The results of long-term (two years) investigation of Cs-137 fixation
in soil suspension are presented. It was shown that the equilibration
time in a soil-water system for this radionuclide can be as long as on
e year. This means that the total caesium distribution coefficient is
nor relevant for mathematical modelling of migration processes with ch
aracteristic times less than several months. The distribution coeffici
ent of the exchangeable fraction of the radionuclide is recommended fo
r modelling of faster processes. Caesium interaction with soils is cha
racterised by three phases with distinctly different time scales: init
ially rapid uptake, relatively slow intermediate uptake and the third
phase with characteristic time about several months. A mechanistic int
erpretation of each phase is proposed.