EP0347625A2 - Method for separating technetium, ruthenium and palladium from solutions of nuclear fuels - Google Patents

Abstract

In the sepn. of Tc, Ru and Pd values from streams of substances obtd. by reprocessing irradiated nuclear fuel, by pptn. and ion exchange from a stock soln. (I) of the values and other fission/activation prods. in HNO3, the novel features are that: a) Pd is pptd. selectively by adding diethylthiourea (II) to (I) and the ppte. is sepd.; b) Tc and Ru are sepd. by passing Pd-free (I) into a bed of strongly acidic cation exchanger (III); c) (III) is washed with dil., pref. ca. 2 m HNO3; d) Tc is recovered by selective oxidative elution with dil. HNO3 soln. (IV) contg. an oxidant; and e) Ru is recovered by selective elution with conc., pref. 6-8 M HNO3. Pref. (I) is treated with ca. 4 mole (II)/mole Pd and also ca. 6 mole (II)/mole Ru present. (III) is a macroporous styrene-DVB copolymer with 2-8% crosslinking, pref. 'AG50W-X2' (RTM) (IIIA) with 2% crosslinking. (IV) contains H2O2, the concns. pref. being 0.05-3, esp. 0.1-1 mole/1 HNO3 and 0.05-3, esp. 0.1-1 mole/1 H2O2. (I) is produced from feed clarified slurry by bringing most of the slurry into soln. by decomposition with carbonate and addn. of NHO3 and sepn. of the insol. Rh oxide from the soln. The ppte. formed from (II) and Pd is converted to Pd oxide at ca. 500 deg.C and Pd metal is produced by calcining the oxide at ca. 900 deg.C. Pd-free (I) is warmed to ca. 70 deg.C ofr ac. 30 min. to accelerate Ru complex formation with (II).

Description

The invention relates to a process for separating the valuable substances technetium, ruthenium and palladium from material flows which arise during the reprocessing of irradiated nuclear fuel, the valuable substances being isolated by precipitation and ion exchange from a nitric acid containing the valuable substances and other fission / activation products.

When nuclear fuels are irradiated in nuclear reactors, fission and activation products are created. Activation products arise from the atoms of the nuclear fuel (uranium, plutonium) by neutron capture; they belong to the group of actinides.

Fission products are isotopes of chemical elements that arise when atoms of the nuclear fuel (uranium, plutonium) are split into two or three fragments. These isotopes can themselves be radioactive, but can also be inactive. Technetium, ruthenium and palladium belong to the group of fission products. Technetium mainly forms the isotope with mass 99, which is weakly radioactive and therefore does not occur in nature and can therefore only be generated artificially by nuclear reactions.

The fission products ruthenium and palladium arise in nuclear reactors in large quantities, whereby both radioactive and non-radioactive isotopes are formed.

Split ruthenium contains about 3% of the isotope Ru-106, which has a half-life of about 1 year and decays into inactive palladium (Pd-106). Fission palladium is very weakly radioactive due to its Pd-107 content. However, this radioactivity does not significantly limit the usability for technical purposes.

During reprocessing, the irradiated nuclear fuel is dissolved in boiling semi-concentrated nitric acid. The majority of the nuclear fuels and the fission and activation products go into solution.

A small remainder of the irradiated nuclear fuel remains in the undissolved form in the dissolver. This residue is known as feed sludge. It contains significant amounts of molybdenum, zirconium, technetium and precious metals.

Unused uranium and the plutonium produced are separated from the solution of the irradiated nuclear fuel by extraction. The remaining fission products are finally melted into glass.

Because of the commercial value of the fission products technetium, ruthenium and palladium, a number of proposals for the separation of these valuable materials from solutions that are produced during the reprocessing of irradiated nuclear fuels have been published (MW Davis, The Extraction of Cs, Sr and the Platinum Group Metals from Acidic High Activity Nuclear Waste, DOE / SR / 10714-T3).

These proposals relate to the use of several columns connected in series, on which the disruptive and later the desired nuclides are fixed to the columns with a retention of between 75% and 95%. This requires a high level of equipment due to the necessary use of several columns.

Furthermore, there is a considerable amount of secondary waste, since used columns are heavily contaminated and have to be disposed of separately.

A process is known from US Pat. No. 3,848,048 in which palladium, technetium, rhodium and ruthenium are separated from acidic fuel solutions by passing these solutions in succession over three carbon beds which are impregnated with different chelating agents. The elements palladium, technetium and ruthenium / rhodium are retained in chelated form on these beds.

The loaded beds are preferably incinerated and the valuable materials are isolated from the ashes.

