Introduction

Brachytherapy is the common method for treating various tumors, and currently the ruthenium-106 and iodine-125 applicators are the most frequently used. Considering that 106Ru is a β − emitter with maximum energy of 3.54 MeV, it is best indicated in the treatment of small melanomas, with up to 20 mm tissue range [1]. It also replaced 90Sr/90Y sources because of it higher energy of emitted β − particles [2] and possibility of simpler source preparation.

106Ru is commercially obtained from neutron irradiated high enrichment 235U target in process of production 99Mo. After isolation of 99Mo radioisotope and decaying of 103Ru, ruthenium is separated from the wastes by multistep procedure. At present, there are only a handful of ageing reactors worldwide capable of producing the 99Mo, therefore alternative strategies for production of this key medical isotope are explored. In our work, we propose to use liquid high-level radioactive waste as a source of high activity of 106Ru.

The potential utilisation of fission-produced platinum metals (fission platinoids) as valuable products has attracted attention in the last few decades, as large amounts of spent nuclear fuel have accumulated worldwide [3]. Table 1 presents the isotopic composition of ruthenium isotopes after 5 years cooling of liquid nuclear waste [4].

Table 1 Isotopic composition of ruthenium originated from fission of 235U after 5 years cooling
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Simple calculations indicate that 1 dm3 of waste solution after reprocessing of nuclear fuel contains about 500 GBq of 106Ru after 4 years of cooling. This amount of activity is enough for production of about few thousands of brachytherapy sources.

During reprocessing of the spent fuel, the metallic Ru is dissolved in concentrated nitric acid and forms stable Ru-nitrosyl complexes [5]. In the high acidity the dominating ruthenium species are the [RuNO(NO2)2(NO3)(H2O)2]0 and [RuNO(NO2)2(H2O)3]+ [6]. The concentration of different species depends mainly on the composition of the medium and also the time of ageing.

Ruthenium metal was efficiently separated from other fission products by oxidation and distillation of RuO4 with absorption in NaOH solution. El-Absy et al. [7, 8] separated Ru radionuclides from a 131I-free fission product acidic solution containing KMnO4, by boiling for 40 min. In other work, ruthenium was electrochemically eliminated from a 3 M HNO3 solution of high-level waste, as RuO4, in the presence of AgNO3 at 60 °C [9]. Gandon et al. [10] co-precipitated ruthenium with copper ferrocyanide neutral solution. D. Banerjee et al. [11] used conventional ion exchangers and chemical precipitation based processes for the effective removal of the 106Ru activity from NH4NO3 effluent generated during wet processing of rejected sintered depleted uranium fuel pellets.

Present communication reports results of our process development studies on the recovery of ruthenium radioisotopes from simulated solution of high level radioactive waste using oxidation-extraction method.

Experimental

Radionuclide

For reasons of availability we used in experiments the 103Ru nuclide instead of 106Ru. The latter nuclide 106Ru is separated in complicated procedure from fission products of 235U, while 103Ru is produced in a simple way by direct thermal neutron irradiation of natural ruthenium. 103Ru was obtained by neutron irradiation of ruthenium salt (NH4)2[RuCl5(H2O)] at a neutron flux 7 × 1013 n cm−2 s−1 for 8 h in the nuclear reactor Maria at Świerk, Poland. The irradiated target was dissolved in 1 M HNO3.

Others radionuclides, 131I in the form of Na131I solutions was obtained from NCBJ-Polatom Świerk and 99mTc in the form of 99mTcO4 − was milked from 99Mo/99mTc generator.

Radioactivity measurements

The 103Ru radioactivity was measured using an ORTEC system with a high resolution HPGe detector using photo peak at E γ = 497.05 keV (88.7 %) and in NaI γ-scintilation counter LG-1b, ICHTJ, Poland.

Reagents

The following commercial chemicals were used without additional purification: H5IO6 was from Fluka and (NH4)2[RuCl5(H2O)] from Alfa Aesar, other reagents, carbon tetrachloride from Chempur, Poland, reductants and acids were from POCh Gliwice, Poland. Desirable concentrations of reagents were obtained by dilution of stock solutions.

Solvent extraction studies

Experiments were carried out under ambient conditions by shaking equal volume (5 ml each) of organic and aqueous phase in a separatory funnel using wrist action shaker. Phase separation was done by centrifugation and suitable aliquots (1 ml) of each phase were assayed. The distribution ratio “D” of the metal was determined as the ratio of metal concentration in organic phase to that in aqueous phase. Percentage extraction of metal ion was calculated by equation:

$$ {\text{\% }}E\; = \;\frac{\text{D}}{{{\text{D}} + 1}}\; \times \;100\;{\text{\% }}. $$

Results

Extraction of 103Ru to CCl4 phase

In oxidizing solutions ruthenium forms tetroxide, RuO4, which is easily extractable to organic phase. Formation of RuO4 is indicated by color change from deep orange to golden yellow. The RuO4 formed was extracted to an organic phase. Unfortunately, the RuO4 is not stable in the CCl4 phase and formation of black RuO2 precipitate is observed after a few hours. To avoid reduction of RuO4 to RuO2 the organic phase was contacted with a solution generating Cl2 molecules: 0.01 M HCl + 0.05 M H5IO6. The Cl2 molecules, formed in aqueous solution, are very soluble in CCl4 and distributed among the two liquid phases keeping ruthenium in the form of RuO4 in the organic phase for several month [12].

