To some extent, rare-earth-doped UO2 is representative of an irradiated nuclear fuel. The two phases we observed previously in neodymium-doped UO2 are now interpreted as the existence of a miscibility gap in the U-Nd-O phase diagram using new results obtained with Raman spectroscopy. Extrapolating the miscibility gap in the U-Nd-O phase diagram to irradiated UO2 opens the path to a new understanding of nuclear oxides in the environment.