GB2227013A

GB2227013A - Separating uranium and plutonium by solvent extraction

Info

Publication number: GB2227013A
Application number: GB8928528A
Authority: GB
Inventors: Hossein Ghafourian; Abdel Ali Sameh; H-Joachim Ache
Original assignee: Kernforschungszentrum Karlsruhe GmbH
Current assignee: Forschungszentrum Karlsruhe GmbH
Priority date: 1988-12-24
Filing date: 1989-12-18
Publication date: 1990-07-18
Anticipated expiration: 2009-12-18
Also published as: FR2640888A1; GB8928528D0; GB2227013B; DE3843887C1; FR2640888B1

Description

kil' -7 0 1 -3 METHOD OF SELECTIVELY SEPARATING URANIUM AND PLUTONIUM The

invention relates to a method of selectively separaing uranium and plutonium, based on an aqueous. nitric acid solution which contains Pu(IV) and U02 2+ ions, wherein the aqueous solution is brought into contact with an organic extracting agent. and plutonium (IV) and uranium are extracted into the organic phase, whereupon plutonium is stripped from the organic phase. in that. by the addition of a reducing agent, Pu(IV) is reduced to Pu(III), while uranium remains in the organic phase, and the organic extracting agent and the reducing agent are recycled.

A method which corresponds to the above method includes partial steps of the known "PUREV' process.

In the PUREX process it is usual for a 30% by vol. tributyl phosphate solution in kerosene to be used as the extracting agent.

Iron sulphamate or hydroxyl ammonium, nitrate was previously used as the reducing agent. However, the C> disadvantace with these reducing agents is that they C> cannot be recycled and, in consequence, they have to be removed as radioactive waste.

For this reason, uranium (IV) is mainly used nowadays to reduce plutonium (IV) to plutonium (III).

Plutonium (III) is not stable under PUREX conditions, and it easily oxidises to PU(N) or Pu(VI).

Since only Pu(III) is insoluble in the organic extracting agent, but Pu(IC and Pu(VI) are not, a stabilising agent for Pu(III) is generally added when plutonium is stripped from the organic phase into the aqueous phase because -otherwise plutonium can only be inadequately separated from uranium. which remains in the organic phase.

Hydrazine is used as the stabilising agent. However, because of its tendency to disintegrate explosively in a relatively high concentration, hydrazine constitutes a risk to safety for the reprocessing process and, more especially, for storage.

Furthermore, hydrazine can be converted to hydrazoic acid, in accordance with the equation:

N2H4 + HN02!5HN3 + 2H20 utilising nitrous acid. Hydrazoic acid and many of its heavy metal salts disintegrate explosively with a considerable development of heat when heated.

In German Offenlegungsschrift No. 3 345 1199, there is disclosed a method of reductively stripping plutonium from an organic reprocessing solution into an aqueous,nitric acid solution, utilising an electrolytic current, wherein plutonium (IV) is electrochemically reduced to plutonium (III) without the aqueous solution containing a stabilising agent for Pu(III). such as hydrazine. for example. However, in such a case, the phase separating device has to be an electrolytic cell, 1 i i 1,' and this necessitates a large amount of apparatus being used to accomplish the method. Furthermore, uranium and plutonium must first be converted into the organic phase.

In European published Patent Application No. 0 224 277,there is also disclosed -a method of selectively separating plutonium from uranium, which is based on an aqueous solution containing Pu4+ and U02 2+ ions, and the two fission substances, as anionic sulphato complexes, are separated from each other by means of cationic surfactants. The various complexing properties of Pu(III) and Pu(IV) are utilised hereby, Iron (II) sulphate is proposed f or reducing Pu (IV) to Pu(III).

One disadvantage of the method is seen to reside in the fact that cationic surfactants present problems when they are subsequently removed, because they make it more diffIcult for waste solutions to be converted into solid cement products.

Based on a Pu(IV)/U02 2+ solution, an object of the invention is to provide a simplified method of selectively separating uranium and plutonium, wherein the addition of hydrazine as a stabilising agent for Pu(III) is unnecessary. In addition, the method should be capable of being accomplished with conventional phase separating devices and should not require the use of an electrolytic current. Substances should not be used whith hinder cementing when they are subsequently removed.

According to the present invention there is provided a method of selectively separating uranium and plutonium, based on an aqueous, nitric acid solution which contains Pu(IV) and U02 2+ ions, wherein' the aqueous solution is brought into contact with an organic extracting agent, and plutonium (IV) and uranium are extracted into the organic phase, whereupon plutonium is stripped from the organic phase. in that, by the addition of a reducing agent, Pu(IV) is reduced to Pu(III). while uranium remains in the organic phase. and the organic extracting agent and the reducing agent are recycled, in which diethylthiourea is added. as the reducing agent, to the organic extracting agent. and the extracting and stripping operations are effected'in one method step.

According to the invention, the extracting agent, e.g. a 30% by vol. solution of tributyl phosphate in kerosene, is mixed with solid N.Wdiethylthiourea (DETH) and DETH is dissolved by stirring.

The solubility of DETH in kerosene/TBP [tributyl phosphate] is more than 600 g/1.

