GB2216886A

GB2216886A - Stabilisation of an element against oxidation during a solvent extraction process

Info

Publication number: GB2216886A
Application number: GB8907772A
Authority: GB
Inventors: John Hedley Miles
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 1988-04-08
Filing date: 1989-04-06
Publication date: 1989-10-18
Also published as: GB8808284D0; GB8907772D0

Abstract

An element (e.g. Pu) is dissolved in a first oxidation state (e.g. as Pu (III)) in a mixed phase system comprising an aqueous nitric acid phase and an organic liquid phase. The element in that state is only partly extractable or is not extractable from the aqueous phase into the organic phase, and is oxidisable in aqueous nitric acid to a higher oxidation state (e.g. as Pu(IV)) in which it is extractable from the aqueous phase into the organic phase. The element is stabilised in its first oxidation state by a substituted urea dissolved in the aqueous phase. Such stabilisation may be used to separate the element from other elements (such as U) which are readily extractable from the aqueous phase into the organic phase in certain oxidation states.

Description

Stabilisation of an element aqainst oxidation This invention relates to the stabilisation of an element against oxidation in a solution containing nitric acid.

When spent nuclear fuel is reprocessed a number of solvent extraction procedures are used to remove unwanted fission products and separate plutonium from uranium. In one such procedure a nitric acid solution containing uranium and plutonium(IV) is first treated with a reducing aqent to reduce the plutonium(IV) to plutonium(III) and then mixed with an organic solvent so that the uranium and plutonium are partitioned between the nitric acid solution and the organic solvent. The plutonium(III) tends to remain in the nitric acid solution whereas the uranium is extracted into the organic solvent, thus enabling the uranium and plutonium to be separated. However, the efficiency of the separation relies on the plutonium being in its + (III) oxidation state rather than its + (IV) oxidation state as plutonium(IV) is extracted into the organic solvent along with the uranium.In practice, the efficiency of the separation is lowered as a result of the nitric acid oxidising some of the plutonium(III) to plutonium(IV).

According to the present invention there is provided a method of stabilising an element in a first oxidation state in a mixed phase system comprising establishing a mixed phase system comprising an aqueous nitric acid phase and an organic liquid phase, the system containing dissolved therein the element in the first oxidation state, in which state the element is only partly or is not extractable from the aaueous phase into the organic phase and which element is oxidisable in aaueous nitric acid to a higher oxidation state, in which higher state the element is extractable from the aqueous Phase into the organic phase, the system being provided with a sufficient concentration of a substituted urea dissolved in the aqueous phase to stabilise the element in said first oxidation state.

It is believed that, in aqueous nitric acid, nitrite ions (for example as nitrous acid) promote oxidation of the element from the first to the higher oxidation state and that, in the invention, the substituted urea acts by scavenging nitrite ions from the aqueous phase.

The element may comprise plutonium and the first and higher oxidation states may comprise + (III) and + (IV) respectively.

The organic solvent may comprise tributyl phosphate and a hydrocarbon such as kerosene, and the mixed phase system may comprise a solvent extraction system.

A reducing agent, for examPle iron(II) or uranium(IV), may be provided in the mixed phase system to reduce the element to said first oxidation state.

Preferably, the urea is substituted on at least one of the nitrogen atoms. The substituted urea may he a monoalkyl or a diallyl substituted urea and the alkyl substituents may have from 1 to 4 carbon atoms. For example, the urea may be N-methylurea, N-ethylurea or N,N'-dimethylurea. The substituted urea may also be N-hydroxyurea.

In a preferred embodiment of the invention, the above method is applied to separating the element ("the first element") from at least one other element ("the other element(s)") by solvent extraction. Thus where the aqueous phase initially contains at least one other element in an oxidation state such that it is readily extractable from the aqueous phase into the organic phase, the other element(s) is extracted into the organic phase whereas the first element remains in the aqueous phase. As a specific example, plutonium is the first element and uranium is included in the other element(s).

The substituted urea reacts rapidly with nitrite ions, which may be in the form of nitrous acid; the reaction product may, for example, be a nitroso derivative. Also, it is preferable that the substituted urea and its nitroso derivative, if formed, do not have extraction factors near unity in any part of the solvent extraction system since, otherwise, organic material may accumulate in the contactors. Further, when the solutions are derived from nuclear fuel, it is preferable that the substituted urea does not react with any pertechnate which may be present.

The invention will now be described, by way of example only, with reference to the following Examples and to the accompanying Figure which is a flowsheet for a U, Pu separation contactor.

