Correct mathematical models of a reactor core in benchmarking of reactor physics experiments require detailed knowledge of exact isotopic composition of fuel pellets, especially the presence of neutron absorbers. Among them, knowledge of 234U and 236U abundance in fuel is important, because their absorption is considerably stronger than that of the most common 238U and their concentration is significantly higher than any other absorbers. Thus, the fuel pellets used in the nuclear research reactor LR-0 have been experimentally investigated using the procedure that involves a method of secondary ion mass spectrometry (SIMS), which allowed for analyses of miniature amount of the pellet material without disturbing its integrity. The pellets were swiped with cotton cloths and the microscopic particles collected on the cloth were analyzed. The isotopic composition of individual uranium particles then provided information required for the reactor core modelling and in addition to that some hints about used source materials, history of nuclear material, and the manufacturing process of the pellets.