Pyrochemistry is a promising technology that can provide benefits for the safe reprocessing of relatively fresh spent nuclear fuel with a short storage time (3–5 years). The radioactive waste emanating from this process is an electrolyte (LiCl–KCl) mixture with fission products included. Such wastes are rarely immobilized through common matrices such as cement and glass. In this study, samples of ceramic materials, based on natural bentonite clay, were studied as matrices for radioactive waste in the form of LiCl–KCl eutectic. The phase composition of the samples, and their mechanical, hydrolytic, and radiation resistance were characterized. The possibility of using bentonite clay as a material for immobilizing high-level waste arising from pyrochemical processing of spent nuclear fuel is further discussed in this paper.
Cementing, i.e., incorporating liquid radioactive wastes into inorganic binders (Portland cement, Portland blastfurnace cement, metallurgical slags), is the simplest and cheapest method of conditioning. However, this method also has 11 drawbacks, one of which is that it is impossible to decrease or even increase the volume when cementing liquid radioactive wastes with salt content of up to 200 g/liter [1]. In this case, the salt content in the solidified wastes is 5-7%.Preliminary investigations on model compositions have shown that cementing can produce quite hard materials, containing up to 26 and 19% dry residue of salt concentrates from nuclear power plants with RBMK and VVI~R reactors, respectively. The hardness under compression of such materials is much higher than 100 kg(force)/cm 2 [2].The determination of the water resistance of cemented materials with a high salt content on 137Cs-marked samples has shown that after 14 days the rate of leaching of t37Cs from the samples containing 15% dry residue from the wastes from a nuclear power plant with a RBMK reactor is 1"10 -4 g/(cm2'day), and the leach rate from samples with 33% dry residue of the wastes from a nuclear power plant with a VVER reactor under the same conditions is I. 10 -3 g/(cm2"day). These preliminary results have shown that it is in principle possible to obtain cement materials with a higher salt content than previously thought [I].To confirm the possibility of obtaining cement compounds with a high salt content, investigations were performed with real concentrates. For this, salt concentrates from nuclear power plants with a VVI~R (Novovoronezh, Kalinin, and Zaporozh'e) and RBMK (Kursk) reactors and wastes from the Scientific and Industrial Association "Radon," a large fraction of which consist of distillation residues of liquid wastes from scientific-research institutes, were performed.As one can see from the data in Table I, the mass of the dry residue from the RBMK nuclear power plant (Kursk) and the Scientific and Industrial Association "Radon" consist mainly of sodium nitrate and the wastes from the VVER nuclear power plant contain, together with sodium nitrate, a large quantity of sodium borates, whose content in the wastes from different stations is different.To ensure a high degree of filling of the cement materials, the salt content of the liquid radioactive wastes must be increased by additionally concentrating them. This is achieved by obtaining high-salt concentrates on the UGU-500 deepevaporation plant (Novovoronezh and Zaporozh'e nuclear power plants) or evaporation of real solutions under laboratory
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...
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