Example I The substituted urea under test was dissolved in nitric acid (1.6M) to give a 0.03M solution. A 5% sodium nitrite solution was prepared and about lml added to 100ml of the 0.03M solution of the substituted urea under test so as to give a solution with a nitrite concentration of about 0.01M. The sample cell (1cam) of a spectrometer was filled with the solution, and the reference cell was filled with nitric acid (1.5M). The characteristic "four-fingered" nitrous acid spectrum between 340nm and 400nm was observed and the time taken for this spectrum to disappear noted for substituted ureas which did not react rapidly with the nitrite.With the following substituted ureas the nitrous acid spectrum disappeared within a minute of mixing and probably much faster: methyl urea; N,N1-dimethylurea; thiourea; methyl thiourea; NN'-dimethyl thiourea; and hydroxyurea.

The rapid disappearance of the nitrous acid spectrum indicated a rapid reaction between the substituted urea and nitrous acid.

Example II An aqueous stock solution of the substituted urea under test (0.pea) was prepared, a 0.5M iron(II) solution was prepared from iron(II) ammonium sulphate and acidified with 0.2% sulphuric acid, and a stock solution of nitric acid (3M) was prepared. In addition a mixture of 30e tributylphosphate and odourless kerosene (TBP/OK) was prepared and equilibrated three times with equal volumes of the 3M nitric acid solution.

An aqueous phase solution was prepared by mixing lOml of the iron(II) solution with 10ml of the stock 0.1M solution of the substituted urea under test and 15ml of water. To this solution 10ml of nitric acid (concentrated) was added in aliquots and the solution made up to 50ml.

This solution (2.5ml) was then placed in a 50ml centrifuge tube together with 50 1 of a concentrated plutonium(IV) nitrate solution (about 300gel~1) and the two solutions mixed. Usually, a sky-hlue coloration was observed (indicating the presence of plutonium(III)), and a 25m1 portion of the equilibrated 30% TBP/OK solution was added.

The resulting two phase system was intensely mixed with the aid of an electric stirrer so that the aaueous phase was lifted into the organic phase as a suspension of droplets.

Every few minutes the stirring was stopped and the colour of the phases observed. The time taken for the blue coloration of plutonium(III) to disappear completely from the aqueous phase, ie the time taken for the aqueous phase to become colourless, was noted as a measure of the extent to which the substituted urea stabilises plutonium(III), and is expressed as the stability time for the substituted urea in the following table (Table 1).

TABLE 1 UREA UNDER TEST STABILITY TIME(MINS) Methyl urea(CH3NHCONH2) 10 NN'-Dimethyl urea(CH3NHCONHCH3) 15 Hydroxyurea(H#NCONHOH) 16 Example III The substituted ureas listed in the above table (1) were also tested for their reactivity with pertechnate ion as this tends to be present in solutions in the reprocessing of spent nuclear fuel.

A solution of the substituted urea under test was prepared so as to be 0.2M in the urea and 0.75M in nitric acid. 20ml of this solution was treated with a solution of NH4Tc04 (0.34M) to give a final solution with a Tc04- concentration of 0.04M. Compounds reacting with Tc04- give a maroon coloured solution with some precipitation and gas evolution. Hydroxyurea reacted very slowly (over 3 hours for any real effect) and was virtually unreactive. Methyl urea and NN'-dimethylurea were completely unreactive, with no reaction even after heating at 600C for 6 hours.

It was also found that the methyl ureas listed in Table 1 could be degraded by boiling in nitric acid, thereby enabling excess reagent to be destroyed following solvent extraction.

Example IV Mixed phase stability tests were carried out in accordance with the flowsheet shown in Fiaure 1. Two sets of mixer-settler batteries were employed each of which comprised twelve tanks arranged in series to form a contactor. A feed solution containing plutonium(IV) and uranium(VI) in an organic solvent was used together with an aqueous acid containing a reductant (uranium(IV)). In the first contactor the plutonium(IV) was reduced to plutonium(III) which was preferentially extracted into the aqueous acid phase, leaving the uranium in the organic solvent. The plutonium-containing acid was then stripped in the second contactor (IBS) with an organic solvent to give a substantially uranium free plutonium solution.

The solvent feed was prepared by dissolving uranyl nitrate hexahydrate (5379) in 3.01 1 30091 of a 30% mixture of tributylphosphate and odourless kerosene (TBP/OK). The aqueous phase which separated was discarded, and 38 ml of concentrated nitric acid added to the organic phase and the aqueous phase produced after this mixing also discarded.

Some of this solution was used in a pre-run to presaturate the organic phase in the mixer-settlers with uranium(VI) but the feed for the run itself were prepared by taking 1.751 of the solution and shaking it with NH4TcO4 (4.8ml, 0.58 Cil-1, 0.021 TBql#1, 34gl~l) and plutonium(IV) nitrate solution (4.0 ml, 356gel 1 in Pu).

