THE EXPERIMENTAL STUDY ON URANIUM ISOTOPE SEPARATION BY THE U(Ⅲ)-U(Ⅳ) CHEMICAL EXCHANGE METHOD

THE EXPERIMENTAL STUDY ON URANIUM ISOTOPE SEPARATION BY THE U(Ⅲ)-U(Ⅳ) CHEMICAL EXCHANGE METHOD

Kim Chung Hyok

Candidate of chemical sciences, Kim Il Sung University,

DPRK, Pyongyang

Choe Gwang Chol

Doctor of chemical sciences, associate Professor,

Kim Il Sung University,

DPRK, Pyongyang

Ri Song Bom

Doctor of chemical sciences, Professor, Kim Il Sung University,

DPRK, Pyongyang

Jon Un Chol

Candidate of chemical sciences, Kim Il Sung University,

DPRK, Pyongyang

Jong Chol Jin

Candidate of chemical sciences, Kim Il Sung University,

DPRK, Pyongyang

ABSTRACT

The system, 0.19mol·L^-1UCl₃-0.01mol·L^-1UCl₄-7mol·L^-1HCl in aqueous phase and 0.25mol· L^-1 UCl₄-40%TBP-kerosene in organic phase, as an example of isotope exchange systems in U(Ⅲ)-U(Ⅳ) solution for uranium isotope separation, was selected.

The diameter, ф and height, H of pulsed extraction column for uranium isotope separation was 20mm and 1500mm respectively, pulsed frequency was 1rps, and the atmosphere of column was Ar gas.

When the column was operated 8 times by simple cascade and 3 times by counter-current recirculation cascade making organic phase as continuous phase and aqueous phase as dispersed phase, the enrichment degree of ²³⁵U reached 0.797, 0.745mol% respectively.

The concentration of uranium and the enrichment degree of ²³⁵U were analysed by spectrophotometer and mass spectrometer respectively.

Keywords: uranium isotope separation, chemical exchange method, extraction.

Introduction

Most of separation methods for enriched uranium production in current uranium enrichment plant worldwide are gas centrifugation and gaseous diffusion. The study on different kinds of uranium isotope separation methods that has low capital costs, low operating costs and high economic effectiveness, including laser isotope separation, plasma isotope separation and chemical exchange have been working.

The study on chemical exchange method provided lower costs for enriched uranium production than gas centrifugation and gaseous diffusion and succeed in the pilot plant, but didn’t realize the large-scale applications yet^[1-8,10].

Some papers on the theoretical base, usable possibilities and view of uranium isotope separation by chemical exchange have already reported^[1-17]. The scraps of data on stage separation factor, production of trivalent uranium, some exchange systems, exchange speed and separation column have reported.

The best things about these are a chemical extraction enrichment process called “CHEMEX” in France and ion exchange chromatographic process called “ACEP” in Japan.

The reaction of uranium isotope exchange in CHEMEX follows as:

²³⁵U(Ⅲ)_W+²³⁸U(Ⅳ)₀ ⇔ ²³⁸U(Ⅲ)_W+²³⁵U(Ⅳ)₀

The equilibrium constant of the isotope exchange reaction has the largest value in uranium chemical exchange system as 1.0025 to 1.0030 and the equilibrium reaching time is very fast as a few seconds. Some researchers have taken the general analysis on the chemical enrichment methods and have estimated the economic comparison^[9-10]. They have reported that uranium isotope separation by chemical exchange method has low energy consumption and simple equipments, and doesn’t use harmful UF₆ gas, so it is helpful to environment production and effective use of uranium resource.

Recently, some researchers has discussed the conceptual design the commercial enrichment plant in 1400tSWU/y of separation capacity, suggesting that they pay attention to redox ion exchange method whose industrialization was stopped by Asahi-Kasei company in 1991^[8].

In France, the pilot plant test in 180SWU/(m³ y) of separation capacity by CHEMEX was taken in 1980s and the plan of factory’s building in 1MSWU/y-scale was established, but the concrete data hasn’t been reported.  Some researchers in other countries have reported from inference the possibilities and characteristics of CHEMEX based on bits of data in France.

Some literatures have suggested that the contents of transition elements such as Co, Ni, Cu, Ag and Pb, oxidizing agents for U(Ⅲ), in aqueous solution should decrease less than 10^-4% because U(Ⅲ) in aqueous solution is very instable^[10-12]. The engineering calculation results have estimated that HETP of the plate pulsed extraction column for uranium isotope separation would be 50cm^[13].

Thus, uranium isotope separation by the U(Ⅲ)-U(Ⅳ) chemical exchange method has been estimated to be reasonable and practical in economic aspects and in industrial application, but is uncertain of the specific feasibleness and characteristics

In the paper, possibility and characteristics of uranium isotope separation by the U(Ⅲ)-U(Ⅳ) chemical exchange method have been checked experimentally.

1. Experiment

1.1 Reagents

The uranium was refined, nuclear purity of uranyl nitrate solution in 3-5mol·L^-1 HNO₃ passing through pulsed extraction column, washing column, stripping column in turns, and was converted with uranyl chloride by passing through the column packed  Cl^--type anion exchange resin. It was converted with U(Ⅲ) or U(Ⅳ) by electrolysis and was used to the feed for uranium isotope separation.

The extractants, tri-n-butylphosphate(TBP), and the solvents, kerosene, hydrochloric acid, nitric acid and sulfuric acid was refined to chemical pure from technical pure and was used.

