RU2696475C1 - Method of dissolving plutonium dioxide to obtain a concentrated solution - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to a method of dissolving plutonium dioxide or mixed oxides of actinides containing plutonium dioxide, any other oxides with a redox potential which is more positive than the potential of the pair Ag²⁺/Ag⁺(-1.98 V). Method comprises loading into electrolysis unit with pulsation chamber a solution of nitric acid, plutonium dioxide, silver in form of metal or nitrate, feeding anode and cathode of electric current, and into pulsation chamber of pulses with period of 0.9 seconds and duration of 0.4 seconds, addition as far as concentration of reacting components in electrolysis unit of plutonium dioxide and nitric acid into anolyte, catholyte is fed with concentrated nitric acid of 630 g/l of 10 ml, after each loading of plutonium dioxide, make-up is carried out, addition of reacting components is carried out after confirmation of complete dissolution of previously added.

EFFECT: invention provides a simple and non-energy-consuming process with a technologically acceptable time of its carrying out at room temperature, absence of extraneous interfering extraction of components and corrosion-hazardous reagents and obtaining a solution with high concentration of plutonium.

1 cl, 2 tbl, 2 ex

Description

The invention relates to methods for dissolving plutonium dioxide or mixed actinide oxides containing plutonium dioxide, any other oxides with redox potential, is more positive than potential of Ag ²⁺ / Ag ⁺ (-1.98 V) pair and can be used in hydrometallurgical technology, including operations dissolving plutonium dioxide-containing materials for subsequent extraction purification of plutonyl nitrate solutions.

It is known that plutonium dioxide (PuO ₂ ) obtained at low temperatures readily dissolves in concentrated hydrochloric, bromic, phosphoric, hydroiodic and perchloric acids. However, plutonium dioxide calcined and aged for a relatively long time is hardly soluble even in these acids. Known chemical methods for transferring calcined plutonium dioxide to a soluble state include the following: 1) preliminary fusion of PuO ₂ with KHSO ₄ ; 2) the interaction of PuO ₂ at 700 ° C with carbon tetrachloride; 3) dissolution in acids: hydrochloric, phosphoric, hydroiodic, perchloric and concentrated chloroacetic acids with the evolution of hydrogen and the formation of the Pu ³⁺ ion; 4) dissolution in dilute sulfuric and nitric acids, while the dissolution of plutonium dioxide is slow, the acids have a passivating effect, and the process is accompanied by the formation of an insoluble precipitate consisting of polymers of plutonium hydroxide or hydrated oxides (I.N.Bekman PLUTONIUM Study Guide http: //profbeckman.narod.ru/Pluton.htm); 5) dissolution in mixtures of mineral acids HNO ₃ , HF and HI (Plutonium / Handbook, edited by O. Vika, Volume 1, M .: Atomizdat, 1971. - P. 23); 6) dissolution methods involving fusion in salt compositions: fusion with potassium pyrosulfate (Analytical chemistry of plutonium. M .: Nauka, 1965, p. 107), fusion with a mixture of potassium nitrate and potassium hydroxide (patent RU 2456687, IPC G21C 3/00 , C01G 56/00, publ. 20.07.2012) and others.

All these methods for the chemical dissolution of plutonium dioxide have one or a combination of the following disadvantages that adversely affect subsequent extraction refining: the inability to completely dissolve plutonium, the formation of plutonium salts with low extraction ratios; non-technological process, expressed in many stages or the use of high temperatures, the corrosive effect of the components used on the materials of the apparatus.

The method of concentration of solutions by evaporation also leads to irreversible polymerization of plutonium and causes the deposition of products on the walls of the evaporator, which leads to frequent shutdowns of the process. The operation of evaporation of a solution of plutonium requires consideration of its corrosive effects on the apparatus.

It is proved that ionic potentials and, accordingly, a tendency to hydrolysis and complexation decrease in the series:

Despite the greater than the rest stability of pentavalent plutonium, PuO ₂ ⁺ in aqueous solutions, it easily disproportionates to Pu (IV) and Pu (VI). Pu (IV) is prone to complexation, in particular with nitric, sulfuric, hydrochloric and acetic acids. Thus, in concentrated nitric acid, Pu (IV) forms Pu (NO ₃ ) ^5- and Pu (NO ₃ ) ₆ ^2- complexes. In aqueous solutions, Pu (IV) is easily hydrolyzed, and plutonium hydroxide is prone to polymerization. Thus, the most acceptable option is to obtain a concentrated solution of plutonium in the Pu (VI) state and transfer it to Pu (IV) immediately before extraction refining, since Pu in the 4+ oxidation state has the highest distribution coefficient.

