GB2289975A

GB2289975A - Process For The Regeneration Of A Degraded Organic Solvent Used For The Reprocessing Of Spent Nuclear Fuel

Info

Publication number: GB2289975A
Application number: GB9509965A
Authority: GB
Inventors: Henri Scoazec; Pierre Blanc; Andre Bathellier
Original assignee: Commissariat a lEnergie Atomique CEA; Compagnie Generale des Matieres Nucleaires SA
Current assignee: Orano Cycle SA; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1994-06-03
Filing date: 1995-05-17
Publication date: 1995-12-06
Anticipated expiration: 2015-05-17
Also published as: FR2720853A1; JPH0843585A; GB2289975B; FR2720853B1; JP3567017B2; GB9509965D0

Abstract

The process comprises a first stage (A) of contacting the degraded solvent with a first aqueous solution containing an organic sequestrant constituted by alpha -hydroxycarboxylic acid or alpha -ketocarboxylic acid, e.g. mesoxalic acid, for extracting the radioactive metallic impurities, then a second stage (C) of contacting the solvent with a second solution comprising a basic reagent, e.g. tetramethyl ammonium hvdroxide, in order to extract acid organic molecules resulting from the degradation of the solvent. Thus, two separate effluents are collected (in 7 and 19), whereof only one (7) is radioactive. <IMAGE>

Description

PROCESS FOR THE REGENERATION OF A DEGRADED ORGANIC SOLVENT-USED FOR THE REPROCESSING OF SPENT NUCLEAR FUELS The present invention relates to a process for the regeneration of a degraded organic solvent used for the reprocessing of spent nuclear fuels containing on the one hand metal ions and on the other acid organic molecules.

The industrial reprocessing of spent nuclear fuels is at present carried out by a chemical process based on liquid - liquid extraction. The universally used solvent is a solution of an extractant, tributyl phosphate (TBP), in an inert organic diluent. The degradation of the solvent during its use, mainly by hydrolysis and radiolysis, in the case of TBP leads to the formation of monobutyl and dibutyl phosphoric acids. Due to the presence of these degradation products, the organic solvent cannot be directly recycled into the extraction installation, because these products disturb the uranium and plutonium extraction process. It is therefore necessary to purify the solvent in order to restore its initial physicochemical characteristics before recycling it to the reprocessing installation.

The treatment used at present for carrying out this purification consists of washing the organic solvent with basic, aqueous solutions immiscible with the solvent, followed by a distillation. Two -different basic solutions are successively used, the first being a sodium carbonate solution and the second a sodium hydroxide solution. The effluents produced by these two treatments, which contain impurities deextracted from the solvent and essentially radioactive contaminants and acid degradation products of the solvent, are then combined. As a result of the radiotoxicity of these effluents and the great difficulty in isolating the radioelements responsible for them such as uranium, plutonium and ruthenium, the present control procedure consists of combining them with fission products for vitrification, which constitutes a disadvantage.

Thus, the thus introduced sodium residue quantity to be vitrified considerably increases the radioactive glass volumes to be stored on a long term basis and is prejudicial to the reprocessing costs. Moreover, the volume of the waste is considered too high. Finally, the increase planned for plant reprocessing capacities makes the existing vitrification installations inadequate.

Research has also been carried out in order to attempt to use reagents other than sodium-containing reagents, with a view to eliminating the radioelements responsible for the radiotoxicity of the effluents resulting from the regeneration treatment of the organic solvent.

Thus, FR-A-2 523 156 envisages firstly performing a washing of the solvent by an oxalic acid solution in order to initially eliminate certain metallic radioemitters such as Pu and U. However, this washing method is not satisfactory, because at the necessary oxalic acid concentration (0.8 M) in order to ensure the decontamination of the solvent, this acid is extracted in significant proportions in the organic solvent. It is therefore necessary to eliminate this acid in the following stages of the regeneration treatment of the solvent using base quantities which are disproportionate with the objective of reducing the volume of radioactive waste. In addition, no development of this process has hitherto taken place.

Rovnyi et al in Soviet Radiochemistry, vol.34, no.2, 1992, pp.221-226 have referred to the possibility of using organic sequestrants such as citric and lactic acids for eliminating the plutonium from organic solvents resulting from the reprocessing of spent nuclear fuels.

However, as in the case of oxalic acid, the use of such acids also causes problems for the subsequent stages of the regeneration treatment of the solvent, because they are extracted in a significant proportion therein.

The present invention relates to a process for regenerating a degraded organic solvent used for the reprocessing of spent nuclear fuels, which obviates the disadvantages of the processes described hereinbefore.

