Use of cambrian clay in cementing radioactive sludge deposits from natural water reservoirs

Abstract

The use of Cambrian clay from Leningrad oblast for cementing radioactive sludge deposits from natural water reservoirs is discussed. The addition of Cambrian clay impedes separation of the mixture of sludge with cement and decreases the leaching of 137 Cs from the solidified products at least five-fold.Most radionuclides in the radioactive contamination of water reservoirs are fixed in sludge deposits. The degree to which isotopes are absorbed by deposits increases as the grain size decreases, i.e., as the external surface area of the deposits increases. It is conjectured [1] that the isotopes of alkali and alkali-earth elements are absorbed by sludge deposits primarily by the cation-exchange mechanism; the absorption of isotopes of hydrolyzing elements probably occurs by a type of sorption extraction or as a result of combined coagulation of colloids (true or pseudo-colloidal forms).When cleaning water reservoirs and extracting from them sludge deposits containing up to 20 wt.% solid phase, it becomes necessary to localize the sludge and to fix the radionuclides in them reliably. The most efficient procedure is to use for this purpose cementing, which is characterized by the simplicity of the apparatus, low energy consumption, cheap and accessible binders [2], as well as high mechanical strength (to 10 MPa and more), water resistance (for the main component Ca 2+ the diffusion coefficient is 1.5·10 -4 cm 2 /day), radiation resistance (to 10 8 rad), and inflammability of the solidified products (withstand heat to 30 min at 800°C) [3].At the same time, when a water phase is included in the solidified product, cementing increases the volume of the products to be buried. Radioactive cement blocks have a high rate of leaching of radionuclides 10 -3 -10 -2 g/(cm 2 ·day), which requires introducing into them sorption additives, of which the most commonly available are clay materials. In addition, the presence of sludge suspensions in water mixed with cement, as a rule, lowers the quality of the cement products [4]. The problem of the present work is to study the possibility of using Cambrian clay to reduce as much as possible the consumption of cement binder and decrease the volume of the radioactive wastes to be buried while preserving their high ecological safety.The tests were conducted on natural mud deposits with moisture content 80 wt.%, containing in the solid phase to 25% clay and to 75% sand fractions. The content of the organic materials in the solid phase of the mud, as determined from the loss of mass of the dry residue during calcination at 900°C, was 4-5 wt.%. The contamination by 90 Sr and 137 Cs reached 8·10 5 and 1.5·10 6 Bq/kg, respectively. The radioactivity of the clay fractions approximately equals that of the sand fractions. Portland cement 400 (GOST 10178-85) was used as the binder; it is recommended for solidifying low-and medium-level wastes [5]. The effect of additions of sodium liquid glass (GOST 11378-81) and granular blast-furnace slag (GOST 3476-74), ground to specific surface area of abou...