Determination of irradiated reactor uranium in soil samples in Belarus using 236U as irradiated uranium tracer

Abstract

This work presents experimental results on the distribution of irradiated reactor uranium from fallout after the accident at Chernobyl Nuclear Power Plant (NPP) in comparison to natural uranium distribution in different soil types. Oxidation processes and vertical migration of irradiated uranium in soils typical of the 30 km relocation area around Chernobyl NPP were studied using ²³⁶U as the tracer for irradiated reactor uranium and inductively coupled plasma mass spectrometry as the analytical method for uranium isotope ratio measurements. Measurements of natural uranium yielded significant variations of its concentration in upper soil layers from 2 × 10⁻⁷ g g⁻¹ to 3.4 × 10⁻⁶ g g⁻¹. Concentrations of irradiated uranium in the upper 0–10 cm soil layers at the investigated sampling sites varied from 5 × 10⁻¹² g g⁻¹ to 2 × 10⁻⁶ g g⁻¹ depending on the distance from Chernobyl NPP. In the majority of investigated soil profiles 78% to 97% of irradiated “Chernobyl” uranium is still contained in the upper 0–10 cm soil layers. The physical and chemical characteristics of the soil do not have any significant influence on processes of fuel particle destruction. Results obtained using carbonate leaching of ²³⁶U confirmed that more than 60% of irradiated “Chernobyl” uranium is still in a tetravalent form, i.e. it is included in the fuel matrix (non-oxidized fuel UO₂). The average value of the destruction rate of fuel particles determined for the Western radioactive trace (k = 0.030 ± 0.005 yr⁻¹) and for the Northern radioactive trace (k = 0.035 ± 0.009 yr⁻¹) coincide within experimental errors. Use of leaching of fission products in comparison to leaching of uranium for study of the destruction rate of fuel particles yielded poor coincidence due to the fact that use of fission products does not take into account differences in the chemical properties of fission products and fuel matrix (uranium).