According to the invention, the process for the regeneration of a degraded organic solvent used for the reprocessing of spent nuclear fuels, containing on the one hand metal ions and on the other acid organic molecules, comprises: - a first stage of contacting the solvent to be regenerated with a first aqueous solution for extracting in said first solution the metal ions present in the solvent, said first aqueous solution comprising an organic sequestrant chosen from among a-hydroxycarboxylic and aketocarboxylic acids, which are soluble in water and having a distribution coefficient D between the organic solvent to be regenerated and the first aqueous solution, below 0.1, and said first aqueous solution not extracting the acid organic molecules contained in the solvent to be regenerated and - the separation of the organic solvent from the said first aqueous solution which has extracted the metal ions.

In general, this process also comprises a second stage of contacting the solvent separated from the first aqueous solution with a second aqueous solution comprising a basic reagent, in order to eliminate from the solvent the acid organic molecules, followed by a separation of the organic solvent from said second solution used for extracting the acid organic molecules.

The process of the invention consequently involves two successive stages having very separate functions, namely a first stage of eliminating the metal ions present in the solvent and a second stage of eliminating the acid organic molecules resulting from the degradation of the solvent.

This manner of proceeding makes it possible to collect two effluent types having different characteristics, namely a first effluent resulting from the first contacting and which contains the metal ions, i.e. the radioactivity, and a second effluent coming from the second stage and containing virtually no radioactivity.

Thus, this process makes it possible to separately collect the disturbing species contained in a degraded organic solvent. Thus, after use in extraction cycles, these disturbing species are of three types: - radioemitters, essentially metallic species in the form of complexes, - degradation products of the extractant, whereof the most disturbing are acids, e.g. phosphoric, monobutyl phosphoric and dibutyl phosphoric acids and - degradation products of the diluent, most of which are chemically neutral With the process of the invention a first aqueous effluent is obtained which contains the radioemitters, in complex form with the organic sequestrant of the type described hereinbefore and which is a completely destructible chemical reagent, which makes it possible to perform a recycling of the dangerous radioemitters into the reprocessing cycle.

Thus, the organic sequestrants used in the first aqueous solution can be easily destroyed, e.g. by oxidation using nitric acid, being transformed into a gas and into water vapour. Therefore the effluent produced after the first contacting can, after an appropriate oxidation treatment, be recycled in the reprocessing installation, which avoids the production of further waste to be stored.

The degradation products of the extractant constituted by acid organic molecules are recovered in the second aqueous effluent produced after the second contacting stage. Thus, as said effluent is less active, it is no longer necessary to treat it by vitrification.

The degradation products of the diluent, most of which are chemically neutral, can then be eliminated by conventional processes, e.g. by distillation, as is normally the case.

The choice of the sequestrant used in the first contacting stage of the process according to the invention is performed in such a way as to ensure the elimination of the metal ions present in the organic solvent, without extracting therefrom the acid organic molecules resulting from its degradation and without contaminating said solvent by said sequestrant.

Said sequestrant must therefore meet the following criteria: - it must be soluble in the first aqueous solution and insoluble in the organic solvent to be regenerated, - it must be destructible into easily eliminatable byproducts, - it must strongly sequester the essentially metallic, radioactive species to be eliminated from the solvent and - it must be acid, so as not to induce the deextraction of acid degradation products from the solvent.

In order to strongly sequester the metallic radioactive species, it is necessary for the sequestrant to have a hydrated ketone or alcohol function in the a position of the carboxylic acid function in order to be able to form, e.g. with the uranyl ion Us2+, complexes of the following type:

with R representing a hydrogen atom or a hydrocarbon group.

This sequestrant can obviously have several carboxylic acid functions, e.g. n COOH functions, but the latter will only be effective for sequestring metal ions if the sequestrant also has n hydrated ketone and/or alcohol functions, each of the ketone or alcohol functions being in the a position relative to a carboxylic acid function.

This complexing agent has a hydrocarbon chain, which is preferably linear when the diluent of the organic solvent is a branched hydrocarbon. However, said chain is preferably branched when the diluent of the organic solvent is a linear hydrocarbon. Preferably it is a saturated hydrocarbon chain, which in preferred manner is a short chain, the sequestrant e.g. containing 2 to 6 carbon atoms.

In order to choose the sequestrant satisfying the other criteria, i.e. insoluble in the degraded organic solvent and sequestring the metal ions without retaining the acid degradation products of the solvent, it is possible to test the properties of various sequestrants which can be used, e.g. by determining the distribution coefficients D corresponding to the ratio between the concentration of a species in the organic solvent and the concentration of said same species in the aqueous phase.

