Experimental evidence of the formation of a new chemical phase in a power ramped UO2 nuclear fuel

We report for the first time the observation of γU4O9 phase in a power ramped UO2 nuclear fuel. This phase is located within strips, a few microns wide, located at the position of the former grain boundaries at the periphery of the UO2 pellet. It has a high content in volatile fission products (Cs, I, Te) that diminish under high electron primary beam in EPMA. Thermochemical calculations allowed its identification as γU4O9.