[en] Highlights: • The presence of radioactive uranium oxide particles containing high specific activity of radionuclides is a distinctive feature of the Chernobyl accident. • The efficiency of the models describing the dissolution/weathering rates of U fuel particles in environment is confirmed. • The biological availability of ⁹⁰Sr in topsoils due to fuel particles dissolution has reached maximum values and further decrease is expected. • The fuel particles' dissolution rate is an important parameter of the source term for radiation protection of the human and the environment. - Abstract: Long-term environmental behaviour of radioactive particles released during the Chernobyl accident and deposited in sandy topsoil in Ivankiv district of Kyiv Region (Ukraine), in radioactive trench waste materials from the Red forest, and in bottom sediments from the Cooling pond has been assessed. The efficiency of the models describing the dissolution/weathering rates of U fuel particles developed 15–20 years ago was tested, and their predictions for the dynamics of remobilization, mobility and plants uptake of ⁹⁰Sr were confirmed. It was found that at present in the topsoil and in radioactive trench waste material, total dissolution of fuel particles of low chemical stability (UO_2+x) has occurred and about half of the non-oxidized chemically stable fuel particles (UO₂) has also dissolved, indicating radiological stabilization of the environment and that the mobile fraction of radionuclides would be reduced in the future. The biological availability of ⁹⁰Sr in topsoil due to fuel particles dissolution has reached maximum values and further decrease is expected. The presence of chemically extra-stable fuel particles (U-Zr_y-O_x) in environments should be taken into account when the total radionuclides activity concentrations are assessed during radioactive materials management. It was shown that nearly half of the ⁹⁰Sr activity remained as part of the non-dissolved UO₂ fuel particles at the time of the study. Taking into consideration that 31 ± 4% of the radionuclide activities were still associated with non-dissolved chemically extra-stable particles (U-Zr_y-O_x) in radioactive trench waste materials from the Red forest, increased dissolution should not be expected in the near future. The physico-chemical form of radionuclides in air exposed sediments from the Cooling pond were determined, and results showed that about 70–80% of total ⁹⁰Sr, ²⁴¹Am and plutonium isotopes activity were associated with U fuel particles. The low dissolution rate of radionuclides from the pond sediments is attributed to prolonged slightly alkaline pH in the medium due to zebra mussel residues. According to new data, the emission value of ²³⁸Pu associated with fuel particles released during the Chernobyl accident amounted to 1.8 × 10¹³Bq (1.2% of the activity in the reactor) and ⁹⁰Sr amounted to 2.6 × 10¹⁵Bq (1.5% of the activity in the reactor).