RU2270172C2 - Tetravalent plutonium stabilization in nitric acid solutions - Google Patents

Abstract

FIELD: electrochemistry.

SUBSTANCE: hydroxylamine is introduced into 0.5-3 M, preferably 1-2.5 M, nitric acid solution containing mixture of plutonium with different valence, namely Pu(VI), Pu(IV), and Pu(III). Solution is conditioned at 20-40°C, preferably at 25-35°C during period sufficient to reduce Pu(VI) to Pu(IV). Then nitric acid concentration is adjusted to 2.5-4 M, preferably to 2.5-3 M and solution is conditioned at 20-40°C up to step-like increasing of redox potential (namely to +0.84-+0.87 V), measured using silver chloride reference electrode. In another embodiment of the invention in nitric acid solution containing mixture of plutonium with different valence, namely Pu(VI), Pu(IV), and Pu(III) nitric acid concentration is adjusted to 2-4 M, hydroxylamine is introduced and solution is conditioned at 20-40°C up to step-like increasing of redox potential (namely to +0.84-+0.87 V), measured using silver chloride reference electrode.

EFFECT: accelerated method for chemical tetravalent plutonium stabilization; decreased process step number, reduced heat consumption, and decreased radioactive gases and aerosol release.

4 cl, 6 ex, 3 dwg

Description

At present, the extraction method based on the extraction of plutonium from aqueous nitric acid solutions with organic solvents, mainly tributyl phosphate (TBP) is used to regenerate and purify plutonium from radioactive fission products in the processing of spent nuclear materials [V.I. Zemlyanukhin, E.I. , AN Kondratyev and others. Radiochemical processing of nuclear fuel of nuclear power plants. M .: Energoatomizdat, 1983, p. 82]. To achieve the most complete extraction and purification of plutonium, it is necessary to stabilize it before extraction in the 4-valence state, which has a maximum distribution coefficient compared to other plutonium valence forms - Pu (III) and Pu (VI).

Known methods for stabilizing plutonium (IV) in aqueous solutions containing a mixture of valence forms Pu (III), Pu (IV) and Pu (VI), using various chemical reagents.

A method is described for stabilizing Pu (IV) in aqueous nitric acid solutions using hydrogen peroxide, H ₂ O ₂ , at a nitric acid concentration of 5 M and a temperature above 50 ° C [S. Stoler, R. Richards. Nuclear fuel reprocessing. M .: Atomizdat, 1964, p.620].

The disadvantage of this method is the inability to stabilize Pu (IV) at low acidity (1-2 M HNO ₃ ) and a temperature of 25-35 ° C, typical for extraction processing. Additional difficulties when using H ₂ O _{2 are} caused by the formation of poorly soluble peroxide compounds Pu (IV) during stabilization, which poses a threat to the safe conduct of the stabilization process, especially with a high content of plutonium in solution.

A method for stabilizing Pu (IV) using solutions of sodium nitrite - NaNO ₂ [Plutonium. Directory. Ed. O. Vika, volume 1. 1971, p. 343, 351]. The disadvantage of this method is the need to heat solutions with plutonium to a high temperature (more than 50 ° C). Another drawback is the introduction of sodium ions into the processed solutions, which excludes the use of this method in the preparation of plutonium solutions for oxalate precipitation. In order to exclude sodium ions proposed [P. Rance, B. Zilberman, Proceed. Int. Conf. "GIOBAL-2001", Paris, France, Sept. 9-13, 2001, paper No. 314] a method for stabilizing Pu (IV) using nitrogen oxides, NO and NO ₂ , which, like sodium nitrite, form nitrous acid when dissolved in nitric acid.

The disadvantage of this method is the low rate of Pu (VI) reduction and, as a consequence, the need to carry out the process at high temperature (at least 60 ° C).

