Spent nuclear fuel (SNF) of light water reactors (LWR) is constituted of a matrix of unfissioned UO2 containing a small fraction of fission products (FP) and minor actinides (MA). These MA are mainly α-emitters with very long half-lives: therefore, SNF will keep α-self-irradiating for millennia after discharge from the reactor, resulting in widespread changes of the thermophysical properties of the material. In order to properly coordinate and license any disposal strategy for SNF, the long-term effect of α-self-irradiation has to be known and anticipated. Due to the lack of real old SNF, a proxy system to study the effect of α-irradiation on the SNF matrix is provided by the synthesis of UO2 doped with short-lived α-emitters. In this way, a significant amount of radiation damage can be stockpiled over a laboratory timescale in a simpler surrogate system that does not combine radiation damage with chemical or density gradients (built-in in real SNF). In the present work, UO2 doped with 238Pu was produced to study the effect of α-self-irradiation on the crystalline disorder and lattice swelling. The composition of the samples was carefully chosen, based on the dopant specific activity, in order to reach saturation of the lattice parameter swelling within the 3 years timespan of the project. Samples were periodically characterized by means of XRD and Raman spectroscopy up to 0.4 dpa, equivalent to a spent nuclear fuel with a 40 GWd/t UO2 burnup stored during 300 years, or representative of a 65 GWd/t spent MOX fuel (45 % Pu) after 25 years of storage. Lattice swelling as a function of dpa was assessed with very good accuracy and benchmarked against literature data: saturation was reached at a value of 0.3 % around 0.4 dpa. For the first time, microstrain was also monitored in (U,Pu)O2 as a function of self-irradiation. Periodic Raman spectroscopy acquisitions on (U,Pu)O2 as a function of the dose represent an innovative probe, and they showed fast and progressive degradation of the structural order up to 0.1 dpa, and a slow but persistent increase up to 0.4. SEM characterizations were additionally performed and highlighted that no loss of structural integrity is associated with this microstructural evolution, at least within the measured dpa range.