The performance of EBR-2 Mark-II fuel at 675/sup 0/C is significantly different from that at 590/sup 0/C. Despite the differences, the elements were all successfully irradiated to 8 at. % burnup before being removed from the reactor unbreached. Some of the more significant differences in performance at the higher temperatures are (1) The fuel-element cladding showed a double-peaking strain profile, with enhanced swelling and creep due to extensive carbide precipitation, an FCCI enhanced fission gas stress, or FCMI. (2) The FCCI zone in the fuel was cracked and represented 25% of the cladding thickness. This zone was substantially larger than observed at 590/sup 0/C. (3) The fuel pin lifted 12 mm to the restrainer dimple, so some type of restrainer may be necessary in future designs. A restrainer had previously been deemed unnecessary based on irradiations at 590/sup 0/C. (4) The fission-gas release is less at a fuel-centerline temperature of 720/sup 0/C compared at 650/sup 0/C. This difference is primarily due to the upper third of the element having very little open porosity. Owing to the lack of bnd sodium infiltration into the fuel at the higher temperature, the fission gas pressure is the same at both temperatures