The most common way to stabilize Pu (IV) in solutions containing a mixture of valence forms of plutonium is to reduce Pu (VI) and Pu (IV) to Pu (III) and then oxidize Pu (III) to Pu (IV) with sodium nitrite. A known method of stabilization of Pu (IV), based on the reduction of plutonium to Pu (III) with hydrazine [E.M. Rogozina, L.F. Konkina, D.K. Popov. Radiochemistry, 1973, v.15, No. 1, p.61].

The disadvantages of this method are the low rate of plutonium reduction with hydrazine (for this reason, the process is carried out at an elevated temperature of 80 ° C) and the formation of ammonium nitrate, which leads to an increase in the volume of liquid radioactive waste entering the disposal site.

In another method, a Fe (II) solution in a mixture with hydrazine is used as a reducing agent [GP Nikitina et al. Radiochemistry, 1975, v.17, No. 4, p.573]. This method allows you to restore plutonium to a trivalent state at room temperature, however, the introduction of iron ions into solutions does not allow using it to stabilize Pu (IV) before oxalate precipitation. In addition, the resulting iron nitrate further increases the salt content in the liquid waste.

Closest to the claimed is a method of stabilization of Pu (IV) (US Patent No. 4011296, class C 01 G 043/00, 1977), which is first carried out the restoration of plutonium to the trivalent state with hydroxylamine at low acidity, after which the resulting solution is introduced into a boiling solution of strong nitric acid and then the combined solution is evaporated 2 times.

Although the description of this invention does not indicate the numerical values of the concentration of HNO ₃ during the stabilization of Pu (IV), it is known that aqueous solutions after re-extraction of plutonium with diluted nitric acid, in which the prototype was reduced with hydroxylamine, have an acidity of less than 1 M HNO ₃ . It also follows from the description of the invention that, after evaporation, plutonium (IV) is present in the form of a hexanitrate complex, Pu (NO ₃ ) ₆ ^2- , and this solution is supplied to the sorption recovery of plutonium. It is known [Egorov E.V., Makarova S.B. Ion exchange in radiochemistry. M .: Atomizdat, 1971, p. 71, 125; Anisimov V.I., Kozlov A.G., Shurmel V.A. et al., Atomic Energy, 1977, v. 42, No. 3, p. 191], that the optimum acidity of 7–7.5 M HNO ₃ and, therefore, the concentration of boiling nitric acid into which the solution is introduced is optimal when plutonium is adsorbed on an anion exchange resin. hydroxylamine-containing plutonium is at least 3.5 M.

The disadvantages of the prototype are:

- the need for a stage of plutonium reduction at low acidity, which creates the conditions for the formation of Pu (IV) polymers, especially in solutions with a high concentration of plutonium (several tens of g / l);

- carrying out the operations following the recovery stage at high temperature (~ 100 ° C), which requires a large heat consumption and leads to the formation of a large number of radioactive vapors, gases and aerosols;

- multi-stage stabilization process, including the recovery of plutonium to Pu (III), acidification of the solution and its evaporation;

- the long duration of the process of transition of plutonium to the trivalent state, due to the strong inhibition of the reaction of Pu (IV) reduction by hydroxylamine as its product - plutonium (IV) [Koltunov BC, Zhuravleva GI, Radiokhimiya, 1978, v.20, no. 94].

An object of the invention is to accelerate the chemical stabilization process of Pu (IV). The solution to this problem allows us to reduce the number of process steps, reduce heat consumption for heating solutions and reduce the release of radioactive gases and aerosols.

In the first embodiment of the proposed method, the problem is solved in that in the known method for stabilizing plutonium in the tetravalent state in an aqueous nitric acid solution (including the reduction of Pu (VI) and Pu (IV) to Pu (III) with hydroxylamine and the subsequent oxidation of Pu (III) at a temperature ~ 100 ° C) in a solution with a concentration of nitric acid from 0.5 to 3 M, preferably from 1 to 2.5 M, containing a mixture of valence forms of plutonium Pu (VI), Pu (IV) and Pu (III), hydroxylamine is introduced , the solution is maintained at a temperature of from 20 to 40 ° C, preferably from 25 to 35 ° C, for a time nor necessary for the reduction of Pu (VI) to Pu (IV) and Pu (III), after which the concentration of nitric acid is adjusted to 2.5-4 M, preferably from 2.5 to 3 M, and the solution is kept at a temperature of from 20 up to 40 ° C until a stepwise increase in the redox potential of the solution and its increase to a value from +0.84 to +0.87 V, measured relative to the silver chloride reference electrode.

