The PUREX process has been designed for the reprocessing of spent nuclear fuel to separate uranium and plutonium from the fission products. In accident scenario, the solvent can ignite itself, releasing in volatile forms PuO₂, PuO₃ or PuO₂(OH)₂. Our theoretical study focusses on the thermodynamics properties of the former two species for which large experimental uncertainties remain. Actinide-containing complexes present formidable challenges for electronic structure methods due to the large number of degenerate or quasi-degenerate electronic states arising from partially occupied 5f and 6d shells. To compute highly accurate thermodynamics properties (enthalpies, entropies) with quantum chemical methods, static and dynamic correlation effects along with relativistic effects have to be treated on equal footing. In particular, the clear multi-reference character of the wave-function of these compounds requires the enthalpies of formation to be computed with multi-configurational quantum chemical methods like CASSCF and CASPT2. Spin-orbit interaction is treated a posteriori with the state-interaction RASSI method. The computed thermodynamics quantities reach a high accuracy allowing us to predict the composition of the released volatile products. However, in the CASSCF/CASPT2 calculations the active spaces had to be truncated, the limit of 18 electrons in 18 orbitals being reached in these plutonium molecules. The Density Matrix Renormalization Group (DMRG) algorithm, which allows us to overcome the previous limit, was used in the second part of this study to handle active spaces that include the full atomic valence shells. Dynamical correlation is treated with DMRG-NEVPT2, and spin-orbit interaction can then be treated with the state-interaction RASSI approach.