Removal of Ruthenium from PUREX Process

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 26[3], pp. 358~364 (March 1989).

Extraction of Ruthenium Tetroxide with Paraffin Oil and Filtration of Ruthenium Dioxide

Kenji MOTOJIMA

Kaken Co., Ltd.*

Received December 24, 1988

Ruthenium in nitric acid solution is oxidized by the addition of ceric nitrate to ruthenium tetroxide, which is extracted with n-paraffin oil. Ruthenium tetroxide in paraffin is immediately reduced by the solvent to black ruthenium dioxide as suspension, which can be readily filtered off through the ordinary cellulose filter paper. Through the combined process of extraction and filtration, the ruthenium in a nitric acid solution can be eliminated.

KEYWORDS: ruthenium, ruthenium tetroxide, ruthenium dioxide, nitrosglruthe- nium complexes, ceric nitrate, n-paraffin oil, solvent extraction, filtration, spent fuel reprocessing, PUREX Procss, fission products

uranium and plutonium are contaminated I. INTRODUCTION by radio-ruthenium. Ruthenium is known to be one of the most As a result, the possibility of the rel- trouble-some nuclides of fission products be- ease of radio-ruthenium in the environment cause it has large fission yield and relatively increases. long half lives. (103Ru: 39.8 d; 106Ru: 368 d) (2) In the process of the concentration of Moreover, the chemistry of ruthenium is very highly radioactive aqueous waste, contain- complicated. It has many oxidation states ing many nitrates of fission products and (0~8), and forms a large number of complexes. nitric acid, ruthenium is oxidized to volatile Ruthenium tetroxide is especially volatile, and tetroxide by nitric acid, and escapes to the it is mainly this property which causes many vapor phase. A large portion of volatil- trouble. ized ruthenium tetroxide is reduced to its Because of these reasons, ruthenium is dioxide at the inner surface of the equip- known as an extremely troublesome nuclide ment and is deposited there. As a result, in the PUREX Process(2), which is utilized the radiation dose of the plant is increased most widely for the reprocessing of spent fuel. and the corrosion of the inner surface of The problems in the PUREX Process associated the equipment is acceleratedc(6). with ruthenium are as follows : In addition, a part of ruthenium tetrox- (1) In the process of tributyl phosphate ide is introduced in the recovered nitric acid. (TBP)-dodecane extraction of uranium and (3) In the process of the solidification of plutonium, the behavior of ruthenium is highly radioactive liquid waste, ruthenium unpredictable and complicated(3)~(5). Prob- tends to escape in off gas as its tetroxide ably, several species of nitrosyl-ruthenium- at high temperature and causes serious nitrato complex remain in TBP-dodecane contamination. after stripping and the product streams of * 1044, Hori-cho, Mito-shi 310.

