Factors affecting the quality of low- and medium-level solidified wastes

Abstract

621.039.7 As in most countries, in our country the concept of burial of low-and medium-level wastes near the surface, i.e., in burial sites located on the earth's surface or at a shallow depth, has been adopted for solidification of such wastes [1][2][3][4]. The solidified wastes placed in such burial sites can be influenced by a change in the composition of the water in contact with them, the variable temperature, the presence of microbiological action, and other factors.Radionuclides escape into the environment mainly because the solidified wastes come into contact with water, which can enter the burial site as a result of a leak in the site. In the process both ground water and surface water (rain or flood waters) can penetrate into the site. In addition, as the water passes through the shielding barriers, it will be enriched by the soluble components of the constructional and biospheric materials.A great deal of attention is being devoted to the study of the factors which influence the behavior of the solidified wastes stored near the surface or in the case of accidents during shipment [5][6][7].The effect of the composition of the contact water on the rate of leaching of the most mobile element -cesium from cement and bitumen compounds, which were obtained during solidification of salt concentrates formed during the operation of nuclear power plants with VVI~R and RBMK reactors, has been studied under laboratory conditions. The cement compounds were prepared using a mixture of Portland cement and bentonite clay in the ratio 9:1. The content of the dry residue of the wastes in the compound was equal to 15 and 30% for wastes from nuclear power plants with RBMK and VVI~R reactors, respectively. Distilled water [8], water which for a period of one month was in contact with the bentonite clay employed in burial sites as a buffer material, or with fragmented hard concrete, and water containing macrocomponents of the wastes leached out of the cement compounds were used as the leaching medium (Table 1). The rate of leaching of the cesium was determined by the method adopted in [8]. The investigations were performed on samples in the form of 8 cm 3 cubes.The results indicated a very small change in the amount of cesium arriving in the leach medium during the time of the experiment. The fraction of 137Cs leached out of the compounds with salt concentrates of a nuclear power plant with a RBMK reactor over a period of 159 days is equal to 3-7% for distilled water and 3-5 % for water in contact with the bentonite clay.Approximately 8% of the 137Cs in the distilled water, 7.5-8% in the water in contact with the bentonite, 11-12% in the water in contact with the hard concrete, and about 9% in the ground water was leached from the cement samples with salt concentrates from a nuclear power plant with a VVI~R reactor over a period of 14 days. As one can see, in this case the difference in the leach rate does not exceed 50%.Another factor that can affect the escape of radionuclides from cement compounds into the environmen...