C2N-C5N fuel swelling problem radiometallurgy examination

Fuel element swelling problems have been experienced in the high-exposure fuel irradiation program in C Reactor, In an effort to determine the mechanism responsible for this swelling, three elements from C Reactor, one of which was a ruptures, were sent to the Radiometallurgy Laboratory for visual and metallographic examination. Because the swelled fuel was not numbered as to column position, the exact position of the examined elements is not known but they were believed to have come from the center of the charge and therefore exposures for these individual elements will be 25--50% higher than the column average exposures. This percentage figure was determined from data from nine tubes of C2N fuel which exhibited swelling. These nine tubes operated under nearly the same conditions as the examined elements. Maximum core temperatures for the examined elements are believed to be between 300--340C. The mechanisms for swelling were different in the high and low-exposure fuels. The high-exposure fuel exhibited grain-boundary tearing along a wide circumferential band over the entire transverse section of the fuel examined. The areas near the OD and ID were relatively free from tearing. The low-exposure fuel showed no grain boundary tearing but it did contain angular voids in the same general shape of the typical carbide particles found in this alloy