5 2 Vol. 26, No. 3 (Mar. 1989) 359

Many studies have been made on these widely utilized in the actual reprocessing problems by many researchers for more than process. In addition, several aliphatic and 30 yr. aromatic hydrocarbons, carbon tetrachloride Nevertheless, a conclusive method to solve and some other solvents were examined. them has not been developed. The author 2. Equipment found that the ruthenium tetroxide in the The equipment used throughout the experi- nitric acid aqueous solution, which is formed ments is shown in Fig. 1. by the addition of ceric nitrate, is extractable Extraction vessel : A(1) 50~2,000 ml beaker in paraffin oil, and the tetroxide extracted in or a conicalflask etc. can be used. The aque- paraffin oil is immediately reduced by the ous and paraffin oil phases are mixed by a solvent, and yields black ruthenium dioxide magnetic stirrer with thermostating heater. suspension. The ruthenium dioxide suspended Coagulation column : It is equipped(2) with in paraffin oil can be readily filtered off a nichrome wire heater and an AC thermo- through the ordinary filter paper made of couple. cellulose fiber. Usually, paraffin oil in the column is heated By combining with these properties, the at about 90dc in order to coagulate the ruthe- ruthenium in nitric acid solution can be re- nium dioxide suspension. moved easily. Filter : (3)A common thimble shape filter (30 mm 0. D., 100 mm high) made of cellulose II. EXPERIMENTAL fiber is used. In order to treat relatively large 1. Reagents amounts of ruthenium (about 100 mg), it is ( 1 ) Ruthenium Solutions preferable to coat the inside wall of filter Ruthenium nitrate (3 M nitric acid) solution with fine filter pulp (about 2 g). and nitrosylruthenium nitrato (3 NI nitric acid) Circulating pump : A (4)small magnetic solution were used. These solutions contained pump made of stainless steel and with a flow 1.0 mg of ruthenium in 1 ml respectively, and rate controller (IWAKI H-2A type) is used. were prepared by the modified methods of Storage vessel of paraffin oil. (5) Fletcher et al.c(3) All of these solution were supplyed by Nippon Engelhard Co., Ltd. The solution of nitrosylruthenium tetranitro complex was prepared by dissolving disodium hydroxo-tetranitro-nitrosylruthenate, Na2[Ru(NO)(NO2),OH]NH2O. The complex was synthesized and purified by the improved method of Sato(8). ( 2 ) Ceric Nitrate Solution The solution of 2 M ceric nitrate (3 IA nitric acid), prepared by Daiichi Kigenso Kagaku Kogyo Co., Ltd. was used. ( 3 ) n-Paraffin Oil The n-paraffin oil supplied by Nisseki Plastic Chemicals Co., Ltd. was used throughout the experiment. Its composition was n-C10: 16.0%, n-C11 : 50.8%, n-C12 : 32.0%, n-C13 : 0.1%, n-C14: 0.1% and n-C15 : 0.0%. The behaviors of the solvents in the experiments mentioned in this paper are essentially equal to that of n-dodecane, which is most Fig. 1 Experimental equipment

53 360 J, Nucl. Sci Technol.,

3. Experimental Procedure evaporation. Finally, the ruthenium concen- From 20 to 2,000 ml of ruthenium solution tration of the solution was determined by was placed in an extraction vessel of suitable spectroscopic analysis using inductively coupled capacity. The solution contains less than 100 plasma or by atomic absorption analysis(9). mg of ruthenium and its nitric acid concen- The determination limit of ruthenium by these tration is usually 3 M. Make the paraffin oil analytical methods was 0.1 mg• ml-1. circulation by pump, and control its flow rate at about 200 ml,min-1 The amount of paraf- III. RESULTS AND DISCUSSION fin oil on aqueous layer was adjusted at 3~5 1. Condition of Extraction cm in thickness. (Oxidant) The extraction vessel was heated at about Though many oxidizing agents can be used 60dc, and the coagulation column at about to form ruthenium tetroxide in the nitric acid 90dc, respectively. solution, ceric nitrate is the most favorable Stir to mix the aqueous and paraffin oil because it does not generate any corrosive phases, the condition of stirring seems most materials, such as halogens, and cerium is suitable when the paraffin oil droplets are a originally contained in fission products, and few millimeter in diameter and disperse in then, its addition does not complicate the the whole aqueous phase. PUREX Process. The necessary amounts of A necessary amounts of the ceric nitrate ceric nitrate is 1.5 times the ruthenium equiv- solution was added to send it into the aqueous alent, but some chemical forms of ruthenium phase, usually, 1.5 times equivalent of ceric nitrosylnitro complexes require more amounts is preferable to oxidize the ruthenium com- of ceric nitrate to oxidize these ligands. pletely. Immediately, ruthenium tetroxide is (Solvent) formed, extracted in paraffin oil and converted All aliphatic hydrocarbons which are oily into black dioxide suspension. The ruthenium at ordinary temperatures seem to be useful dioxide suspension was transfered through for the proposed method, but for chemical the coagulation column and filtered off. The stability and safety, n-paraffin in which the particle of ruthenium dioxide formed in the number of carbons is 11 to 15 and their mix- paraffin phase was very fine, its diameter is tures are preferable. The reaction of ruthe- observed to be less than 0.1 mm by an elec- nium tetroxide and n-paraffin has not been tron microscope. It was found as amorphous clear in detail. The author has found that at by X-ray diffraction analysis. Since these least five isomers of n-dodecanone are formed particles had strong adsorption properties on by the oxidation of pure n-dodecane. The cellulose fiber, filtration was not so difficult. amounts of them are nearly equal, and at the On the other hand, the particle is hydrophobic proposed condition each of them are less than and does not disperse in the aqueous phase, 0.02% of dodecane when 50 mg of ruthenium because it might absorb paraffin strongly. is extracted with 150 ml of dodecane. The The cellulose filter-fiber holding black suspen- detailed investigation of the oxidation reac- sion material can be easily burned away by tions is being performed at present. heating them at about 800dc, while powdery Aromatic hydrocarbons, such as benzene, ruthernium dioxide and metallic ruthenium toluene and xylene, and halogenated solwents, are remained. such as carbon-tetrachloride and chloroform, At a definite time after the extraction has extract ruthenium tetroxide, but they do not been started, the stirring is stopped temporari- react as aliphatic hydrocarbons and do not ly and about a 1.5 ml portion of the aqueous give ruthenium dioxide suspension. phase was transferred into a test tube using In addition, aromatic hydrocarbons have a a small pipet, and then the solution was treated disadvantage in that they form troublesome with a small droplet of hydrogen peroxide to nitro-compounds, and halogenated compounds reduce ceric ion in order to prevent ruthenium have a tendency to form corrosive halogen

