydraulic Rabbit, indicating there is no marked decrease in thermal conductivity of the UO₂ fuel in irradiations up to 379 hours. (author)