AFIP-6 Breach Assessment Report

Analysis of the AFIP-6 experiment is summarized in this report in order to determine the cause of gaseous fission product release observed during irradiation. During the irradiation, a series of small fission product releases were observed. In order to limit the potential for primary coolant contamination, the operating cycle was terminated and the AFIP-6 experiment was removed for examination. Both in-canal and post-irradiation examination revealed the presence of an unusually thick oxide layer and discrete surface blisters on the fuel plates. These blisters were the likely cause of fission product release. Subsequent detailed thermal hydraulic analysis of the experiment indicated that the combination of the high operating power and test vehicle configuration led to high nominal operating temperatures for the fuel plates. This elevated temperature led to accelerated surface corrosion and eventually spallation of the fuel plate cladding. The thermal insulating nature of this corrosion layer led to significantly elevated fuel meat temperatures that induced blistering. Analysis was performed to validate a corrosion rate model and criteria for onset of spallation type surface corrosion were determined. The corrosion rate model will be used to estimate the oxide thickness anticipated for experiments in the future. The margin to the spallation threshold will then be used to project the experiment performance.