54 Vol. 26, No. 3 (Mar. 1989) 361

compounds. similar method described above. The results Therefore, the study of these solvents has are shown in Fig. 3. been rejected. (Concentration of Nitric Acid) The effects of nitric acid concentration on the extractability of ruthenium were examined in a batchwise test. In a test tube with a glass stopper, 10.0 ml of the ruthenium nitrate solution containing 0.2 mg of ruthenium and having a definite nitric acid concentration, 0.05 ml of the 2 M ceric nitrate solution and 10.0 ml of paraffin oil are added and shaken vigorously by hand for 30 s. After standing for 10 min, take approximately 1.5 ml of aqueous phase was withdrawn and ruthenium in the solution was determined by the same method mentioned in the experimental procedure. The relationship between the distribution of ruthenium, that is, the ratio of the amounts Fig. 3 Effects of temperature of ruthenium in paraffin oil per in aqueous on Ru-extraction phase, and the concentration of nitric acid is shown in Fig. 2. The extraction of ruthenium increases with temperature. In the present experiments, the extraction was performed at 60dc considering the vaporization and flash point of the paraffin oil used. (Stirring) Generally, vigorous mixing is effective for solvent extraction. In the recommended process, however, vigorous stirring to form the emulsion is not preferable, in this case because the ruthenium dioxide formed in paraffin oil tends to be reoxidized to tetroxide by ceric nitrate, and ceric is consumed uselessly by this reaction. 2. Effects of Chemical Species of Ruthenium Fig. 2 Effects of HNO3 concentration In a nitric acid solution, ruthenium will on Ru-extraction (at 20dc) exist as ruthenium ion (Re ) and several kinds of nitrosylruthenium nitro or nitrato com- The extraction of ruthenium is more effec- plexes. The individual adaptability of the tive when the nitric acid concentration is proposed method for these ruthenium com- lower, but in the experiments, 3 M nitric acid pounds is as follows : concentration is selected because its concen- (Ruthenium Nitrate [Ru(NO3)3]) tration of major aqueous phase in the PUREX When ruthenium exists as Ru3+ ion, it can Process is 2.5~3 M. be readily and quickly removed by the pro- (Temperature) posed method. The effects of temperature on the extrac- Experimental results are illustrated in tion of ruthenium were examined by the Fig. 4.

