A core of bottom sediments from Lake Krugloe located within the 30 km influence zone of the Siberian Chemical Plant (located in the city of Seversk “Tomsk-7”) was studied to determine scales and rates of migration of artificial radionuclides 137 Cs and 241 Am in organomineral sediment. It was found that the main portion of 137 Cs and 241 Am was contained in the sediment interval above 10 cm. This means that the horizon of 10 cm corresponds to 1950—the time of the start of widespread tests of nuclear weapons. The 210 Pb ex dates also confirm that this particular horizon was formed in the 1950s. Pore waters in the core above the 10 cm horizon are in oxidizing conditions. The depth of the oxidized/reduced boundary was determined from the distribution of redox-sensitive elements Fe and U dissolved in the pore solution. The core distribution of 137 Cs is a slightly sloping step, with the lower edge at the 10 cm level. The smearing of the lower boundary of this distribution showing the scale of 137 Cs migration made it possible to estimate the diffusion mobility of 137 Cs. Its diffusion coefficient turned out to be of the order of 10 −8 cm 2 s −1 . As shown by measurements, the scale of migration of 241 Am and the scale of migration of 137 Cs have similar values. Theoretical analysis carried out in this work shows that the most probable mechanism of 137 Cs and 241 Am migration in the pore solution of lake sediment is the migration of colloidal particles to which these radionuclides are strongly bound. Calculation of the diameter of such particles by the Stokes–Einstein formula shows that they have submicron dimensions (d ≈ 400 nm). No evidence was found that the change in redox conditions in the sediment had an effect on migration of Am ions in pore solution.