As a result of the Chernobyl accident in April 1986 a large quantity of radioactive substances entered the cooling pond. Since most of the radionuclides are now in the bottom deposits, the radioactive contamination in the pond is not a serious hazard either for the population outside the restricted zone or for the personnel who operate the cooling pond. However, the insoluble particles deposited in the bottom deposits break down with time, and this could become a source of additional contamination of the water. The objective of the present work is to predict the change in the average yearly radionuclide concentration in the pond water with time. The prediction is made by the method of mathematical modeling. The construction of the mathematical model is based on the results of running modeling of the state of the pond, performed by the radiation safety services.The cooling pond lies near the right-hand bank of the Pripyat' River and consists of a 22.8 km 2 and 0.151 km 3 pond. It has no direct runoff into the river, but constant filtering occurs through the body of the dam as a result of the difference in the water levels. The design rate of filtration, according to data from the Institute of Hydroengineering Constructions, is equal to 0.2 km3/yr. According to measurements, the filtration rate is now equal to 0.107-0.120 km3/yr. Water losses are made up from the Pripyat' River with the aid of a pump station located on the northern part of the dam.In the present work we study the average yearly concentration of radionuclides in the water, averaged over the area of the pond. This simplifies the construction of the mathematical model. For calculations we employed a model which is based on the following simplifying assumptions.Four phases of contamination are considered: by radioactive substances dissolved in the volume of the water; by dissolved substances sorbed by solid particles in the bottom deposits; insoluble particles which are part of the bottom deposits; and, insoluble particles suspended in the volume of the water. Radioactive substances are exchanged between the phases. Exchange between the contamination dissolved in the water volume (phase 1) and sorbed dissolved contamination (phase 2) occurs by means of sorption and desorption from the surface of particles in the bottom deposits, exchange between phases of insoluble compounds (phases 3 and 4) occurs by means of sedimentation and wind agitation, and exchange between the insoluble phases (3, 4) and soluble phases (1, 2) occurs by means of leaching. It is assumed that the sorption equilibration time between the dissolved and adsorbed phases as well as the local equilibration rate between the suspended phase and the bottom deposits are negligibly small compared to the averaging time. This latter assumption leads to the conclusion that the concentration of dissolved adsorbed radionuclides is described by the relation q = KO, where q is the radionuclide concentration in the water and p is the radionuclide concentration on the solid particles (Henry's law) wit...
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