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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...

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At the present time vitrification is the main method of localization of high-level liquid wastes. The high-level liquid wastes which are to be vitrified and which are obtained by extraction recovery of nuclear fuel from BBI~R and research reactors can be represented in the form of the system HNO 3 --H20 --NaNO 3 --AI(NO3)yTo obtain phosphate glass phosphoric acid is added [1] and correspondingly we obtain the system HNO3 _ H20 _ NaNO 3 _ AI(NO3)3 m H3PO4.Ordinarily, high-level solutions are relatively dilute. Therefore to decrease the water load on the vitrification apparatus and to increase its glass capacity the solutions are first concentrated by evaporation. The degree of evaporation is determined by the ratio of the salt concentration in the evaporated solution to the concentration in the initial solution and the degree of concentration is determined as the ratio of the salt concentration in the solution entering the vitrification apparatus to the initial concentration. The degrees of evaporation and concentration may or may not be equal to one another. The degree of evaporation (concentration) is determined by the solubility of the salts. In the process of evaporation the volatile components (nitric acid and water) are driven off, i.e., denitration of the solution occurs. Therefore, the data on the solubility and liquid-vapor equilibrium in the indicated nitric-acid (1), nitric-phosphoric-acid salt systems (2), and in their component subsystems form the physical-chemical basis of the processes of evaporation and denitration of high-level wastes.In the present paper we present additional data or data not available in the scientific-technical literature.The experimental results are presented in Tables 1-9, where the concentration of nitrates and phosphoric acid is given in mass %, the concentration of free nitric acid is given in mass % relative to the sum of the water and nitric acid, and the concentration of bound nitric acid is expressed in terms of the nitric acid deficit in %, relative to the acid that should be bound in sodium and aluminum nitrates, with a minus sign.Data on the Solubility. It is well known that the solubility of sodium and aluminum nitrates decreases with increasing nitric acid concentration [5] and the solubility of sodium nitrates decreases with the addition of phosphoric acid [6]. The data obtained with constant ratios of the nitrates and phosphoric acid, which is observed in the vitrification technology [1], and at a temperature higher than in [5,6], specifically, at the boiling point at atmospheric pressure, are presented in Tables 1-3. These data determine the maximum degree of evaporation.As one can see from Tables 1 and 2, as phosphoric acid is added and its concentration increases, the solubility of sodium nitrate decreases (in agreement with [6]) and the solubility of aluminum nitrate also decreases. The solubility of aluminum nitrate, just as the mixture of sodium and aluminum nitrates, in the presence of phosphoric acid [the system (2)] increases as the nitric acid concent...