There is an electrolytic method for the extraction and purification of metallic plutonium in a melt of an equimolar mixture of potassium and sodium chlorides. In this method, after electrolytic refining, plutonium is subjected to electrochemical dissolution in 1-5 M nitric acid (electrolyte) at a temperature of from 20 ° C to 80 ° C and anode current density of 0.5-10 A / cm ² using plutonium as an anode. As the cathode material, tantalum, titanium, tungsten, and acid-resistant stainless steel can be used (I.N. Bekman PLUTONIUM Study Guide Chapter 13 http://profbeckman.narod.ru/Pluton.htm).

The disadvantage of this method of electrochemical dissolution is that when a large concentration of plutonium is obtained at high anode potentials, the formation of polymer plutonium hydroxides is possible, in this case, the anodic dissolution of plutonium ceases.

The known method described in the patent of the Russian Federation for invention No. 2171506 (IPC G21C 19/46, C01G 43/00, C01G 56/00, publ. July 27, 2001). The invention relates to a method and apparatus for dissolving a powder consisting of a mixture of oxides of uranium, plutonium and / or mixed oxides of uranium and plutonium. The powder is dissolved in a solution of nitric acid using divalent silver obtained by electrolysis. The method includes two stages, which are carried out sequentially in the same device for dissolution. The reacting solution circulates in a closed loop: at the first, continuous, stage, uranium oxide dissolves and the solution stream containing the dissolved oxide is diverted; in the second, periodic, stage, plutonium oxide accumulated during the first stage is electrolytically dissolved.

In this method, the inventors experimentally confirmed the low solubility of PuO ₂ . The maximum concentration obtained by this method was 50 g / l for plutonium (the value obtained from the calculation of the dissolution of 3.25 kg in a volume of 65 l).

The disadvantages of this method, in part of the second stage of dissolution, include the following: the relative complexity of the implementation due to the applied design: the use of a mechanical mixing device - a mixer, as well as a filter.

Known electrolyzer for the dissolution of metal oxides (patent RU 2404130, IPC C01G 56/00, C01G 43/00, C25C 7/00, publ. 20.11.2010). The description of the invention discloses a method for dissolving oxides of uranium and / or plutonium, taken as a prototype. A solution of nitric acid is poured into the electrolyzer, silver is introduced in the form of a metal or nitrate, plutonium (uranium) dioxide or products containing their oxides are poured. An electric current is supplied to the anode and cathode, and pulses are sent to the pulsation chamber. The process is conducted either until the calculated amount of current is passed, or to a predetermined concentration of the target element in the solution. Periodically, as the concentration of reacting components in the electrolyzer changes, soluble elements are added, as well as nitric acid to adjust the acidity of the solution.

The main disadvantage of the prototype is that the method does not provide for feeding catholyte with concentrated nitric acid to minimize the process of evolution of explosive hydrogen. In the process of dissolution of plutonium dioxide, nitric acid decomposes in the cathode space (catholyte), leading to electrolysis of water, accompanied by the release of hydrogen. It should also be noted that there is a lack of information on the modes (criteria) for the implementation of the processes described in the description, in particular: the absence of the frequency and duration of a pulse of compressed air that mixes the electrolyte to maintain soluble particles in suspension; the frequency and loading criteria of the next portion of the oxide powder, if necessary, to obtain highly concentrated solutions; the concentration and volume of nitric acid used to fuel the cathode space of the cell.

The objective of the invention is to develop a technologically advanced method for dissolving plutonium dioxide to obtain a concentrated solution.

The technical result is a high concentration of plutonium in the final solution, which makes it possible to reduce the amount of radioactive waste generated during subsequent extraction refining of plutonium.

