FUEL ELEMENT DEVELOPMENT PROGRAM FOR THE PEBBLE BED REACTOR. Quarterly Progress Report, May 1, 1960-July 31, 1960

Fabrication of alumina-coated UO/sub 2/ and pyrolytic carboncoated UC/ sub 2/ particles was studied. Some reaction was noted between alumina and graphite at 2500 deg F. For UC/sub 2/ particles coated with carbon at 2000 deg F, the coatings were found to crack at temperaturss above 2000 deg F, whereas 2450 deg F deposition gave fewer failures at 3600 deg F, more rapid deposition- ratss, and absence of excess soot formation. A Pebble Bed Reactor fuel element consisting of a 1.5-in. graphite sphere fueled with alumina-coated UO/sub 2/ particles was irradiated at 1400 deg F to a burrup of 3.3 at.% U/sup 235/. Up to 2.5 at.% burnup, the fission product leakage factors (rate of release/rate of production) ranged between 10/sup -9/ and 10/sup -5/ for 10 isotopes and were ascribed to trace amounts of uranium contamination outside the particle coatings. In the latter part of the quarter, ths leakage factor for Xe/sup 133/ rose to lO/ sup -3/ while several other shorter lived fission products increased to a smaller extent, indicating the beginning of diffusion through the coatings due to radiation damage of the alumina. Carbon-coated UC/sub 2/ particles coated at 2000 deg F were found to have good fission product retention up to 2000 deg F, and less than 4.5 x 10/sup -6/ of the Xen/sup 133/ was release d in a 4-hr test. Graphite bodies of density>2.0 g/cm/sup 3/ were prepared using natural graphite powder as filler material, and their crushing strength was on the same order as that of AGOT synthetic graphite. (D. L. C.