In the particular case of the implementation of the first variant of the invention, the initial concentration of hydroxylamine is chosen equal to from 0.02 to 0.2 M, preferably from 0.05 to 0.2 M.

The lower limit of HNO ₃ concentration (0.5 M) is limited by the polymerization of plutonium, especially in solutions with a high concentration of it (several tens of g / l) and at elevated temperature. The upper limit of the concentration of HNO ₃ (3 M) is limited by the rapid decomposition of hydroxylamine by its reaction with nitric acid, which leads to incomplete reduction of Pu (VI), especially at a low initial concentration of hydroxylamine.

The initial concentration of hydroxylamine should be selected so as to ensure complete recovery of Pu (VI), taking into account the additional consumption of hydroxylamine in its parallel reactions with nitric and nitrous acids. The value of the initial concentration of hydroxylamine is selected depending on the acidity of the solution and temperature. With a low acidity of the solution (0.5-1 M HNO ₃ ) and a low temperature of 20-25 ° C, the minimum initial hydroxylamine concentration is 0.02 M; at low acidity and high temperature (40 ° C), as well as at high acidity (3 M HNO ₃ ) and low temperature, this value is 0.05 M; at high acidity and high temperature, it should be at least 0.1 M.

The time required for the reduction of Pu (VI) is determined either experimentally (for example, by spectrophotometric method for absorbing light in the wavelength region of 830 nm) or by calculation (according to the kinetic equations of chemical reactions in solution).

The process of plutonium oxidation is carried out at a nitric acid concentration of from 2.5 to 3 M at a temperature of 25 to 40 ° C until the value of the redox potential of the solution is reached from +0.84 V to +0.87 V, measured relative to the silver chloride reference electrode. The numerical value of the potential depends on the concentration of HNO ₃ (it increases with increasing acidity).

It is proposed to use hydroxylamine nitrate as a reducing agent for plutonium ions, which decomposes under the action of nitric acid to gaseous nitrous oxide, NaO, and water and does not increase the amount of liquid radioactive waste. Hydroxylamine quite quickly reduces Pu (VI) even at room temperature (20-25 ° C) and at relatively high acidity (more than 1 M HNO ₃ ), while the complete transition of plutonium to the trivalent state under these conditions proceeds much more slowly. The main feature of the recovery process according to the proposed method is that, unlike the prototype, it is not necessary to transfer all plutonium to the trivalent state, but rather to restore Pu (VI) to the lower valence forms Pu (IV) and Pu (III), which can significantly reduce recovery stage duration.

It is proposed to oxidize Pu (III) to Pu (IV) either by acidifying the solution to a concentration of HNO _{3 of} at least 2.5 M (if the acidity of the solution after the reduction step was less than 2.5 M HNO ₃ ), or by heating the solution to a temperature above 20 ° C (if the temperature of the solution after the reduction step was less than 20 ° C), or at the same time by acidification and heating of the solution.

The lower limit of the concentration of HNO ₃ in this embodiment of the invention is associated with the behavior of hydroxylamine in solutions of nitric acid, due to the competition of two parallel reactions of hydroxylamine with nitric and nitrous acids. The upper limit of the concentration of HNO ₃ is associated with the need to ensure a safe oxidation process (without peak emissions of gaseous products).

The duration of the oxidation process (it decreases with increasing nitric acid concentration and temperature and with decreasing hydroxylamine concentration) is determined by the change in the redox potential of the solution, measured relative to the silver chloride reference electrode. It was experimentally established that the transition of plutonium to the tetravalent state is accompanied by an increase in the redox potential of the solution, as measured by a platinum electrode relative to the silver chloride reference electrode. The stabilization of plutonium in the tetravalent state is completed after an abrupt increase in the potential to a value equal to +0.84 to +0.87 V (depending on the concentration of HNO ₃ ).