55 362 J. Nucl. Sci. Technol.,

Curve (1): 100 ml of 3 M nitric acid solution of ceric nitrate and extended extraction time. containing 10 mg of ruthenium (Ru: 100 mg. It is effective to convert these complexes to ml-1) was treated with 1 ml of 2 M ceric the extractable chemical form by the addition nitrate solution, and ruthenium is removed by of hydrogen peroxide and ageing for a period the proposed method. of several hours. Curve (2): 1,000 ml of 3 M nitric acid solution containing 50 mg of ruthenium (Ru: 50 mg,ml-1) was treated with 3 ml of 2 M ceric nitrate solution and ruthenium in the aqueous solution is removed similarly. In these experiments, ruthenium was reduced to less than 0.1 mg ml-1, within about 1 h.

Fig. 5 Removal of Ru from Ru (NO) (NO3)3- HNO3 solutions (2), compared with Ru (NO3) 2 solution (1)

In this study, after the extraction is made for about 30 min by the proposed method 1% v/v of hydrogen peroxide (30%) is added to the solution, allowed to stand for a day at 60-C, and then the extraction is continued. Fig. 4 Removal of Ru from Ru (NO3)3- By this process, ruthenium in the aqueous HNO3 solutions solution can be reduced to half or less, and (Nitrosylruthenium Trinitrato Complex by repeating it 2 or 3 times, the ruthenium [Ru(NO)(NO3)3]) concentration can be reduced to less than a Various results have been obtained from few 10-1 mg ,ml-1. Experimental results are the different preparation method of the com- shown in Fig. 5-(2). plex. When the solution was prepared by (Hydroxo-tetranitro-nitrosylruthenate passing nitrogen oxide gas into ruthenium (disodium salt) Na2[Ru(NO)(NO2)4OH12H2O) nitrate in 3 M nitric acid solution (Ru : 100 mg, This complex is the most stable among ml-1), ruthenium in the solution could be easily nitrosylruthenium complexes, and ruthenium removed using the same method as for removal in this form is difficult to be removed by the of the simple ruthenium (Ru+3) nitrate, as proposed method. shown in Fig. 5-(1). Some part of the complex can be decom- When the complex solution was prepared posed only by heating in nitric acid at over by dissolving nitrosylruthenium hydroxide 60-C for several hours, and the reaction is with nitric acid(3), a low percentage of ruthe- accelerated by the addition of hydrogen per- nium was removed by this method ; the reason oxide. Nevertheless, complete decomposition may be that chemical forms of ruthenium in of the complex is difficult by using the single the solution are types of nitronitrato com- hydrogen peroxide treatment, and considerable plexes. In this case, the effectiveness for the amounts of ruthenium remain after extrac- removal of ruthenium is weakened by addition tion.

56 Vol. 26, No. 3 (Mar. 1989) 363

Several repeated treatments are necessary the proposed method. However, the exami- to remove ruthenium in the aqueous phase nation using those radioactive solutions is until it becomes less than 0.1 mg,ml-1. Some difficult to perform in the author's limited of the experimental results are shown in laboratory facilities. Therefore, preliminary Fig. 6. Each 100 ml of 3 M nitric acid solu- tests using only g-irradiated ruthenium nitrate tions containing hydroxotetranitro ruthenate solution have been made. The solution of disodium salt (Ru: 100 mg,mt-1) were treated ruthenium nitrate in 3 M nitric acid was irra- with (1): 0 ml, (2): 1 ml, (3) : 3 ml and (4) : diated by g-ray to 1.1x107 rad at the 60Co- 5 ml of hydrogen peroxide (30%) respectively Irradiation Facility in Tokai Establishment of and heated at 60dc for a day. The removal Japan Atomic Energy Research Institute. The of ruthenium from these solutions was tested extraction behavior of ruthenium in the irra- by the proposed method. After the first ex- diated solution is similar to that of nitrosyl- traction, the aqueous phase was treated with ruthenium trinitrato complex containing small 1 ml of hydrogen peroxide and aged as men- amounts of nitro complex, and ruthenium in tioned above. the solution can be eliminated by the same procedure mentioned in the nitrosylruthenium nitrate experiment. 3. Others (Effect of Diverse Materials) Several 3 M nitric acid solutions containing simulated fission and corrosion materials were prepared by dissolving definite amounts of each metal nitrates. The metals and their approximate concentration (mg,ml-1) were as follows : Cs(800), Mo(500), Ba(400), La(400), Ce(200), Sr(200), Fe(200), Cr(30), Ni(20). In the case of molybdenum, sodium mol- ybdate was employed. By used of those solutions, extraction tests were performed and it was found that these materials do not inter- fere with the proposed method. Fig. 6 Removal of Ru from In the presence of large amounts of uranium [Ru (NO) (NO2)4OH]2- HNO3 solution (as UO2+2more than 100 mg,ml-1, determination of micro amounts of ruthenium is difficult, The behavior of ruthenium complexes is therefore, quantitative information about the quite curious, and the mechanism of the re- ruthenium elimination could not be obtained. actions described here have not been thoroughly But, in this case, the phenomenon of ruthe- investigated. nium extraction into n-paraffin oil was Development of a simple method to convert observed. such stable ruthenium complexes to the com- The influences of plutonium and technetium pounds containing easy extractable ruthenium have not been examined because of limited would be potentially beneficial. laboratory facilities. (g-Irradiated Ruthenium Nitrate) (Corrosion-resistance Materials) The chemical species of ruthenium in the The ceric nitrate solution in nitric acid is actual aqueous solution in the PUREX Process corrosive even for stainless steel which is are not clearly understood. commonly used for the PUREX Process equip- Experiments using the actual solution are ment. Therefore, for the extraction vessel, needed in order to confirm the usefulness of special materials having better corrosion

