Environmental monitoring of actinides and evaluation of the contamination source (nuclear weapons tests, nuclear power plant and nuclear reprocessing plant accidents, etc.) requires precise and accurate isotopic analysis of actinides, especially uranium and plutonium. Double-focusing sector-field inductively coupled plasma mass spectrometry (ICP-SFMS) using a low-flow microconcentric nebulizer with membrane desolvation, “Aridus”, was applied for isotopic measurements of uranium and plutonium at the ultratrace level. The detection limit (3σ) for ²³⁶U and ²³⁹Pu after chemical extraction was 0.2 pg l⁻¹ in aqueous solution and 0.04 pg g⁻¹ in soil, respectively. ²³⁵U/²³⁸U, ²³⁶U/²³⁸U and ²⁴⁰Pu/²³⁹Pu isotope ratios were measured in soil samples collected within the 30 km zone around the Chernobyl nuclear power plant. The average ²⁴⁰Pu/²³⁹Pu isotope ratio in contaminated surface soil was 0.396 ± 0.014. The burn-up grade and the portion of spent uranium in the spent uranium/natural uranium mixture in soil were calculated using the iteration method. A slight variation in the burn-up grade of spent reactor uranium was revealed by analyzing ²³⁵U/²³⁸U and ²³⁶U/²³⁸U isotope ratios. A relationship between the ²⁴⁰Pu/²³⁹Pu isotope ratio and burn-up of spent uranium was observed.