Chemical fractionation of 90Sr, 106Ru, 137Cs and 144Ce in Chernobyl-contaminated soils: an evolution in the course of time

The sequential extraction procedure was used for the evaluation of the chemical species of 90Sr, 106Ru, 134Cs, 137Cs and 144Ce nuclides in highly contaminated soils inside a 30-km restricted zone around the Chernobyl nuclear power plant. Chemical leaching and weathering of fuel particles have resulted in a rapid increase in the relative contents in soil exchangeable 90Sr and amount of 144Ce easily extracted by diluted hydrochloric acid, whereas the ageing effect has helped to make caesium gradually more strongly fixed. In all cases, the behaviour of 134Cs and 137Cs in soils was identical. The fact that only partial extraction of 106Ru by diluted and strong acid has occurred reflects the low solubility of ruthenium, probably included in fuel particles as an intermetallic impurity. The variations in radionuclides speciation with time have been used for estimating the rate of their release from fuel particles and fixation in soil. Average leaching rate constants for 90Sr, 144Ce and 106Ru are 1.1 × 10−3, 6.9 × 10−4 and 9.4 × 10−5 day−1, respectively, and the rate constant for caesium fixation is about 7.3 × 10−4 day−1.