Observations on the microstructures of mixed carbide fuel irradiated up to 10% burn up of heavy atoms

Mixed uranium-plutonium carbide fuel has been examined by optical metallography after DFR irradiation from 1 to 10 percent burnup. Second phase depletion, fission gas bubble formation, and solid fission product precipitation are described. Dicarbide completely disappeared during irradiation while sesquicarbide disappeared from hot regions only. Onset of optically visible fission gas bubbles was temperature and burn up dependent. Below 10000C peak center temperature, grain boundary gas bubble linkage is believed to have occurred above about 9 percent burnup. All grains in low temperature fuel were fissured at 10 percent burnup; the stresses associated with an asymmetrical temperature distribution are believed to have been responsible. Precipitates rich in intermetallic fission products were present in the hot regions of fuel above 4.6 percent burnup but not in specimens irradiated below about 9000C. They were often concentrated in a band around the limit of visible gas bubble formation suggesting vapor phase migration down the temperature gradient along the interconnected fission gas passages. Microprobe analysis of large inclusions revealed the presence of Pu, U, La, Ce, Nd, Pr, Ru, Rh and Pd. 22 fig