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A factorial study is made of precipitation of ammonium polyuranates from nitric acid solutions of uranium in the presence of urea, including measuring the effects of the temperature and urea and uranium concentrations in the initial solution on the sedimentation and filtration characteristics of the precipitates. The conditions of the process are optimized to obtain crystalline, readily filterable precipitates.Precipitation of ammonium polyuranates with ammonia from concentrated uranium backwash solutions finds wide use in the commonly accepted extraction3 precipitation reprocessing of spent nuclear fuel [1,2]. That is why intensive studies have been made of the mechanism of precipitation of ammonium polyuranates and of the properties of the resulting precipitates as influenced by the precipitation conditions [3,4].In the extraction3precipitation reprocessing of highly enriched (weapon-grade) uranium at the Siberian Chemical Combine, ammonium polyuranates are precipitated from solutions containing considerable concentrations of urea (urea is used in the backwashing stage, to obtain uranium concentrates) [5,6].There are only limited data in the literature on precipitation of ammonium polyuranates in the presence of urea [7]. Therefore, in this work we thoroughly studied the effect of the urea concentration in nitric acid solutions of uranium on the precipitation of ammonium polyuranates with ammonia. EXPERIMENTAL Stock uranium solutions were prepared by dissolving uranyl nitrate hexahydrate in 0.2 M HNO 3 [ultrapure grade, OST (Branch Standard) K-03-265376]. Then, a fixed amount of urea [GOST (State Standard) 2081392, grade A] was dissolved in the uranium solution.Precipitation of ammonium polyuranates from urea-containing uranyl nitrate solutions was carried out in a continuous precipitator with automatic control of the temperature and pH. Ammonia (25 wt % solution, GOST 6221-82E) and the stock uranium solution were fed simultaneously to the precipitator (slurry residence time in the reactor 1 h). The flow rate of the initial solution was controlled using a batcher, and that of the ammonia solution was automatically controlled by pH. Fixed pH was maintained with a BAT-15 automatic titration unit connected to a pH-121 pH meter (ESL 63-07 pH-metric glass electrode; EVL-1MB Ag/AgCl reference electrode). Stirring was carried out with a propeller mixer. The temperature control was realized using a contact thermometer (GOST 9871361) connected to a temperature controller. Slurry was transferred through an upper discharge to a settling tank. After 3 h of continuous operation of the system, samples of the slurry were taken, and the aqueous phase composition and physicochemical characteristics of the precipitates were analyzed.The completion of precipitation was characterized by the uranium concentration in the mother liquors, and properties of the precipitate, by its moisture content and permeability coefficient. The relative volume of the precipitate was estimated as the ratio of the precipitate volume after settling to ...
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A mathematical model of the physicochemical processes occurring in a reservoir bed with simultaneous disposal of organic and aqueous radioactive wastes in deep-lying porous geological formations is presented. The model includes a description of filtration, convective-dispersive mass transfer, sorption and desorption of radionuclides by the surrounding rock, radioactive decay, decomposition of organic components, and convective heat transfer and heat transmission. The numerical implementation of the method is used as a basis for developing computer software that makes it possible to perform predictive calculations of the change in the state of a reservoir bed of radioactive wastes. The results of the simulation of the dynamics of the thermal fields and the behavior of the components of the spent organic extracting agent and aqueous radioactive wastes in the reservoir bed in the deep-disposal test area are presented.In the course of reprocessing fissioning materials, the organic extracting agent, consisting of a solution of tributylphosphate (TBP) in a hydrocarbon diluent, undergoes radiation-chemical decomposition, as a result of which it loses its operational properties and is removed from operation. The spent extracting agent contains fission products, uranium and plutonium, and for this reason is classified as a radioactive waste. One method of isolating such wastes is deep burial in porous geological formations [1]. This served as a prerequisite for examining the burial of spent extracting agent together with aqueous radioactive wastes. Laboratory investigations and experimental-commercial tests showed that burial of spent extracting agent in underground reservoir beds is reliable, and have made it possible to determine the injection conditions. It was established that the organic impurities which are part of the liquid water wastes effectively decompose in reservoir bed [2]. In connection with the higher safety requirements for underground burial and the need for optimizing safety, it is important to give a quantitative description of the behavior of organic wastes taking account of their migration, which is considered to be transport and redistribution of chemical elements accompanying geochemical processes, and predict the change in the state of the underground stratum.Spent organic extracting agent is injected into an underground reservoir bed through wells, used for storing alkaline water wastes, specifically, from the Siberian Integrated Chemical Plant. The combined filtration of water wastes and spent extracting agent in the porous medium of a reservoir bed is accompanied by a large number of intercoupled nonequilibrium physicochemical processes. The data from observation of the state of the reservoir bed in the deep burial area shows that the
Safety codes require that open surface disposal sites for liquid low-level wastes at radiochemical works be closed down. At the Siberian Chemical Works, the first operation was to stop the flow of wastes into the pulp repository. The presence of burial sites predetermined the choice of precisely this method as an alternative for dumping wastes into open disposal sites. The results of a study of the physicochemical characteristics of the wastes flowing into the pulp repository PKh-1,2 from four plants at the Works made it possible to develop a unified scheme for preparing them for disposal -wastes are put into a deep repository and maximum use is made of the individual properties of reprocessed wastes, which decreases the consumption of additional reagents substantially, i.e., it decreases the cost of the preparation process. The scheme developed has successfully passed commercial prototype tests and is now in the design stage.The production operations at the Siberian Chemical Works produce liquid radioactive wastes with different salt composition and content of radionuclides and suspensions. At the first stage, surface disposal sites, which keep radionuclides out of the open hydrographic network, were built to salvage liquid wastes. Subsequently, the favorable hydrogeological conditions at the plant site made it possible to implement on a commercial scale the underground method for disposing of wastes in sandy collector formations deep underground and to avoid building additional surface repositories [1].The first geological exploration work performed in our country to validate the safety of deep disposal of liquid wastes was begun in the mid-1950s at the Siberian Chemical Works by the Novosibirsk Geological Office. Subsequently, the wastes were transferred to the enterprise Gidrospetsgeologiya. The results of the investigations made it possible in 1963 to begin operation of an experimental section of the site and in 1967 to switch to a commercial variant of deep disposal of liquid low-level wastes. The purpose of the site is to localize and store (hold) wastes in deep collector formations in order to allow the concentration of radionuclides to drop below the intervention level by decaying [2].The site lies in the southeastern part of the Ob' artesian basin. A section of the region, from bottom to top, shows the water-bearing system of the Paleozoic basement, the bottom water-bearing system of sedimentary sandy-clayey Mesozoic rock (bottom and top Cretaceous), and the upper water-bearing system of sandy-clayey rock of the Cenozoic era (Paleogene, Neogene, and Quaternary). The bottom water-bearing system of Mesozoic rocks includes the water bearing levels I, II, and III and the confining beds A, B, C, and D (arbitrary enumeration) separating them. The upper water-bearing system of
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