In order to fulfil the non-solubility condition of the sequestrant in the organic solvent, it is necessary for the distribution coefficient D of said sequestrant between the organic solvent and the first aqueous solution to be as low as possible and in particular below 0.1.

In order to test the sequestring properties, it is possible to determine the distribution coefficient of the radioactive species between the organic solvent and the first aqueous solution containing the tested sequestrant.

This coefficient must be as low as possible in order to ensure a good extraction.

In order to test the properties of the sequestrant with respect to the acid organic molecules resulting from the degradation of the solvent, it is possible to determine in the same way the distribution coefficients of the acid degradation products of the solvent or acids generated by the process. These distribution coefficients must be as high as possible.

Examples of sequestrants meeting these requirements, reference can be made to mesoxalic acid, namely 2,2dihydroxypropanedioic acid, also known as hydrated ketomalonic acid, 2,3-dihydroxybutanedioic acid or tartaric acid, as well as a-hydroxyethanoic or glycolic acid.

In the case of hydrated ketomalonic acid, the two acid and alcohol functions can participate in the formation of a complex with the UQ2+ ions in the following way:

if there are sufficient Uo22+.

The sequestrant concentration of the first aqueous solution is chosen as a function of the sequestrant used and the decontamination factor to be achieved. Generally it is preferable for said concentration to be low, e.g. in the range 0.01 to 0.2 mole/t.

In the second contacting stage of the process according to the invention, use is made of a highly basic reagent which is inextractable in the organic solvent to be regenerated.

In view of the fact that this basic reagent is used after the elimination of the radio contaminants, it is possible to use as the basic reagent, sodium-containing reagents used at present such as soda. Thus, the control of the resulting sodium-containing effluent is facilitated by its inactive character, so that its vitrification can be avoided.

However, the preferred basic reagent is a decomposable organic base, in order to reduce the volume of basic effluents. The strong organic bases which can be used are hydroxides and quaternary ammonium salts, which are inextractable in the organic solvent to be regenerated, e.g.

tetraalkyl ammonium hydroxides of formula R4NOH, in which R is a hydrocarbon radical, e.g. an alkyl group. The quaternary ammonium salts can e.g. be carbonates or oxalates. As an example of a usable organic base reference can be made to tetramethyl ammonium hydroxide.

The basic reagent concentration of the second solution is also chosen as a function of the reagent used and the level of the acid organic species to be eliminated.

Generally the said concentration is 0.1 to 1 mole/4.

According to a preferred embodiment of the process according to the invention, there is a complementary stage of washing with water the organic solvent separated from the first aqueous solution prior to contacting it with the second aqueous solution.

This washing with water makes it possible to eliminate from the solvent traces of sequestrant possibly extracted during the first contacting stage. This washing water can be recycled for performing the first contacting stage, being added to the first aqueous solution.

The process can be performed continuously or discontinuously. In order to perform each of the stages of this process, it is possible to use conventional equipment such as mixer-settlers, exchange columns, pulsed columns and to work at ambient temperature or higher temperatures, e.g. 20 to 700 C.

For the two contacting stages and for the washing, the quantities of organic solvent and aqueous solutions contacted or the flow rates are chosen as a function of the distribution coefficients of the species to be extracted, the solubility of the sequestrant in the organic solvent and the decontamination factor to be achieved.

In general, the organic solvent quantities to be regenerated and aqueous solution or water contacted are such that the volume ratio of the organic solvent to the aqueous solution is 5 to 100.

The organic solvents which can be regenerated by the process according to the invention can be constituted by solutions of an acid or neutral organic extractant in an inert diluent. The extractants can e.g. be ethers, organophosphorus compounds or organonitrogen compounds. The diluent can be a hydrocarbon, a neutral or acid organic compound. The process of the invention is more particularly applicable to the treatment of an organic solvent constituted by tributyl phosphate in an inert organic diluent such as tetrapropylene hydrogen (trade name TPH).

Other features and advantages of the invention can be gathered from reading the following description given in non-limitative manner and with reference to the attached drawing, which diagrammatically illustrates a way of performing the process according to the invention.

The drawing diagrammatically illustrates the three stages A, B and C of the process according to the invention, the first stage A representing the first contacting of the solvent with the first aqueous solution, stage B being an intermediate washing stage and stage C corresponding to the second contacting of the solvent with the second aqueous solution.

In the drawing the continuous line illustrates the circulation diagram of the organic solvent to be regenerated, whereas the dotted lines relate to the circulation diagrams of the aqueous solutions and the washing water.

Thus, in the first stage A, countercurrent contacting takes place of the organic solvent introduced by the pipe 1 with the first aqueous solution introduced by the pipe 3.

