Radioactive contamination in the upper part of the Techa river: stirring-up of bottom sediments and precipitation of suspended particles. Analysis of the data obtained in 1949-1951.

A hydrodynamic model of the upper part of the Techa river was developed on the basis of the river valley geometry as well as data of hydrological conditions and of the granulometric composition of bottom sediments. The model describes the transport of radioactivity by suspended sediments with different granulometric compositions (clay, silt) in the early 1950s. It includes the stirring-up of bottom sediments and the precipitation of suspended sediments as a function of water discharge rate and water level in the investigated part of the river. The results allow to specify the development of the river system contamination as a result of inflow of suspended sediments contaminated with radionuclides. In the period of liquid radioactive waste (LRW) discharges, the water of the Techa river contained a large fraction of finely dispersed particles of less than 5 micro m diameter. At the site of LRW discharge 80% of the discharged activity was adsorbed to these particles. Depending on the water flow, 40-80% of the suspensions precipitated at the bottom of subsequent sedimentation reservoirs. A total of about 1.6 MCi adsorbed to the suspended particles entered the open hydrographic system of the Techa river. The conclusion that the largest part of the activity was adsorbed on the suspended particles contradicts the assumption in the Techa river dosimetry system, TRDS-2000, that most of the released activity entered the Techa river in soluble form. For a correct reconstruction of the doses received by the Techa river population it is, therefore, essential to consider hydrodynamic models that take into account the transport of radionuclides adsorbed on the suspended particles.