This process has several disadvantages. The chelating agents can contaminate the solution which has been freed from the valuable substances and in this way can severely disrupt the removal of further valuable substances or the conditioning of the remaining ingredients. Because of their flammability, the carbon beds represent a significant hazard potential. They are burned after loading, whereby radioactive components are released as gases or aerosols and must be retained by an effective exhaust gas cleaning system with the help of scrubbers and filters. Washing liquids and filters must be disposed of as secondary waste.

The object of the invention is to selectively separate the valuable materials technetium, ruthenium and palladium from acidic solutions of irradiated nuclear fuel and with high efficiency. The separation of these valuable substances should not cause any additional problems in the further treatment of the radioactive substances; in particular, only substances should be used which do not contaminate the solution freed from the valuable substances or which can be removed by simple measures such. B. remove by heating or extracting the solution. Easily flammable substances should not be used. The process should be simple to carry out, with as little secondary waste as possible.

The object is achieved by the features listed in the characterizing part of the main claim.

The subclaims indicate advantageous developments of the method.

The process according to the invention can in principle be carried out with all nitric acid solutions which are produced during the reprocessing process and which contain technetium, ruthenium and palladium.

Feed clarification sludge is a preferred source of these valuable substances because it contains the valuable substances in a concentrated form.

The feed sludge can be brought into solution by treating it in a manner known per se with reducing gases such as CO or H₂ and annealing it with carbonates. The residue on ignition is taken up with 3-7 molar nitric acid and the solution is adjusted to 1 mol HNO₃ / l. Rhodium remains as Rh₂O₃ in the residue.

The solution freed from solid Rh₂O₃ forms the stock solution of the process according to the invention which, in addition to the valuable materials technetium, ruthenium and palladium, depending on the origin and pretreatment of the irradiated nuclear fuel, also varying amounts of the elements Pu, U, Am, Mo, Zr, Ce and other fission and may contain activation products.

Diethyl thiourea (DETH) is added to the stock solution in solid form or as an aqueous solution. The amount of DETH depends on the amount of palladium and ruthenium in the stock solution. 4 moles of DETH are added per mole of palladium and an additional 6 moles of DETH are added per mole of ruthenium. Palladium selectively forms an insoluble precipitate with the DETH reagent, in which more than 99% of the palladium present is ent are holding. Spectrophotometric studies allow the assumption that it is polymeric Pd-DETH complexes.

The precipitate containing palladium is separated off in a customary manner and roasted at about 500.degree. This forms PdO, which can be reduced to metal by annealing at 900 ° C.

The Pd precipitate filtrate is heated to a temperature of about 70 ° C for about 30 minutes to accelerate complexation of the DETH with ruthenium.

The cooled solution is passed through a strongly acidic cation exchanger. The Ru (NO) -DETH compounds present exclusively in cationic form are quantitatively retained on the adsorber together with the TcO²⁺ ions present in the medium in the tetravalent state, while the accompanying impurities are only partially adsorbed and by washing the column with about 2-molar HNO₃ can be desorbed again.

The strongly acidic cation exchanger AG 50 W-X2 proved to be particularly efficient in terms of capacity and sorption kinetics; it consists of a macroporous co-polymer of polystyrene divinylbenzene with 2% crosslinking.

After the cation exchanger has been freed from the adsorbed undesirable substances by washing, the technetium is selectively and quantitatively eluted. The elution is preferably carried out with a solution of about 0.1 - 1 mol H₂O₂ / l and 0.1 - 1 mol HNO₃ / l. Technetium is present as pertechnetate after elution.

Ruthenium is then eluted; preferably 6-8 molar HNO₃ is used as the eluent.

An essential advantage of the process according to the invention is that, in addition to the nitric acid which is in any case necessary for the dissolution of irradiated nuclear fuel, only chemicals are used which can be removed from the solution freed from the valuable substances by simple boiling or extraction. Therefore, the further treatment of these solutions is not made difficult. The process can be integrated into the process diagram of the reprocessing without the reprocessing process having to be changed.

The chemicals used make no higher demands on the corrosion resistance of a plant for carrying out the method according to the invention than the reprocessing process. The method can be carried out simply and inexpensively. Because of the use of a single separation column, which - in contrast to the process according to US Pat. No. 3,848,048 - is reused, no significant amounts of secondary waste are produced.

The water solubility of the chemicals used makes the addition of flammable organic solvents unnecessary.

The method according to the invention is characterized by a high effectiveness; The high loading levels that can be achieved allow the construction of compact, easy-to-use systems.

The invention is explained in more detail below with the aid of two implementation examples.

