The effect of lattice disorder on the low-temperature heat capacity of (U(1−y)Th(y))O(2) and (238)Pu-doped UO(2)

The low-temperature heat capacity of (U(1−y)Th(y))O(2) and (238)Pu-doped UO(2) samples were determined using hybrid adiabatic relaxation calorimetry. Results of the investigated systems revealed the presence of the magnetic transition specific for UO(2) in all three intermediate compositions of the uranium-thorium dioxide (y = 0.05, 0.09 and 0.12) and in the (238)Pu-doped UO(2) around 25 K. The magnetic behaviour of UO(2) exposed to the high alpha dose from the (238)Pu isotope was studied over time and it was found that 1.6% (238)Pu 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.