Any aqueous phases separating were ignored.

The A3 and A4 feed solutions were prepared from a stock solution of uranium(IV) stabilised with dimethyl urea. The stock solution was prepared by dissolving dimethyl urea (12.3g), uranyl nitrate hexahydrate (88.6 g) and concentrated nitric acid (64ml) in about 500my of water and then diluting by adding water to give a volume of 70Oml. Reduction at a mercury cathode gave a solution containing 125.8all uranium(IV) (some evaporation had taken place). The A3 feed was prepared by adding 33ml of concentrated nitric acid to 1001 of water and adding the solution (after cooling) to 228ml of the uranium(IV) stock solution and making the volume of the solution produced up to 500ml. The solution was stored in the dark.The A4 feed was prepared by dissolving dimethylurea (18.9p) in water (500ml), adding concentrated nitric acid (4my), followed by 64.9ml of the uranium(IV) stock solution. The solution produced was mixed, the volume made up to 1200m1 and stored in the dark.

The solvent strip feed comprised 30% TBP/OK which had previously been washed with sodium hydroxide and sodium carbonate solutions and then with water.

The contactors used were of a size such that each stage or tank contained about 5.5ml of each phase and hence each contactor contained 66ml of each phase. In order to attain steady-state conditions the run was continued for 7 hours. Samples were then collected of the aqueous product and organic product and the samples analysed. In the case of the aqueous product the samples were first diluted and then analysed by liquid scintillation counting of the e emissions from plutonium -241. The organic samples were anlaysed without dilution bya - scintillation counting.

Under steady state conditions the aqueous phases in the IBS contactor were all dark and green with reddish-brown tinges in places, whereas the organic phases were colourless at the aqueous product end but yellow in the 4 stages near the feed point. In the IBX contactor the aqueous phase was emerald green in the two stages near the feed point and dark brown for the 2 stages near the organic product. There was little colour in between and the organic phase was yellow throughout. The species responsible for all these colours are not known with certainty but the lack of colour in the organic phases near the plutonium product end indicate that the uranium(VI) and uranium(IV) have extracted and the plutonium is in its inextractable +III state. Also, it was found that the plutonium levels in the organic product were very low and were at least as low as those achieved in a similar run using hydrazine ( l.3mgl#1) in place of dimethyl urea.

Plutonium levels in the aqueous product remained steady at about 3.6awl~1 and the solutions retained their hlue/violet colour on standing.

Thus, substituted ureas may be used to scavenge nitrite ions and hence reduce the rate of oxidation of an element by nitric acid.

Claims

1. A method of stabilising an element in a first oxidation state in a mixed phase system comprising establishing a mixed phase system comprising an aqueous nitric acid phase and an organic liquid phase, the system containing dissolved therein the element in the first oxidation state, in which state the element is only partly or is not extractable from the aqueous phase into the organic phase and which element is oxidizable in aqueous nitric acid to a higher oxidation state, in which higher state the element is extractable from the aqueous phase into the organic phase, the system being provided with a sufficient concentration of a substituted urea dissolved in the aqueous phase to stabilise the element in said first oxidation state.

2. A method according to claim 1 wherein the element comprises nlutonium and the first and higher oxidation states comprise +(III) and +(IV) respectively.

3. A method according to either of the preceding claims wherein the organic solvent comprises tributyl phosphate and a hydrocarbon.

4. A method according to claim 3 wherein the hydrocarbon is kerosene.

5. A method according to any of the preceding claims wherein a reducing agent is provided in the mixed phase system to reduce the element to said first oxidation state.

6. A method according to claim 5 wherein the reducing agent is iron(II) or uraniu:m(IV).

7. A method according to any of the preceding claims wherein the urea is substituted on at least one of the nitrogen atoms.

8. A method according to claim 7 wherein the urea is a monoalkyl or a dialkyl substituted urea.

9. A method according to claim 8 wherein the or both alkyl groups have from 1 to 4 carbon atoms.

10. A method according to claim 9 wherein the substituted urea is N-methylurea, N-ethylurea or N,N -dimethylurea.

11. A method according to claim 7 wherein the substituted urea is N-hydroxyurea.

12. A method according to any of the preceding claims applied to separating the element as a first element from one or more other elements, the aqueous phase initially containing at least one other element in an oxidation state such that it is readily extractable from the aqueous phase into the organic phase and is so extracted, whereas the first element remains in the aqueous phase.

13. A method according to claim 12 wherein the first element is plutonium and the at least one other elements includes uranium.

14. A method according to claim 1 substantially as described herein with reference to any of the examples.