GB1574874A - Method for increasing the lifetime of an extraction medium used for reprocessing spent nuclear fuel and/or breeder materials - Google Patents

Description

(54) A METHOD FOR INCREASING THE LIFETIME OF AN EXTRACTION MEDIUM USED FOR REPROCESSING SPENT NUCLEAR FUEL AND/OR BREEDER MATERIALS (71) We, KERNFORSCHUNGSZEN TRUM KARLSRUHE GESELLSCHAFT MIT BESCHRAENKTER HAFTUNG, a formerly Gesellschaft Fuer Kernforschung M.B.H., of 75 Karlsruhe 1, Postfach 3640, 5 Weberstrasse, Germany (Fed. Rep.); a German Body Corporate, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a method for increasing the lifetime of an extraction medium containing an organophosphorus acid ester and a hydrocarbon and being used for reprocessing spent nuclear fuel and/or breeder materials, by removing from the extraction medium impurities and interfering compounds of such impurities with radionuclides resulting from chemical and/or radiolytic decomposition, comprising bringing the extraction medium, after use, into intimate contact with an aqueous hydrazine hydrate solution having a concentration of from 0.1 to 1.0 molar at a temperature of from 20 to 750C, and then separating the aqueous hydrazine hydrate solution from the extraction medium. Such a method is claimed in Claim I of our prior Application No. 42931/75 (Serial No.

1,507,170).

During the washing of the extraction medium, there are formed at the boundary surface of the organic phase with the aqueous phase in certain circumstances, precipitates containing fission material.

Such precipitates are voluminous and may lead to stoppages at the constricted points of the apparatus utilized for carrying out such method which, besides being connected with considerable disturbances in the course of the method, also introduces the risk of criticality. Moreover, the efficiency of the washing of the extraction medium is unfavourably affected.

It is an object of the invention to provide a method by which the forming of such precipitates is avoided and the washing of the extraction medium can be carried out without any functional disturbances.

According to the present invention there is provided a method as claimed in Claim 1 of parent Application No. 42931/75 (Serial No. 1,507,170), wherein the aqueous hydrazine hydrate solution is charged with carbon dioxide.

This charging with CO2 depends upon the content of metal ions having a tendency to form a precipitate in the organic phase and the flow ratio of the organic phase relative to the aqueous phase. Preferred CO2 additions are in the region of from 0.1 to 1.5 mol CO2 per litre of aqueous wash solution, especially 0.5 mol/l. The aqueous phase after the washing of the extraction medium should still have a pH value of about 8. This means, that the aqueous phase had to contain an excess of hydrazine of from 0. 1 to 0.2 mol beyond that quantity required for neutralising the acid in the organic phase.

If the extraction medium contains much acid and little uranium, then the wash solution, with constant flow ratio, has a relatively high content of hydrazine and is charged with a relatively small quantity of CO2. A separate adjustment of the pH value of the aqueous wash solution and its efficiency in avoiding precipitations is made possible by the method in accordance with the invention which is not attainable with the Na2CO3 wash of the prior art.

By charging the hydrazine hydrate solution with CO2, the metal ions which have a tendency to form precipitates are retained in solution.

The present invention will be further illustrated, with reference to the following examples, it being understood that the invention is not limited thereto. In such examples, the washing operation is carried out on a continuous basis as compared with the batch process disclosed in our abovenumbered parent application. Accordingly, having regard to the flow ratio of organic solution (1 litre per hour) as compared to the flow ratio of the aqueous phase (0.05 litres per hour), it is to be noted that in order to obtain comparable values as specified in our parent application, it is necessary to divide the concentration of the hydrazine hydrate by 20 since, in a single stage mixing settler 20 times as much organic solution is passed through continuously and without reflux as compared to the quantity of the hydrazine hydrate solution.

Example 1 This example shows the efficiency of a CO2 introduction into the aqueous phase.

Washing of the organic extraction medium (30 /" per volume tributylphosphate, 70% per volume alkane) with 2.67 molar N2H4H2O solution in a single stage mixing settler.

The organic extraction medium contained: U: 73 mg/l HNO2: 0.052 mol/l p-y-activity 11700 Imp/min (impulses per minute) (mainly Ru-106) Dibutylphosphate: 280 mg/l The external flow ratio between organic and aqueous phase was 1 l/h:0.05 I/h.

During the experiment a precipitate formation occurred. The precipitate was deposited at the boundary between the organic and aqueous phases and besides uranium also contained a large proportion of A-r-activity (102,000 Imp./min).

The experiment was interrupted after a 16 hour operating period. By introducing CO2 the precipitate was dissolved again.