However, DETH only needs to be used concentration of about 1 to 10 g DETH/1.

in a The DETH concentration is preferably so selected that the DETH: Pu ratio is in the range of 1:1 to 2:1.

The organic phase thus produced is brought into contact with the aqueous solution, which contains uranium and tetravalent plutonium.

I- The tetravalent plutonium is thereby reduced to trivalent plutonium and kept in solution in this valency or, if Pu(IV) has already been converted into the organic phase, it is brought into solution as Pu(III) in the aqueous phase, while uranium passes into the organic phase as hexavalent uranyl nitrate.

The phases are separated from each other as usual. Because of the low solubility of DETH in water, DETH follows the organic current.

The aqueous phase, which contains Pu(III) and a small proportion (approx. 1%) DETH, may be freed of the traces of the reducing agent DETH through extraction, utilising a suitable solvent, e.g. chloroform. DETH and chloroform can be separated from each other in a cleaning stage, so that the two substances can be recycled.

Uranium can be removed from the organic phase, which contains uranium and the extracting agent together with the main proportion of DETH, by washing with 0.05 to 0.1 molar nitric acid, whereupon uranium is in pure form in nitric acid solution, and the organic phase can be reused.

17hen washed with nitric acid and/or when in contact with fresh starting solution, the reduced. monomeric reducing agent is also formed again from the oxidised dimeric form of DETH.

The method of the invention permits more than 99% of the original plutonium to be successfully separated from uranium.

A substantial advantage of DETH is that Pu(M) is stabilised. With a DETH: Pu ratio of 1: 1, plutonium is still in its trivalent form after 6 days; with a DETH: Pu ratio of 1:2, Pu(III) continues to be "stable for an additional day.

According to the invention, therefore. DETH serves not only as a reducing agent, but also as a very effective stabilising agent for Pu(III). which can also still be very easily converted again into its original.

C:- reduced form.

In consequence, no time limits have to be observed when effecting the extraction step of the invention.

The oxidised, dimeric DEETH is reduced rapidly and completely into its monomeric form by nitric acid or nitrous gases, which are always produced in a reprocessing plant by the influence of the radiation from nitric acid.

The invention is hereinafter with reference to one accomplishing the method.

Example explained more fully example for A 3-molar nitric acid was used as the starting solution. in which 50 g uranium/1 and 2 mg plutonium/1 were dissolved in a first experiment and 50 g uranium/1 and 1 g plutonium/1 were dissolved in an additional experiment.

Solid diethylthiourea was dissolved in 30% by vol. TBP/kerosene mixture.

In both experiments, the - molar DETH concentration corresponded to the molar plutonium concentration in the starting solution.

ml of the aqueous starting solution and 5 ml of the organic DETH/TBP solution were intensively mixed for 5 minutes. The two phases were subsequently separated.

The aqueous phase contained more than 99% of the plutonium used in both experiments. Furthermore, it contained substantially 1% of the originally introduced DETH, which could easily be re moved by washing the aqueous phase with chloroform.

It was advisable to separate DETH only shortly prior to the reprocessing of the plutonium. because it stabilises trivalent plutonium.

The organic phase, which contains uran-Lum, dissolved in a DETH/TBP/kerosene mixture. was washed withl substantially 0.1 molar nitric acid for the removal of uranium.

The organic phase could subsequently be recycled without additional treatment steps.

a C1.

1? J

Claims

A method of selectively separating uranium and plutonium, based on an aqueous, nitric acid solution which contains Pu(IV) and U02 2+ ' ions, wherein the aqueous solution is brought into contact with an organic extracting agent. and plutonium(I'V) and uranium are extracted into the organic phase. whereupon plutonium is stripped from the organic phase. in that. by the addition of a reducing agent, Pu(N) is reduced to Pu(III), while uranium remains in the organic phase. and the organic extracting agent and the reducing agent are recycled, in which diethylthiourea is added, as the reducing agent. to the organic extracting agent. and the extracting and stripping operations are effected in one method step.
2. A method as claimed in claim 1, in which the ratio of the molar concentration of diethylthiourea in the extracting agent to the molar concentration of plutonium in the starting solution is in the range between 1:1 and 2:1.
3. A method as claimed in claim 1 or 2, in which the volume of the extractinc, acent corresponds to the t> volume of the starting solution.
4. A method as claimed in claim 1. 2 or 3, in which uranium is extracted from the organic phase by means of 0.05 to 0.1 molar nitric acid, and the organic phase, which is freed of uranium, is recycled.

i:

1 a f
5. A method as claimed in any one of claims 1 to 41 in which traces of diethylthiourea are extracted from the aqueous, plutonium-containing phase by means of a suitable soivent. preferably chloroform. and the diethylthiourea and the solvent are recycled after they have been separated.
6. A method of selectively separating uranium and plutonium, as claimed in any preceding claim, substantially as hereinbefore described and exemplified.
7.

A method of stabilising trivalent plutonium in aqueous solution, comprising adding diethylthiourea thereto.
8. A method as claimed in claim 7, substantially as hereinbefore described and exemplified.

Published 1990 atThe Patent Ofnee, State House.88171 Holbom London WC1R 4TP.Purther copies mavbeobtatr=Th@Ptt=oe.