The implementation examples are based on a stock solution which is obtained from feed sludge by carbonate digestion. Here, rhodium remains behind. An inactive simulate was used for the feed sewage sludge, which is based on published data on the composition of the feed sewage sludge (K. Naito et al, Recovery of Noble Metals from Insoluble Residue of Spent Fuel, J. Nucl. Sc. And Tech., 23 (6) , pp. 540-549 (June 1986); H. Kleykamp, composition of residues from the dissolution of irradiated LWR- (U, PU) O₂ with recycled Pu, nuclear industry, July 1982).

The inactive simulate was carried with radioactive isotopes of the corresponding elements. Isotope 239 was used for plutonium; the rare earths are represented by the element cerium.

Table 1 shows the average composition of the feed sludge in% by weight. Table 1 Average composition of the feed sludge with a burn-up of 33000 MWd / t. Fission and activation products Average composition weight percent molybdenum 15 Technitium 3rd Ruthenium 40 Rhodium 4th palladium 8th Actinides uranium 4th plutonium 0.1 Other (Zr, Fe, Cr) 26 total 100

Tables 2 and 3 show the molar concentrations of the individual elements in the stock solution used. Table 2 (Experiment 1) element Concentration (mol / l) Ru / external nuclide ratio Decontamination factor DF Ru 5 x 10⁻³ - 3 x 10³ U 5 x 10⁻⁴ 10th 4.6 x 10² Pu 1.25 x 10⁻⁵ 400 > 1 x 10³ At the 3 x 10⁻⁶ 333 > 6 x 10² Mon 1.87 x 10⁻³ 2.67 > 1 x 10⁶ Tc 3.75 x 10⁻⁴ 13.33 1 x 10⁴ Pd 1 x 10⁻³ 5 > 99 Zr 3.75 x 10⁻⁵ 133.33 6.25 x 10² Ce 1.25 x 10⁻⁴ 40 5 x 10⁴ (Experiment 2) element Concentration (mol / l) Ru / external nuclide ratio Decontamination factor DF Ru 1 x 10⁻² - 2.8 x 10³ U 1 x 10⁻³ 10th 5.1 x 10² Pu 2.5 x 10⁻⁵ 400 > 1 x 10³ At the 6 x 10⁻⁶ 333 > 6 x 10² Mon 3.7 x 10⁻³ 2.67 > 1 x 10⁶ Tc 7.5 x 10⁻⁴ 13.33 1.3 x 10⁴ Pd 2 x 10⁻³ 5 > 99 Zr 7.5 x 10⁻⁵ 133.33 5.8 x 10² Ce 2.5 x 10⁻⁴ 40 4.8 x 10⁴ The decontamination factor DF indicates the proportion of the elements removed when carrying out the method according to the invention.

The valuable materials ruthenium, technetium and palladium are separated with decontamination factors from 2800 - 3000 or 10000 to 13000 or> 99.

The separated materials are only very slightly contaminated by the undesired fission products and actinides, as can be seen from the decontamination factors of these elements.

Implementation examples (experiment 1, experiment 2)

A 1 molar nitric acid solution was used as the stock solution. The element concentrations for test 1 are shown in table 2, for test 2 in table 3.

With constant stirring, the organic complexing agent N, N′-diethylthiourea was added to the stock solutions at room temperature. 4 moles of DETH were added per mole of palladium present in the stock solution and 6 moles of DETH were added per mole of ruthenium present in the stock solution. After about 15 minutes, more than 99% of the palladium has precipitated as a Pd-DETH complex. The residue was separated and the solution freed from the residue was heated in a thermostated water bath at 70 ° C. for 30 minutes. Under these conditions, the ruthenium nitrosyl nitrate complexes, which are present in different Ru values, are quantitatively converted into the double and triple positively charged ruthenium nitrosyl diethyl urea complexes, while technetium, which originally existed as pertechnetrate, is reduced to TcO²⁺.

The solution pretreated in this way was applied to a column loaded with AG50W-X2, with ruthenium and technetium being completely retained.

The column was washed with 4-5 column volumes of 2-molar HNO₃, the interfering substances partially retained on the column (Zr, Ce, U, Pu, Am, Mo) being removed.

The column was then oxidized with 4 column volumes of an aqueous solution, each containing HNO₃ and H₂O₂ in a concentration of 0.5 mol / l, the technetium being released as the pertechnetrate.

The column was then eluted to release Ru with 14 column volumes of a 7 molar HNO₃.

The results of experiments 1 and 2 are shown in Tables 2 and 3.

