How can Raman spectroscopy be used to study nuclear fuel?

Abstract Raman spectroscopy was recently used to characterize irradiated, doped, or implanted UO_2. In all cases, the Raman spectrum evidenced three additional peaks, called defect peaks, whose interpretation is still under debate. Here, we evidence that defect peaks are Raman resonant peaks with different resonance energy using Raman multiwavelength analysis of He-irradiated UO_2. Moreover, we demonstrate by phonon calculations that they correspond to a local symmetry, in which at least F-centering is lost. These findings provide a new framework for the interpretation of the Raman spectrum of modified compounds having initial fluorite structure. Impact statement Nuclear fuel is a major environmental, energetic, and geostrategic issue. Its identification and characterization is mandatory for controlling both its dissemination and its behavior in any situation. Raman spectroscopy is a new and promising option for that purpose, because it can easily be used in situ , even in a radioactive environment, thanks to optical fibers for allowing remote operation. Nevertheless, its development requires an in-depth understanding of the information that can be gained from the Raman method about this material. This article provides an explanation for the interpretation of the so-called defect peaks. The main outcome of this interpretation is that there is no straightforward manner to interpret the defect peaks, but, in some simplified cases, the defect peaks can be the signature of one single effect, whose concentration could then be determined quantitatively. The identification and modeling of all defects that can generate the defect peaks is a promising area of research. This framework for interpretation could also be applied to other compounds having fluorite structure. Graphical abstract