The low-temperature heat capacity of (U_1−yTh_y)O₂ and ²³⁸Pu-doped UO₂ samples were determined using hybrid adiabatic relaxation calorimetry. Results of the investigated systems revealed the presence of the magnetic transition specific for UO₂ in all three intermediate compositions of the uranium-thorium dioxide (y = 0.05, 0.09 and 0.12) and in the ²³⁸Pu-doped UO₂ around 25 K. The magnetic behaviour of UO₂ exposed to the high alpha dose from the ²³⁸Pu isotope was studied over time and it was found that 1.6% ²³⁸Pu affects the magnetic transition substantially, even after short period of time after annealing. In both systems the antiferromagnetic transition changes intensity, shape and Néel temperature with increasing Th-content and radiation dose, respectively, related to the increasing disorder on the crystal lattice resulting from substitution and defect creation.