Usually highly active raft'mates evaporate, the residue is stored, the tramuranium elements are removed, and the remainder is vitrified [1, 2]. Economically and technically, it is advantageous to evaporate as much as possible. However then the hard-to-dissolve materials contained in the raffinates remain in the residue, so data on the solubility of these matedals is of practical interest.The highly active raft'mates are nitric acid solutions that contain various metal nitrates (from fission products, corrosion, etc.). Of these, one of the least soluble is barium nitrate.Below are our data on the solubility of barium nitrate in nitric acid solutions. To some degree these solutions imitate the composition of highly active raft'mates under evaporation conditions. These data either are not available in the scientific and engineering literature or extend what is available [3].The solubility was determined for isothermal (30 and 60"C) or isobaric (200, 465, and 1000 GPa) conditions. For the isothermal measurements, the barium nitrate was introduced into the solution as a salt; for the isobaric measurements, it was introduced as a water solution with a known concentration (8.4% by weight) and then evaporated until a precipitate started to form. Equilibrium between the saturated solution and the precipitate occurred after 2 h. Samples of the saturated solution were immediately diluted with water and analyzed for barium nitrate by chromate titration [4].The data in Tables 1-3 show that the solubility of barium nitrate increases with temperature and pressure, but decreases with increasing concentrations of nitric acid and the nitrates of sodium, calcium, and aluminum. Analogous results were also obtained for solutions containing a combination of nitrates of Fe 3+, Cr 3+, Ce 3+, lanthanum, cesium, strontium, and zirconium.

Nitric-acid raffinates from the reprocessing of spent nuclear fuel are usually evaporated. Molybdenum is one of the components of raffinates that on evaporation can end up in the residue, thereby complicating the process and subsequent reprocessing of the vat residue [1]. Studying the evaporation of raffinates, we noted some characteristic features of the behavior of molybdenum.The experiments were performed in a glass evaporator, whose construction is described in [1]. Evaporation was conducted under a residual pressure of 350 mm Hz with a yield of -0.2 liters/h, numerically equal to the vat solution in the apparatus (which reproduces the conditions of industrial evaporation).The initial solution, a simulator of raffinate, had the following composition (in g/liter): nitric acid 190, Mo 0.6, Zr 0.7, Ce 3+ 4.1, Cs +, Ba 2+, Sr 2+, Cr 3+, and Fe 3+ 2.7.Technical grade nitric acid (-57 mass%), nitric-acid salts, and powders of metallic molybdenum were used. The solutions were prepared according to a weighed quantity of the nitrates, the volume of the nitric-acid solutions, and nitric-acid solutions of molybdenum and zirconium of known concentration. The concentration of the nitric acid was determined by alkalimetric titration, the molybdenum concentration was determined photocolorimetrically [3], and the zirconium concentration was determined spectrophotometrically [4].In a semicontinuous process (continuous feeding of the initial solution without extraction of the vat residue) the molybdenum started to precipitate with degree of evaporation n -5; for n = 50 its concentration in the vat solution was equal to 8.5 g/liter, i.e., -72% precipitated into the residue. Similar results were obtained with initial solutions, in which the concentration of molybdenum and salt was 1.5-2 times lower, and also in the absence of salts and with addition of AI 3+, Ca 2+, UO22+, and Ni 2+. The results correspond qualitatively with the low (at the level of grams per liter) solubility of molybdenum, which can be estimated from the data in [5], though, quantitatively, they probably exceed it.Vat residues with the molybdenum concentration clearly higher than its solubility were obtained when evaporation was conducted with a solution of acid ("cushion"), whose concentration was equal to approximately the concentration of acid for a continuous process with n = 40 (i.e., -500 g/liter), added beforehand into the apparatus. The results of such evaporation are presented in Table 1, whence it follows that in the case of semicontinuous evaporation of the raft'mate simulator on the cushion, molybdenum does not precipitate into the residue. A high concentration of molybdenum is also achieved by evaporating nitric-acid solutions of molybdenum and zirconium, while in the absence of zirconium (experiment 5) almost all of the molybdenum ends up in the residue. Therefore zirconium promotes retaiument of molybdenum in the solution. At the same time it was established that when the vat residues are subsequently boiled, molybdenum and to a lesser extent zi...