Legend for the figures:

• Fig. 1:
  Flow diagram of the recovery of valuable materials from a stock solution, which is obtained by carbonate digestion and subsequent dissolution in nitric acid.
• Fig. 2:
  Dynamic retention of technetium R. Ordinate: R as a percentage of the amount of technetium added. Abscissa: amount of technetium given in mg / g of cation exchanger. Cation exchanger: AG 50 W-X2 (50 - 100 mesh). Temperature: 24 ± 0.1 ° C.
• Fig. 3:
  Technetium concentration C _Tc in the eluate depending on the eluate volume V. Cation exchanger: AG 50 W-X2. Eluent: 0.5 M H₂O₂ / l M HNO₃.
• Fig. 4:
  Dynamic Ru-DETH retention R. Ordinate: R in% of the amount of ruthenium added. Abscissa: amount of ruthenium given m _Ru in mg Ru / g cation exchanger. Cation exchanger AG 50 W-X2 (200 - 400 mesh) 500 mg; Feed speed 0.76 ml / min. Temperature: 22 ° C.
• Fig. 5:
  Dependence of the ruthenium concentration C _Ru in the eluate from a column loaded with 28 mg Ru / g cation exchanger. Ordinate: C _Ru in mol / l. Abscissa: eluate volume; V as a multiple of the column volume. Cation exchanger: AG-50W-X2 (200 - 400 mesh). Column volume 5 ml. Eluent 6 m HNO₃.
• Fig. 6:
  Ruthenium elution yield (A _Ru ) in percent of the fixed ruthenium as a function of the eluate volume (V: multiple of the column volume).

The following figures relate to the elution yield A of adsorbed undesirable substances during the washing of the cation exchanger. A is given as a percentage of the fixed undesirable substance.

• Fig. 7: Elutionsausbeute A_U für Uran
• Fig. 8: Elutionsausbeute A_Pu für Plutonium
• Fig. 9: Elutionsausbeute A_Am für Americium
• Fig. 10: Elutionsausbeute A_Zr für Zirkonium
• Fig. 11: Elutionsausbeute A_Ce für Cer.

The multiples of the column volume are plotted on the abscissa. AG 50W-X2 was used as the cation exchanger; the eluent (washing medium) consisted of 1-molar HNO₃ for uranium, for the other undesirable substances of 2-molar HNO₃.

• Fig. 7: Elution yield A _U for uranium
• Fig. 8: Elution yield A _Pu for plutonium
• Fig. 9: Elution yield A _Am for americium
• Fig. 10: Elution yield A _Zr for zirconium
• Fig. 11: Elution yield A _Ce for cerium.

Claims (7)

1. Process for the separation of the valuable materials technetium, ruthenium and palladium from material flows which arise during the reprocessing of irradiated nuclear fuel, the valuable substances being isolated by precipitation and ion exchange from a nitric acid containing the valuable substances and other fission / activation products, characterized by the following characteristics: a) addition of diethylthiourea (DETH) to the stock solution, whereby palladium is selectively precipitated, and separation of the precipitate; b) introducing the palladium-freed solution into a bed of a strongly acidic cation exchanger to separate technetium and ruthenium; c) washing the cation exchanger with dilute, preferably about 2-molar nitric acid; d) selective oxidizing elution of the technetium with a dilute nitric acid solution containing an oxidizing agent; e) selective elution of the ruthenium with the aid of concentrated, preferably 6-8 molar nitric acid. 2. The method according to claim 1, characterized in that about 4 moles of DETH per mole of palladium present in the stock solution and additionally about 6 moles of DETH per mole of ruthenium present in the stock solution are added. 3. The method according to claim 1, characterized in that a macroporous Ko-poly as a strongly acidic cation exchanger merisat of polystyrene divinylbenzene with 2-8% crosslinking, preferably AG50W-X2 with 2% crosslinking is used. 4. The method according to claim 1, characterized in that a hydrogen peroxide-containing nitric acid solution is used for the selective oxidizing elution of the technetium, both hydrogen peroxide and nitric acid in the concentration range 0.05-3 mol / l, preferably 0.1-1 mol / l contains. 5. The method according to claim 1, characterized in that the stock solution is made from feed sewage sludge, the majority of the feed sewage sludge being brought into solution by carbonate digestion and addition of nitric acid and the solution being separated from non-soluble rhodium oxide. 6. The method according to claim 1, characterized in that the precipitate formed from DETH and palladium is converted into palladium oxide at about 500 ° C and is produced from the palladium oxide by annealing at about 900 ° C palladium metal. 7. The method according to claim 1, characterized in that the freed of palladium solution to accelerate the complexation of the ruthenium with DETH is heated to a temperature of about 70 ° C for about 30 min.

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