Effect of adding clay on the properties of cement compounds employed for localization of radioactive wastes

Abstract

One of the main properties which guarantee safety of the storage, shipment, and burial of solidified radioactive wastes is their water resistance, which is determined mainly by the quantity of radionuclides which are transferred into a water medium when the water comes into contact with the compound. From this standpoint, the element cesium which forms easily soluble compounds is most dangerous. The content of this element in some types of wastes, for example, in salt ~oncentrates from nuclear power plants, is equal to 90% and higher of the total radioactivity. Simple cementing of liquid radioactive wastes containing cesium compounds does not fix the cesium reliably, and cesium can be almost completely leached out of small samples (8 cm 3 cubes) within several days on contact with water.The leaching of cesium from cement compounds can be decreased by two methods: preprocessing of the wastes in order to form insoluble cesium ferrocyanide or using sorption additives. The latter variant is cheaper and technologically simpler. Different types of clays, especially bentonites, have good sorption properties with respect to cesium [1][2][3][4]. However, introducing clay into a cement compound and at the same time improving its water resistance can change other properties of the compound. One such property, determining the safety of the handling of the wastes, especially at the stages of reloading and shipment, is mechanical strength. Another important parameter of cementing, which can be affected by the presence of clay in the mixture, is the plasticity of the cement test compound, characterized by the degree of cracking. Lower plasticity can lead to difficulties in preparing a homogeneous mixture and on loading it from the mixer into containers.Our objective in the present paper is to examine these questions. The main investigations were performed for salt concentrates from a nuclear power plant with a VVI~R reactor (Novovoronezh nuclear power plant) and a RBMK reactor (Leningrad and Kursk nuclear power plants). The water resistance was determined by the standard procedure (GOST 29114-91. Radioactive wastes. Method for measuring the chemical stability of solidified radioactive wastes by means of prolonged leaching). A comparative assessment of different materials was made according to the rate of leaching of radionuclides and the amount of radioactivity which has transferred into the water phase over a definite time interval.The results of tests on samples obtained by cementing salt concentrates from a nuclear power plant with a RBMK reactor are presented in Table 1. In these and subsequent tests Portland cement and slag Portland cement were used as binders and bentonite clay was used as a sorption additive. Adding to the binder bentonite clay in amounts of 5-10% by mass decreases the leaching of the radionuclides by a factor of 10, irrespective of the type of binder and the degree of filling of the compound within the limits investigated.The water resistance of cement compounds with salt concentrates from a nuclear pow...

Development of a unified technological process for cementing liquid radioactive wastes

“…The determination that the cement mixes (cement and additions to it) have similar compositions was made on the basis of an analysis of the cementing of different liquid wastes: nuclear power plants with RBMK and VVER, spent oils, ionexchange resins and filter perlite, and residues with high surfactant content [6][7][8][9][10][11]. Liquid wastes from certain nuclear power plants and scientific and industrial objects were used in the work.…”

Inclusion of Radioactive Ion-Exchange Resins in Inorganic Binders

“…The degree to which the solidified products are filled with respect to dry resin reaches [7][8][9][10] 12, respectively, with adequate strength being maintained (at least 5 MPa); the cesium diffusion coefficients are 9.3·10 -4 , 1.2·10 -4 , and 7.2·10 -5 cm 2 /day with the normative value 6.7·10 -4 cm 2 /day. The degree to which the solidified products are filled with respect to dry resin reaches [7][8][9][10] 12, respectively, with adequate strength being maintained (at least 5 MPa); the cesium diffusion coefficients are 9.3·10 -4 , 1.2·10 -4 , and 7.2·10 -5 cm 2 /day with the normative value 6.7·10 -4 cm 2 /day.…”