The deionized water, having less than 4MΩ resistance, that produced when distilled water passed through the H⁺ -type and OH^-- type ion exchange column was used.

Transition metal impurities such as Fe, Co, Ni, Cu, Ag and Pb in all the refined materials were analysed by the atomic absorption photometer “PERKIN ELMER 5100PC”.

1.2 Preparation of uranium(Ⅲ)

Uranium(Ⅲ) was prepared by electrolysing UO₂Cl₂ in the cell.

0.5mol··L^-1 H₂SO₄ solution as the anolyte and 0.4mol·L^-1UO₂Cl₂ in 7mol·L^-1 HCl as the catholyte were used. Self-created fluororesin cation exchange membrane as the diaphragm was used. Graphite as the anode and mercury as the cathode were used, and 3.5V of electrolytic reduction voltage and argon atmosphere were provided.

1.3 Column for isotope exchange

The pulse-packed extraction column as main equipment for uranium isotope exchange in U(Ⅲ)-U(Ⅳ) system was used. The column contains a part of extraction exchange, two chambers of phase separation and a pulsator. The diameter, ф and height, H of a part of extraction exchange was 20mm and 1500mm respectively, and pulsed frequency was 1rps. The quartz rings were packed in the column. The porous diaphragm between a chamber of phase separation and a part of extraction exchange was put and argon atmosphere in the column was provided.

1.4 Composition of  U(Ⅲ)-U(Ⅳ) extraction system

A composition of aqueous and organic phase, based on the extraction isotherm of hydrochloric acid and the extraction isotherm uranium(Ⅳ)  by 40% of TBP-kerosene in HCl, provided in the pulsed extraction column in inert atmosphere were selected as follows:

The composition of aqueous phase: 0.19mol·L^-1UCl₃-0.01mol·L^-1UCl₄-7mol·L^-1HCl

The composition of organic phase: 40%TBP-kerosene-0.25 mol·L^-1UCl₄

1.5 Isotope separation process

The organic phase containing U(Ⅳ) as continuous phase was fed into the bottom of pulse-packed extraction column, the aqueous phase containing U(Ⅲ) as dispersed phase was fed into the top of the column and then two phase formed the counter-current flow in the column.

The aqueous phase containing U(Ⅲ) and the organic phase containing U(Ⅳ) after the pulsed extraction column were recycled in the isotope separation process after they were gone through extraction, stripping, redox and adjust of acidity similar to CHEMEX process. All the separation columns were operated in stable condition.

2. Measurement

The concentration of uranium was measured by spectrophotometer, PD-303 and the enrichment degree of ²³⁵U was measured by mass spectrometer.

3. Result and discussion

3.1 Uranium isotope separation by simple cascade

To consider uranium isotope enrichment effect, the concentration of uranium and the enrichment degree of ²³⁵U were measured at  way out of each column when the uranium isotope separation column was operated 8 times continuously by simple cascade.

The measurement results of the first and eighth column were listed in Table 1. It was found that, in the condition of selected aqueous phase, organic phase and pulse-packed column, there was very large effect of uranium isotope separation form Table 1.

Also, the cascade operation went on favorably.

Table 1.

 The measurement of uranium isotope enrichment degree when operated 8 times by simple cascade

3.2 Uranium isotope separation by counter-current recirculation cascade

The effect of uranium isotope separation was checked by practical counter-current recirculation cascade, basing on the confirmation in effct of uranium isotope separation in U(Ⅲ)-U(Ⅳ) extraction system by simple cascade. A cascade of single-stage rise and two-stage down, as an example of counter-current recirculation cascade, was selected.

An example of counter-current recirculation cascade of single-stage rise and two-stage down was shown in Fig. 1.

Figure 1. Counter-current recirculation cascade of single-stage rise and two-stage down

As shown in Fig.1, raw materials are fed into the top and bottom of ith column. The enriched material(organic phase) in ith column is fed into the i+1th column(aqueous and organic phase) and the depleted material is fed into the i-2th column(aqueous and organic phase). The depleted material in i+1th column and the enriched material in i-2th column are fed into the i-1th column.

A cascade contains the parts of enrichment and depletion, each parts of enrichment and depletion contain 4 pulse-packed extraction column and they are the same isotope exchange column in section 2.3.

There are equipments for extraction and stripping of aqueous and organic phase, redox and adjust of acidity, similar to CHEMEX process, between the columns in cascade. All the separation columns were operated in stable condition.

Operating such the cascade for enrichment continuously, the concentration of uranium and the enrichment degree of ²³⁵U were analysed in samples of parts of enrichment and depletion periodically. The enrichment degree of ²³⁵U in enriched fraction of the final output in parts of enrichment and in depleted fraction of the final output in parts of depletion were 0.745mol% and 0.684mol% respectively. From the simple and counter-current recirculation cascade experiments,  HETP of the pulse-packed extraction column was about 25 to 30cm.

4. Conclusion

The possibility of uranium isotope enrichment by U(Ⅲ)-U(Ⅳ) chemical exchange method was checked by using selected the composition of aqueous and organic phase, the pulse-packed extraction column, the simple cascade and the counter-current recirculation cascade. It was found that it had high effect of uranium isotope enrichment and the operation of cascade was going all right. The important things were the refining of used materials, the production of U(Ⅲ) by electrolysis, the composition of stable U(Ⅲ)-U(Ⅳ) exchange system and the stable operation of pulse-packed extraction column in Ar atmosphere.

It was found that HETP of used column is less than the predicted values of previous reporter.

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