On leaving said first contacting stage, recovery takes place in the pipe 5 of the organic solvent which has been freed from the metal ions, and in pipe 7 of the first aqueous solution containing the extracted metal ions.

In the washing stage B, the organic solvent is freed from the sequestrant optionally extracted by means of the washing water introduced by the pipe 9, thus, on leaving said stage recovery takes place in the pipe 11 of the organic solvent and in the pipe 13 of the sequestrantcontaining washing water. This washing water can be re cycled to stage A, being added to the first solution in the pipe 3.

Following this washing stage, the organic solvent is introduced into C by the pipe 11 for countercurrent contacting with the second aqueous solution introduced by the pipe 15. Thus, following said stage, the regenerated organic solvent is recovered by the pipe 17 and a second aqueous solution having extracted the degradation products from the solvent by the pipe 19.

The first aqueous solution passing out of stage A by pipe 7 can then be treated by oxidation in D in order to decompose the sequestrant and thus obtain an aqueous solution containing metallic impurities, which can be recycled into the spent nuclear fuel reprocessing installation.

The oxidation treatment can consist of a chemical oxidation using nitric acid, which brings about the decomposition of the sequestrant into water and carbon dioxide gas.

The organic solvent passing out through the pipe 17 can then undergo a distillation treatment in order to eliminate the degradation product from the diluent, before being recycled to the reprocessing installation.

In order to perform this process, it is firstly appropriate to investigate which sequestrant can be used in the first contacting stage A.

To this end, determination takes place of the distribution coefficients of the sequestrant between the organic solvent to be regenerated and an aqueous solution containing 0.1 mole/ of said sequestrant. The organic solvent to be regenerated is constituted by 30 vol.% tributyl phosphate in TPH and these distribution coefficients are determined by contacting one volume of solvent with one volume of aqueous solution, under stirring, at 230C and for 5 minutes. After settling, determination takes place of the sequestrant contents of the solvent and the solution and the distribution coefficient Corg D = Caq with Corg representing the concentration in the solvent and Cq representing the concentration in the aqueous solution.

The distribution coefficients obtained under these conditions with different sequestrants are given in the attached Table 1. The latter also gives the results obtained with an aqueous solution also containing 0.1 mole/ of nitric acid.

On the basis of this Table, it is immediately clear that oxalic, citric and lactic acids are not suitable, because they are excessively extracted in the organic solvent to be regenerated. However, mesoxalic, tartaric and glycolic acids can be used in the process according to the invention.

Determination then takes place of their properties for the extraction of uranium(VI), plutonium(IV), as well as 1 o 6 Ru, 127Cs and 15 4 Eu determining the distribution coefficient of said radioactive species between the organic solvent to be regenerated and an aqueous solution containing the tested sequestrant at a concentration of 0.1 mole/ and at 230 C, operating in the same way as hereinbefore.

The results obtained when using as the sequestrant mesoxalic or tartaric acid are given in Table 1.

It is therefore clear that the distribution coefficients of the radioactive species are very low and are suitable for performing the process according to the invention.

Determination then takes place of the extraction power of said sequestrants with respect to the acid species present in the degraded organic solvent, which result either from the degradation of the solvent, or acids generated by the process. Thus, determination takes place of the distribution coefficient of hydrazoic acid HN3 and dibutyl phosphoric acid between the organic solvent to be regenerated and an aqueous solution containing 0.1 mole/ of mesoxalic acid, at 230C and operating as hereinbefore.

The results are given in Table 1.

It can be seen that the distribution coefficients are very high for these species. Therefore they will not be reextracted in the first aqueous solution containing the mesoxalic acid.

For the second aqueous solution, the basic reagent used is chosen as a function of its capacity to extract acid species present in the degraded solvent. This can be evaluated as hereinbefore by determining the distribution coefficients of said species between 1 volume of aqueous solution comprising 0.1 mole/l of basic reagent and 1 volume of organic solvent at 230 C.

The following results are obtained with tetramethyl ammonium hydroxide: - D (dibutyl phosphoric acid) : < 1.5g10- - D (butyric acid) : < 1.5-10- 2 - D (hydrazoic acid HN3) : 5 2.5010-2 - D (nitrobutane) : < 0.1.

It can also be seen that the neutral character molecules, such as the nitrate compounds of type RNO3, are not extracted by the second aqueous solution.

A description is given hereinafter of the results obtained on performing the process of the invention for the treatment of 30 vol.t tributyl phosphate in tetrapropylene hydrogen (TPH) from a reprocessing plant, which has been aged and doped with radiocontaminants using as the first aqueous solution a 0.1 mole/ solution of mesoxalic acid in water, as the pure water washing solution, and as the second aqueous solution a 0.1 mole/ tetramethyl ammonium hydroxide solution. Contacting takes place in mixersettlers having three layers in stage A, two layers in the washing stage B and three layers in the second contacting stage C.