The following examples 1, 2 and 3 describe the implementation of the proposed method for stabilization of tetravalent plutonium according to the first embodiment in an installation, the scheme of which is shown in figure 1, where

1 - jacketed reactor;

2 - salt bridge;

3 - silver chloride reference electrode;

4 - measuring electrode made of platinum;

5 - potentiometer;

6 - recording device.

Figure 2 - change in the redox potential of the solution (E _s ) during the oxidation of Pu (III) in a solution of 2.7 M HNO ₃ containing hydroxylamine.

Figure 3 - change in the redox potential of the solution (E _s ) during the oxidation of Pu (IV) with hydroxylamine in a solution of 2.6 M HNO ₃ .

Example 1. In a solution with a concentration of nitric acid 1 M containing 0.023 M Pu (VI) and 0.10 M Pu (IV), hydroxylamine is introduced to a concentration of 0.2 M, the solution is aged at a temperature of 25 ° C for 30 minutes. Then, a measuring electrode of platinum is placed in the solution to measure the value of the redox potential of the solution (E _s ) and the solution is acidified to a concentration of HNO ₃ equal to 3 M. During acidification, a jump in E _s occurs to a value of +0.846 V. Spectrophotometric analysis of the resulting solution indicates the absence of other valence forms of plutonium in the solution, except for plutonium (IV).

Example 2. In a solution with a concentration of nitric acid 1 M, containing 0.020 M Pu (VI) and 0.084 M Pu (IV), hydroxylamine is introduced to a concentration of 0.1 M, and the solution is kept at a temperature of 25 ° C for 35 minutes. Then, a measuring electrode made of platinum is placed in the solution to continuously record changes in the redox potential of the solution (E _s ), and the solution is acidified to a concentration of HNO ₃ equal to 2.7 M. After 20 minutes, a jump in E _s to +0.844 V occurs (Fig. .2). Spectrophotometric analysis of the resulting solution indicates the presence of plutonium in the solution only in the tetravalent state.

Example 3. In a solution with a concentration of nitric acid of 2.5 M, containing 0.017 M Pu (VI) and 0.077 M Pu (IV), hydroxylamine is introduced to a concentration of 0.1 M, the solution is maintained at a temperature of 25 ° C for 30 minutes. Then, a measuring electrode of platinum is placed in the solution, and the solution is heated to a temperature of 40 ° C; at the same time, continuous recording of changes in the value of the redox potential of the solution (E _s ) is performed. After 15 min, a jump in E _ov occurs to a value of +0.845 V. A spectrophotometric analysis of the resulting solution indicates the absence of other plutonium forms other than plutonium (IV) in the solution.

In the second embodiment of the proposed method, the problem is solved in that in the known method for stabilizing plutonium in the tetravalent state in an aqueous nitric acid solution (including the reduction of Pu (VI) and Pu (IV) to Pu (III) with hydroxylamine and the subsequent oxidation of Pu (III) at a temperature ~ 100 ° C) in a solution of nitric acid containing a mixture of valence forms of plutonium Pu (VI), Pu (IV) and Pu (III), a concentration of nitric acid from 2 to 4 M is created, hydroxylamine is introduced into the solution and the resulting solution is kept at a temperature from 20 to 40 ° C to spasmodic in zrastaniya redox potential of the solution and increasing it to values from 0.84 to 0.87 V, measured against a silver-silver chloride reference electrode.

In the particular case of the second embodiment of the invention, the initial concentration of hydroxylamine is selected to be equal to from 0.05 to 0.2 M, preferably from 0.1 to 0.2 M.

