RU2107959C1 - Method for cleaning uranium from plutonium - Google Patents

Abstract

FIELD: fuel for nuclear reactors. SUBSTANCE: uranium and plutonium are extracted in solutions of tributyl phosphate in inert thinner. Uranium-saturated extract is rinsed with nitric acid solution containing iron II and hydrazine. Then extract is rinsed again with 0.5 mol/l solution of nitric acid containing 0.5-1 g/l of oxalic acid and same amount of diethylene triaminepentaacetic acid. EFFECT: facilitated procedure; improved cleaning efficiency. 1 dwg , 2 tbl

Description

 The proposed method is intended for purification of highly enriched uranium from plutonium with a view to its further use as fuel for fast and thermal neutrons.

 Due to the high content of plutonium in it (1 - 30 microns / kg of uranium), this uranium cannot be sold both on the domestic and foreign markets. Plants producing fuel and fuel assemblies have stringent requirements for plutonium content. So, for example, American firms initially limited the plutonium content to no more than 1 μg / kg, and later set the condition for the plutonium content in uranium at 0.1 μg / kg.

Thus, the coefficients of purification of uranium from plutonium should be more than - 10 ⁵ .

Existing extraction methods for the separation of uranium and plutonium with TBP solutions in an inert diluent in only three extraction purification cycles make it possible to achieve plutonium purification coefficients of 3 • 10 ⁵ [1, 2].

A known method of purification of uranium from plutonium by extraction with a 30% solution of tributyl phosphate at a high degree of saturation of the extractant with uranium, since the latter helps to increase the coefficient of purification of uranium from plutonium. This method is taken by us as a prototype / 3 /. In accordance with this method, after the operation of joint extraction of uranium and plutonium, the uranium-saturated extracts are washed with a solution of nitric acid containing, as a reducing agent, for converting plutonium from the extracted tetravalent state to the non-extractable ferric ions. Hydrazine or sulfate ion is used as a stabilizer of ferrous iron. Using iron (II) ions as a reducing agent for the separation of uranium and plutonium using this technology, it is possible to obtain the coefficients of purification of uranium from plutonium at the level of 10 ⁴ .

 The disadvantages of this method include the insufficient purification of uranium from plutonium despite the extraction process with a high degree of saturation of the extractant with uranium and the use of a washing solution containing iron (II) ions as a reducing agent for plutonium (IV).

The aim of the present invention is to increase the coefficients of purification of uranium from plutonium to the level of 10 ⁵ - 10 ⁶ to obtain a uranium concentrate (90 - 100 g / l) and a plutonium content of less than 1 μg / kg of uranium.

 This goal is achieved by the fact that they carry out an additional washing of the extract with a solution of 0.5 ± 0.1 mol / L nitric acid, containing 0.5-1 g / L of oxalic and diethylene triamine pentaacetic acids. We have established static (distribution coefficient) and dynamic experiments using a laboratory setup (purification coefficient) that the joint use of two well-known complexing agents leads to the achievement of this goal.

 The essence of the method is as follows.

 A solution with a concentration of uranium of 100 - 150 g / l, plutonium up to 30 mg / kg of uranium, nitric acid 2.5 ± 0.5 mol / l is dosed in the 10th stage of the 16-stage extraction washing unit. In the 1st stage, 30% of TBP enters and is saturated with uranium. The extract is washed successively at 4 steps with 2 to 3 mol / L nitric acid containing 1 to 2 g / L of iron (II) and 2 to 5 g / L of hydrazine, and then at 2 steps with a solution of 0.5 mol / l of nitric acid containing 0.5 - 1.0 g / l of oxalic and diethylene triamine pentaacetic acids. The flow rate of washing solutions is 5-7 times less than the extractant flow rate.

 The washed extractant enters the 12-stage unit for re-extraction of uranium with water, and then the extractant enters the washing with soda. The drawing shows a schematic diagram of the purification of uranium from plutonium.

 Example 1

 The distribution coefficients of plutonium depending on the composition of the washing solutions are presented in table. one.

The addition of DTPA or oxalic acid in addition to iron (II) in an aqueous solution does not change the distribution coefficient of plutonium. In a 3 mol / L HNO ₃ medium, no complexes are formed.

 When using an aqueous solution of a composition of 0.5 mol / L nitric acid and 1 g / L complexon (experiments 4, 5), almost equal distribution coefficients (0.015 - 0.02) were obtained.

 The combined use of complexones of 1 g / l (experiment 6) led to a decrease in distribution coefficients by 5–7 times.

When the plutonium content in the experiment was 50 mg / L, the concentrations of H ₂ C ₂ O ₄ and DTPA both individually and together are many hundreds of times higher than the concentration of plutonium and the synergism of sharing cannot be explained only by an increase in the concentration of the complexing reagent due to the second. The use of two reagents leads to the formation of a double complex, the instability constant of which is much less than that of individual complexes.

 The conclusions of static experiments on determining the distribution coefficients of plutonium during the extraction of 30% TBP from aqueous solutions of various compositions were confirmed by experiments on an extraction unit. The composition of the initial solution is as follows: uranium concentration of 150 g / l, plutonium 29 mg / kg of tap, nitric acid - 2.5 mol / g (example 2).

 Example 2

 The coefficients of purification of uranium from plutonium depending on the composition of the second wash solution and the number of steps are presented in table. 2.

The composition of the washing solution of the first washing is as follows: - HNO ₃ 2 mol / L, Fe ²⁺ 1 g / L, hydrazine 2.5 g / L, the same in all experiments. The composition of the washing solution of the additional washing is as follows: nitric acid 0.5 mol / g, oxalic acid 1 g / l (experiments 1 and 2) and additionally DTPA 1 g / l (experiments 3 and 4).

 An increase in the number of steps to 5 and 3 (experiment 2) compared with experiment 1 does not increase the coefficient of purification of the valve from plutonium. The introduction of an additional DTPA wash solution (experiment 3) increased the purification coefficients of uranium from plutonium by 8–10 times, which follows completely from the conclusion of example 1. Additional studies found that reducing the concentration of DTPA to 0.5 g / l does not reduce the coefficients purification of uranium from plutonium. Reducing the number of steps to the initial (experiment 4) did not affect the cleaning coefficients.

 The implementation of the proposed method will allow for deep cleaning of uranium from plutonium.

Bibliography:
1. Vdovenko V.M. Modern radiochemistry, Moscow: Atomizdat, 1969, p. 459 - 465.

 2. Chemical technology of irradiated nuclear fuel. Ed. Shevchenko V.B. - M.: Atomizdat, 1971, p. 169 - 180.

 3. Zemlyanukhin V. I. et al. Radiochemical processing of nuclear fuel of nuclear power plants. - M .; Energoatomizdat, 1989, p. 90 - 94.

Claims (1)

 A method for purifying uranium from plutonium, including the extraction of uranium and plutonium in tributyl phosphate solutions in an inert diluent, washing the uranium-saturated extract with a solution of nitric acid containing iron (II), hydrazine, characterized in that the second washing of the extract with a 0.5 mol / l solution nitric acid containing 0.5 - 1 g / l of oxalic and diethylene triamine pentaacetic acids.

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