The mixer-settlers are kept at a temperature of 500C and the organic solvent is countercurrent contacted with the aqueous solutions or washing water using solvent/ aqueous solution flow rate ratios of 10.

The composition of the organic solvent to be regenerated is as follows: - tributyl phosphate : 30 vol.% in TPH, - dibutyl phosphoric acid : 603 mg/E, - butanoic acid : 0.0035 mole/e, - HNO3 : 5s10- 4 mole/4, - uranium : 9.2 mg/Q, - plutonium : 0.75 mg/l.

Thus, after the first stage there is a plutonium decontamination factor equal to or higher than 150 and a uranium decontamination factor of 2500.

In stage C there is a dibutyl phosphoric acid decontamination factor of 17 and a butanoic acid decontamination factor equal to or above 20. It can also be seen that the sequestring reagent used is absent from the organic phase following the water washing phase B.

Table 2 gives the percentages of starting products resulting from the solvent and which can be collected in the first aqueous solution leaving stage A, the washing solution leaving stage B, the second aqueous solution leaving stage C and the organic solvent leaving stage C.

The results of Table 2 illustrate the effectiveness of the process according to the invention.

TABLE 1 Mesoxalic Tartaric Glycolic Oxalic Citric Lactic acid acid acid acid acid acid D(sequestrant) 0.038 0.038 0.075 0.3 0.095 0.37 D(sequestrant) (+0. 1M HNO3) 0.034 0.035 0.058 DU(IV) < 1.5.10-2 < 1.5.10-2 D Pu(IV) 4.9.10-4 10-3 D106Ru 8.8.10-2 8.8.10-2 D137Cs 6.5.10-2 1.5.10-2 D154Eu 0.1 4.10-2 D HN3 > 20 DHDBP > 20 TABLE 2 HDBP Pu U HNO3 Butanoic acid First aqueous solution leaving A 0.4% #100% #100% 100% < 5% Washing solution leaving B 0.8% 1% < 5% 0 0 Second aqueous solution leaving C 92% 04% 0 0 > 90% Organic solvent 6% 0.6% 0.4% 0 < 5%

Claims

1. Process for the regeneration of a degraded organic solvent used for the reprocessing of spent nuclear fuels, containing on the one hand metal ions and on the other acid organic molecules, comprises: - a first stage of contacting the solvent to be regenerated with a first aqueous solution for extracting in said first solution the metal ions present in the solvent, said first aqueous solution comprising an organic sequestrant chosen from among a-hydroxycarboxylic and aketocarboxylic acids, which are soluble in water and having a distribution coefficient D between the organic solvent to be regenerated and the first aqueous solution, below 0.1, and said first aqueous solution not extracting the acid organic molecules contained in the solvent to be regenerated and - the separation of the organic solvent from the said first aqueous solution which has extracted the metal ions.

2. Process according to claim 1, characterized in that it also comprises a second stage of contacting the solvent separated from the first aqueous solution with a second aqueous solution comprising a basic reagent, in order to eliminate from the solvent the acid organic molecules, followed by a separation of the organic solvent from said second solution having extracted the acid organic molecules.

3. Process according to claim 1, characterized in that the sequestrant has n acid functions and n alcohol and/or ketone functions, each of the alcohol or ketone functions being in the a position relative to an acid function.

4. Process according to claim 1, characterized in that the organic sequestrant is mesoxalic acid.

5. Process according to claim 1, characterized in that the sequestrant is tartaric or glycolic acid.

6. Process according to any one of the claims 1 and 3 to 5, characterized in that the sequestrant concentration of the first aqueous solution is 0.01 to 0.2 mole/.

7. Process according to claim 2, characterized in that the basic reagent is an organic base chosen from among quaternary ammonium hydroxides and salts, which are inextractable in the organic solvent.

8. Process according to claim 7, characterized in that the organic base is tetramethyl ammonium hydroxide.

9. Process according to any one of the claims 1 to 8, characterized in that it also comprises a water washing stage with respect to the organic solvent separated from the first aqueous solution before performing the second contacting stage of the solvent with the second aqueous solution.

10. Process according to any one of the claims 1, 2 and 9, characterized in that the organic solvent and aqueous solution or washing water quantities contacted are such that the volume ratio of the organic solvent to the aqueous solution or washing water is 5 to 100.

11. Process according to any one of the claims 1 to 10, characterized in that the organic solvent incorporates tributyl phosphate.