One of the challenges of multi-recycled Pu, to be used to produce MOx fuel, lies in its isotopic composition. Further recycling enriches the isotopy toward $^{238}$Pu, $^{240}$Pu, and $^{241}$Pu, which have much higher specific activities than the $^{239}$Pu isotope, meaning that those fuels are subjected to strong self-irradiation, provoking defect accumulation in the (U,Pu)O$_2$ crystal lattice. A combination of three different techniques, XRD, EPMA, and RS (x-ray diffraction, electron probe micro-analysis, and Raman spectroscopy, respectively) was implemented to characterize a particular self-irradiated, $^{238}$Pu, $^{240}$u, and $^{241}$Pu-enriched MIMAS (MIcronized-MASter blend)-type MOx fuel sample, which had been stored for 15 years at room temperature under an inert atmosphere, to maximize irradiation effects. For comparison purposes, a specimen from the same batch was submitted to a thermal treatment and was completely analyzed in the two months following this treatment. Two of these methods (EPMA and RS) were used in their imaging mode. In particular, four spectral characteristics could be extracted from the Raman spectra. However, because of the inherent heterogeneity of this particular MOx material, the results had to be analyzed in part in a rather statistical way. This combination of techniques first allowed for determining the local Pu content. Then, the effects of self-irradiation were analyzed in terms of lattice parameter swelling, defect injection, and resonant scattering. The merits and uncertainties associated with these methods are discussed in terms of macro- and/or micro-strains. Finally, the Raman spectroscopy of (U,Pu)O$_2$, in the 0%–40% range, was revisited in part, in an indirect way, however.