The technical result is achieved in a method for dissolving plutonium dioxide, including loading into a nuclear-safe execution cell equipped with a pulsation chamber of a solution of nitric acid, plutonium dioxide, silver in the form of metal or nitrate, supplying an electric current to the anode and cathode, and adding pulses to the pulsation chamber as the concentration of reacting components in the plutonium dioxide and nitric acid electrolyzer changes in the anolyte, catholyte concentrate is fed after each loading of plutonium dioxide with 630 g / L of 10 ml nitric acid, pulses are fed into the pulsation chamber with a period of 0.9 seconds and a duration of 0.4 seconds, the addition of reacting components is carried out after confirmation of the complete dissolution of the previously added.

The method is as follows.

Example 1

A dry plutonium dioxide powder was fed to the electrochemical dissolution in a mass calculated per load. The powder was charged through a loading nozzle using a funnel. To eliminate plutonium losses, the feed funnel was washed with the stock solution. This operation was carried out in a solution with a volume of up to 4.96 l, containing 210-320 g / l HNO ₃ and 5-10 g / l silver at a temperature not exceeding 30 ° C with vigorous stirring with pulsating air for 6-10 hours. Silver was introduced into the solution in the form of silver nitrate. The catholyte was strengthened by adding a solution of nitric acid with a concentration of 630 g / l HNO ₃ . The criterion for the strengthening of catholyte with nitric acid is the presence of hydrogen in the exhaust gases of more than 0.2% vol (hydrogen was monitored continuously). The generation of divalent silver in the electrolyzer was carried out with a current load of up to 25 A.

The pulsation was carried out with a period of 0.9 seconds and a duration of 0.4 seconds. The experimental results are presented in table 1.

At the end of the dissolution process, the plutonium solution was aged for 10 hours while stirring with pulsating air. The power of the electrodes during shutdown was turned off. The yield of plutonium was 87%.

In order to completely dissolve plutonium, an additional (control) dissolution was carried out within 10 hours. Control dissolution was carried out with a solution in a volume of 2.46 l containing 300 g / l of nitric acid and 5 g / l of silver. According to the results of the control dissolution, the total yield of plutonium was 100%, which shows the possibility of technological organization of the complete dissolution of plutonium dioxide at these process parameters.

Example 2

To obtain the maximum possible concentration of Pu in the solution, four weights of plutonium dioxide were dissolved in one volume of the solution. Samples were loaded sequentially into the reactor-electrolyzer — each subsequent sample was loaded upon receipt of the result of analysis of the concentration of plutonium in the selected sample. Dissolution was carried out in a solution of 4.2 l, containing 310 g / l HNO ₃ and 10 g / l silver. After each loading of plutonium dioxide, catholyte was fed with concentrated nitric acid 630 g / l in 10 ml. The dissolution was carried out with vigorous stirring with pulsating air with a pulsation period of 0.9 seconds and a duration of 0.4 seconds. Sampling was carried out every 2-3 hours with de-energizing the electrolyzer. It was noted that a long shutdown of the dissolution process, more than 30 minutes, leads to a decrease in the yield of plutonium at each subsequent stage (loading) of the process. The parameters of the dissolution process of the second operation are presented in table 2.

The final solution is dark brown, not transparent, without sediment. The resulting solution with a plutonium concentration of 173.0 g / l was kept for more than two days; according to the results of observations, the presence of a precipitate of undissolved plutonium was not detected.

The advantages of the described method for dissolving plutonium dioxide to obtain a concentrated solution include: the relative simplicity of organizing the dissolution process in one or two stages; relatively low-energy process; technologically acceptable process time; room temperature of the process; the absence of extraneous interfering extraction components; lack of corrosive chemicals; high concentration of plutonium in the final solution, which makes it possible to reduce the amount of radioactive waste generated during subsequent extraction refining of plutonium.

Claims (1)

A method for dissolving plutonium dioxide, comprising loading into a electrolyzer of nuclear safety design, equipped with a pulsation chamber, a solution of nitric acid, plutonium dioxide, silver in the form of a metal or nitrate, supplying an electric current to the anode and cathode, and adding pulses to the pulsation chamber as it changes the concentration of reactive components in the electrolyzer of plutonium dioxide and nitric acid in the anolyte, characterized in that after each loading of plutonium dioxide, catholyte is fed with concentrated azo hydrochloric acid 630 g / l to 10 ml, in the pulsation chamber is fed with pulses of period of 0.9 seconds and 0.4 seconds duration, the addition of the reactants is carried out after confirmation of complete dissolution of the previously added.