No abstract

Nitric-acid raffinates produced during extractive reprocessing of spent nuclear fuel are ordinarily evaporated and nitric acid is recovered from the liquor vapors, obtained during evaporation, by rectification [1]. Raffinates can contain organic substances, which end up in them as a result of dissolution and removal of the extracting agent and technological additives (as complexing agents and reducing agents). Ordinarily, under conditions of evaporation of raffinates organic substances interact with nitric acid, and some of them are simultaneously distilled off with the liquor vapor, which could influence the technological regimes and the quality of the products of the evaporation and rectification operations. For this reason, it is of practical interest to investigate the behavior of organic substances under conditions of evaporation of raffinates. The results of such investigations reported in this paper supplement existing results (see, for example, [2-4]) or are published for the first time.We determined the kinetics of the interaction of organic substances with nitric acid in water solutions. The concentration regions studied are determined by technological necessity. The investigations were performed in a glass vessel with a reflux cooler and electrical heating of the walls. A vacuum system was connected to the cooler in order to maintain a prescribed boiling point of the solution and to a system of flasks filled with saturated NaCI solution for the purpose of measuring the volume of the gases released, with NaOH solutions for absorption of the acid gases (CO 2 and NO2), and with hydrogen peroxide for absorption of NO. The concentration of nitric acid was determined by alkalimetric titration and that of the organic substances was determined by the standard methods.Interaction of Lactic and Nitric Acids. The acid concentration was determined by potentiometric titration with an alkali in water medium [5] and in acetone [6]. It follows from Table I that the interaction of nitric and lactic acids is noticeable with an initial concentration of the former (X o) of 3.44 moles/liter. It intensifies with increasing X o and m o (it was determined that Mn 2 + and Fe 3 + also increase the interaction rate). The reaction terminates approximately after 4 h (X and m become constants). To identify the organic matter remaining after completion of the reaction, a portion of the solution was distilled off. This yielded condensates which were analyzed to determine the equilibrium distribution of organic material contained in them between the liquid and vapor (equilibrium was determined by the method of [7]). The composition of the condensates fell into the range X = 0.5-1.4 moles/liter and m = 0.2-0.3 moles/liter. At atmospheric pressure the distribution factor of the organic matter between the vapor and the liquid was equal to 0.6-1, which was close to the distribution factor of the acetic acid determined under the same conditions. The presence of traces of acetic acid in the condensates was confirmed by qualitative analysis...

In the extraction reprocessing of spent BBI~R nuclear fuel, a large fraction of the tritium contained in the fuel is transferred into a nitric-acid solution of the fission products. This solution is ordinarily concentrated by evaporation, and a slightly acidic condensate and regenerated nitric acid are obtained from the liquor vapors of rectification. It is assumed that tritium is present in the solution in the form of tritium water, which during the evaporation and rectification processes becomes distributed between the bottoms, condensate, and the regenerated acid [1,2]. The physical-chemical basis of this distribution is characterized by the data on the liquid-vapor equilibrium in systems which simulate the composition of the technological solutions. We have not found such data in the scientific-technical literature. In this connection we present below data on the equilibrium distribution of tritium in the system nitric acid-water.The liquid-vapor equilibrium was determined by the circulation method on a Otmera apparatus (the construction and method of working with it are described in [3]) at pressures of 26.6 and 53.3 kPa and atmospheric pressure and tritium concentrations of 10 -3-1 Ci/liter. Distilled water, distilled water solutions of nitric acid and "A" (TU9G-976.82) tritium water were used as the initial waters. The acid concentration was found by titrating with alkali, and the tritium concentration was determined radiometrically on a RZhS-05 apparatus with a ZhS-8 scintillation liquid.Four parallel determinations of the equilibrium were performed for each composition of the liquid. The concentration of the water and then the distribution factors of t~itium, water, and acid were calculated according to the arithmetic-meanvalues of the acid and tritium concentrations: ,~ ~-:a.,. 7";% ~., where Ty, Yw, and Ya are, respectively, the concentrations of tritium (Ci/liter), water, and acid (moles/liter) in"the va]9or-ph]se condensate: T x, x w, and x a are the same quantities in the liquid.Data obtained for a pressure of 53.3 kPa are presented in Table I. The quantity s T varied within its limits of reproducibility (+7 rel. %) in the range studied with increasing or decreasing pressure. One can see that as the acid concentration increases up to I0-I2 moles/Iiter the distribution factors of all components increase, and c~y and c~ w are close to one TABLE 1. Distribution Factors of the Components in the System Nitric Acid-Water Containing Tritium.