Influence of the various oxidants and acids on ruthenium oxidation-extraction process were studied to optimize the process. Table 2 presents results of 103Ru extraction from solutions containing various oxidizing agents. Concentration of used oxidants was the same taking into account the number of electrons involved in the reaction.

Table 2 Efficiency of Ru extraction in various oxidizing solutions
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As show in Table 2 the obtained results indicate that the best oxidant is orthoperiodic acid (86.0 % extraction), a somewhat worse, but also possible to use is a potassium metaperiodate (75.1 %) and potassium permanganate (79.5 %). The obtained results well correlate with oxidation potential of reagent used.

An important parameter was the selection of a suitable amount of oxidant to obtain complete oxidation of ruthenium to RuO4 and thus its extraction into the organic phase. We have studied the 103Ru extraction depending on the concentration of orthoperiodic acid. The results are presented in Fig. 1.

Fig. 1
figure 1

Extraction of 103Ru from 1 M HNO3 solution containing various concentration of H5IO6

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In concentration range from 5 to 40 g l−1 of H5IO6 only insignificant increasing of 103Ru extraction is observed. Therefore, it can be assumed that that solution containing only 10 g l−1 of H5IO6 should be sufficient for effective extraction of 103Ru to CCl4 phase.

In the next step, influence of various acids and acid concentrations on 103Ru extraction were studied. We examined the following acids: nitric acid, sulfuric acid, hydrochloric acid and perchloric acid. The results are presented in Tables 3 and 4.

Table 3 Efficiency of Ru extraction in various acid solutions
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Table 4 Extraction of 103Ru in various HNO3 concentrations, concentration of H5IO6—10 g l−1
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In the solutions of HNO3, H2SO4, and HClO4 extraction of 103Ru was comparable. Only in HCl solution extraction was significantly lower. Additionally, the use of HCl solution is not desirable due to the formation of Cl2 gas by reaction of orthoperiodic with hydrochloric acid. For further experiments HNO3 solution was selected. This choice was dictated by the fact that the high-level radioactive waste are generally in the form of a HNO3 solution. Table 3 presents dependence of the 103Ru extraction on the HNO3 concentration in range of 1–5 M.

As shown in the Table 4 only very small increasing of 103Ru extraction was observed when HNO3 concentration increased from 1 to 5 M. Summarizing our results on optimization of 103Ru extraction process, we can conclude that 86 % of extraction could be obtained for using H5IO6—10 g l−1 as oxidant and 1 M HNO3 solution. Using of higher H5IO6 and HNO3 concentrations gave only insignificant increasing of the process efficiency.

The PUREX raffinate contains also other long-lived fission products like 135,137Cs, 90Sr, 241Am, 99Tc, 129I, 97Zr, among which 99Tc and 129I could be potentially co-extract with 106Ru. In oxidizing solution technetium could be extracted as HTcO4 and iodine in I2 or interhalogen form. The 135,137Cs, 90Sr, 241Am and other metallic radionuclides in HNO3 solution, not containing complexing agents, are present in either cationic form or nonextractable species.

For co-extraction studies of 99Tc and 129I we used short-lived isotopes 99mTc and 131I. The extraction of both radionuclides were performed in solution of concentration of 10 g l−1 H5IO6 in 1 M HNO3. We did not observe extraction of radionuclide studied, radioactivity of the 99mTc and 131I in the organic phase was below the background level.

Since the 106Ru sources for brachytherapy are usually obtained by electrochemical deposition from aqueous solutions [13], we investigated the possibility of ruthenium transfer from the organic to aqueous phase. Because RuO4 is the only form of ruthenium, which is stable in CCl4 phase, for re-extraction of 103Ru we decided to reduce RuO4 to Ru(III) and Ru(II) oxidation state. The following compounds were selected as reductants: sodium sulfite, hydroxylamine, hydrazine and sodium borohydride. Results of 103Ru extraction from the organic into aqueous phase are shown in Table 5.

Table 5 Percent of re-extraction of 103Ru from CCl4 phase to solution containing reducing agent
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As shown in Table 5, the best results were obtained for 0.1 M aqueous solutions of hydrazine and for hydroxylamine hydrochloride. These reductants are most sufficient, because of their relatively high solubility in the organic phase, where reduction of RuO4 to the Ru(III) and Ru(II) took place. Reduced forms of ruthenium are insoluble in CCl4 phase and passed immediately to the aqueous phase.

Kinetic studies were carried out in the system 103RuO4 in CCl4 (organic phase) and Na2SO3 0.1 M HCl (aqueous phase). The results presented in Fig. 2 indicate that the process is relatively fast and after 40 min equilibrium state is achieved.

Fig. 2
figure 2

Kinetic of 103Ru re-extraction from CCl4 to aqueous phase (0.1 M hydrazine)

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Conclusion

A highly effective and flexible process for the separation of 106Ru from simulated high-level liquid waste was elaborated. It was found that the optimal way for extraction of 103Ru to CCl4 organic phase is oxidation of ruthenium nitrozyl complexes to RuO4 by 10 g l−1 H5IO6 in 1 M HNO3 solution. It was found that in re-extraction process to aqueous phase the most efficient compounds for reduction of RuO4 in CCl4 phase are hydrazine and hydroxylamine hydrochloride. The overall recovery of 106Ru is estimated at more than 80 %.

Production batches of hundreds GBq of 106Ru radioisotope separated from 1 l of PUREX raffinate can be achieved using the above-mentioned separation technique. For verification of the obtained results further experiments with real wastes solutions is necessary.