Example 2 This example shows the efficiency of the method in accordance with the invention in a single stage mixing settler.

A wash of the organic extraction medium (300/, by volume tributylphosphate, 70% by volume alkane) with 5.2 molar N2H4H2O solution, which was charged with 1.25 M CO2/l. The organic extraction medium contained: U: 75 mg/l HNO3: 0.044 molli ,B-y-activity: 6760 Imp/min (main activity Ru-106) Dibutylphosphate: 230 mg/I The flow ratio corresponded to Example 1.

During this experiment no precipitate formation occurred.

The discharge sample had the following composition: Org.phase: U: < 50 mg/l (Detection limit) HNO3: 0.01 mol/l A-y-activity: 5700 Imp/min Dibutylphosphate: 41 mg/l Aqueous phase: U: 1.4 g/l N2HsNO3: 0.68 mol/l P-y-activity: 34730 Imp/min (mainly Ru106: Do~2) Dibutylphosphate: 3800 mg/l (substantially quantitative).

WHAT WE CLAIM IS: 1. A method as claimed in Claim 1 of parent Application No. 42931/75 (Serial No.

1,507,170), wherein the aqueous hydrazine hydrate solution is charged with carbon dioxide.

2. A method as claimed in Claim I, in which CO2 is added in an amount of from 0.1 to 1.5 mol CO2 per litre of aqueous wash solution.

3. A method as claimed in Claim 1 or 2, substantially as hereinbefore described and exemplified.

4. An extraction medium containing an organophosphorus acid ester and a hydrocarbon and being used for reprocessing spent nuclear fuel and/or breeder materials, whenever treated by a method as claimed in any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. the batch process disclosed in our abovenumbered parent application. Accordingly, having regard to the flow ratio of organic solution (1 litre per hour) as compared to the flow ratio of the aqueous phase (0.05 litres per hour), it is to be noted that in order to obtain comparable values as specified in our parent application, it is necessary to divide the concentration of the hydrazine hydrate by 20 since, in a single stage mixing settler 20 times as much organic solution is passed through continuously and without reflux as compared to the quantity of the hydrazine hydrate solution. Example 1 This example shows the efficiency of a CO2 introduction into the aqueous phase. Washing of the organic extraction medium (30 /" per volume tributylphosphate, 70% per volume alkane) with 2.67 molar N2H4H2O solution in a single stage mixing settler. The organic extraction medium contained: U: 73 mg/l HNO2: 0.052 mol/l p-y-activity 11700 Imp/min (impulses per minute) (mainly Ru-106) Dibutylphosphate: 280 mg/l The external flow ratio between organic and aqueous phase was 1 l/h:0.05 I/h. During the experiment a precipitate formation occurred. The precipitate was deposited at the boundary between the organic and aqueous phases and besides uranium also contained a large proportion of A-r-activity (102,000 Imp./min). The experiment was interrupted after a 16 hour operating period. By introducing CO2 the precipitate was dissolved again. Example 2 This example shows the efficiency of the method in accordance with the invention in a single stage mixing settler. A wash of the organic extraction medium (300/, by volume tributylphosphate, 70% by volume alkane) with 5.2 molar N2H4H2O solution, which was charged with 1.25 M CO2/l. The organic extraction medium contained: U: 75 mg/l HNO3: 0.044 molli ,B-y-activity: 6760 Imp/min (main activity Ru-106) Dibutylphosphate: 230 mg/I The flow ratio corresponded to Example 1. During this experiment no precipitate formation occurred. The discharge sample had the following composition: Org.phase: U: < 50 mg/l (Detection limit) HNO3: 0.01 mol/l A-y-activity: 5700 Imp/min Dibutylphosphate: 41 mg/l Aqueous phase: U: 1.4 g/l N2HsNO3: 0.68 mol/l P-y-activity: 34730 Imp/min (mainly Ru106: Do~2) Dibutylphosphate: 3800 mg/l (substantially quantitative). WHAT WE CLAIM IS: 1. A method as claimed in Claim 1 of parent Application No. 42931/75 (Serial No.

1,507,170), wherein the aqueous hydrazine hydrate solution is charged with carbon dioxide.

2. A method as claimed in Claim I, in which CO2 is added in an amount of from 0.1 to 1.5 mol CO2 per litre of aqueous wash solution.

3. A method as claimed in Claim 1 or 2, substantially as hereinbefore described and exemplified.

4. An extraction medium containing an organophosphorus acid ester and a hydrocarbon and being used for reprocessing spent nuclear fuel and/or breeder materials, whenever treated by a method as claimed in any preceding claim.