UO$sub 2$ SWAGED POWDER FUEL ROD FAILURES AT HIGH THERMAL PERFORMANCE CONDITIONS. Preliminary Results

The initial two irradiations of Euratom Program fuel assemblies containing compacted powder fuel rods resulted in fuel rod failures during GETR startup. The first failure occurred on December 20, 1982, when the reactor power rise from 20 to 25 Mw was initiated. The failure was found to be a longitudinal split in the cladding of the maximum power rod in the assembly, approximately 3/4 of an in. long and 5 in. from the bottom end of the rod. The calculated peak surface heat flux in this rod, 4 in. above the point of failure, was 720,000 Btu/ hr-ft/sup 2/. These initial 4 rods in the assembly were all Zircaloy-2 clad, arc- fused UO/sup 2/, vibrated and swaged to a density of 90.5%. Two rods from the assembly, including the failed rod, were replaced by similar UO/sup 2/powder rods fabricated to 86% density by vibratory compaction alone. The replacement assembly was inserted in the PWL for startup on January 8, 1983. Again a failure occurred during startup, this time when the reactor power rise from 25to 27 Mw was initiated. The failure again occurred in the higher power, swaged powder fuel rod. Its calculated peak surface heat flux at the timemore » of failure was approximately 750,000 Btu/hr-ft/sup 2/. Both swaged rods experienced clad swelling and could not be removed from the assembly. Both vibratory compacted rods appeared satisfactory with no clad swelling. The higher power, vibratory compacted rod attained a peak surface heat flux in excess of 800,000 Btu/hr-ft/ sup 2/ during the startup. The currently available results from the examination of these two failures are described and the preliminary conclusions concerning the failure mechanism presented. (auth)« less