The stabilization of Pu (IV) at low acidity (2 M HNO ₃ ) is preferably carried out by heating the solution to a temperature of 35-40 ° C and at a hydroxylamine concentration of 0.05-0.1 M (depending on the concentration of plutonium), and at high acidity (4M HNO ₃ ) - at room temperature and a hydroxylamine concentration of 0.2 M.

Examples 4, 5 and 6 describe the implementation of the proposed method for stabilization of tetravalent plutonium according to the second embodiment in the laboratory setup shown in figure 1.

Example 4. In a solution with a concentration of nitric acid of 2.6 M, containing 0.025 M Pu (VI) and 0.078 M Pu (IV) and having a temperature of 25 ° C, a measuring electrode of platinum is placed, hydroxylamine is introduced to a concentration of 0.1 M, and the solution is heated to a temperature of 40 ° C with continuous recording of changes in the value of the redox potential of the solution (E _s ). After the jump of E _s to a value of +0.856 V (Fig. 3), the absorption spectrum of the solution is recorded, which indicates the absence of other valence forms of plutonium in the solution, except for plutonium (IV).

Example 5. In a solution with a concentration of nitric acid of 4.1 M, containing 0.016 M Pu (VI) and 0.050 M Pu (IV) and having a temperature of 20 ° C, a measuring electrode of platinum is placed and hydroxylamine is introduced to a concentration of 0.2 M. After 48 min, a jump in Е _s occurs to a value of +0.870 V. The absorption spectrum of the final solution indicates the absence in the solution of other valence forms of plutonium, except for plutonium (IV).

Example 6. In a solution with a concentration of nitric acid 2 M, containing 0.024 M Pu (VI) and 0.044 M Pu (IV) and having a temperature of 22 ° C, a measuring electrode of platinum is placed and hydroxylamine is introduced to a concentration of 0.1 M. After 16 minutes, the World War II jumps up to +0.840 V. The absorption spectrum of the final solution indicates the absence in the solution of other valence forms of plutonium, except for plutonium (IV).

Compared with known methods, including the prototype, the proposed method allows the stabilization of Pu (IV) in one stage in a wide range of solution acidity (from 2 to 4 M HNO ₃ ) at low temperatures (from 20 to 40 ° C) in a short time (in optimal conditions for 20-30 minutes). Plutonium (IV) in the solutions obtained by the proposed method is stable during storage and does not change its valency for at least 15 days at room temperature.

Claims (4)

1. The method of stabilization of plutonium in the tetravalent state in a nitric acid solution using hydroxylamine, characterized in that in a solution with a concentration of nitric acid from 0.5 to 3 M, preferably from 1 to 2.5 M, containing a mixture of valence forms of plutonium Pu (VI) Pu (IV) and Pu (III), hydroxylamine is introduced, the solution is kept at a temperature of from 20 to 40 ° C, preferably from 25 to 35 ° C, for the time required to restore Pu (VI) to Pu (IV ) and Pu (III), after which the concentration of nitric acid is adjusted to 2.5-4 M, preferably from 2.5 to 3 M and maintained the solution at a temperature of from 20 to 40 ° C until an abrupt increase in the redox potential of the solution and increase in its value to + 0.84 ÷ + 0.87 V, measured relative to the silver chloride reference electrode. 2. The method according to claim 1, characterized in that the initial concentration of hydroxylamine is chosen equal to from 0.02 to 0.2 M, preferably from 0.05 to 0.2 M. 3. A method for stabilizing plutonium in the tetravalent state in a nitric acid solution using hydroxylamine, characterized in that a concentration of nitric acid is created in a nitric acid solution containing a mixture of valence forms of plutonium Pu (VI), Pu (IV) and Pu (III) from 2 to 4 M, hydroxylamine is introduced into the solution and the solution is kept at a temperature of from 20 to 40 ° C until the redox potential of the solution jumps up and its value increases to + 0.84 ÷ + 0.87 V, measured relative to the silver chloride reference electrode. 4. The method according to claim 3, characterized in that the initial concentration of hydroxylamine is chosen equal to 0.05 to 0.2 M, preferably from 0.1 to 0.2 M.

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