57 364 J. Nucl. Sci. Technol., resistance against the solution are necessary. Japan Atomic Energy Research Institute for At present, results of preliminary corrosion their kind discussion through the experiments. tests show that zirconium and titanium- The author gratefully acknowledges the valu- tantalum alloys(6) seem to be the most appro- able gifts of many ruthenium compounds by priate materials for the extraction vessel in Nippon Engelhard Co., Ltd., ceric nitrate by the engineering scale. On the other hand, Daiichi Kigenso Kagaku Kogyo Co., Ltd., paraffin oil containing ruthenium dioxide n-paraffin oil by Nisseki Plastic Chemicals suspension has not have any problems with Co., Ltd., pumps by Iwaki Co., Ltd. and other corrosion. equipment by Kobayashi Rikakikai Co., Ltd. The author also thanks Mr. I. Matsuda, a W. CONCLUSION lecturer of Ibaraki University, for his kind Fundamental experiments have been per- correction of English. formed to remove ruthenium, one of the most troublesome nuclides in the nuclear fuel - REFERENCES- reprocessing process. (1) SEDDON, E. A., SEDDON, K. R.: "The Chemistry The basis of the developed method is of Ruthenium", (1984), Elsevier Amsterdam. simple and needs only three main agents ; (2) MCKIBBEN, J. M.: Chemistry of the Purex process, Radiochim. Acta, 36, 3 (1984). n-paraffin oil, ceric nitrate and hydrogen (3) FLETCHER, J. M., et al.: Nitrato and nitro com- peroxide. n-paraffin oil is a commonly util- plexes of nitrosylruthenium complexes, J. Inorg, ized solvent in the PUREX Process, and ceric Nucl. Chem., 1955, Vol. 1, 378~401 . nitrate does not seem to affect the process, (4) FLETCHER, J. M., et al.: Nitrosylruthenium ni- because cerium is a main fission nuclide and trato complexes in aqueous nitric acid, J. Inorg. Nucl. Chem., 12, 154 (1959). is already present. Hydrogen peroxide is (5) WALLACE, R. M. : The composition of some ni- readily decomposed and gives no troublesome trato nitrosylruthenium complexes, J. Inorg. residues. Chem., 20, 283 (1961). Though it is not so easy to enlarge the (6) NAKAMURA,Y. : Private communication, (1979); technique to large engineering scale, the FURUYA, T., et al.: Corrosion resistance of zirconium and titanium alloy in HNO3 solutions author heartly expects to use this method , ANS Mtg. Proc. Fuel Reprocessing and Waste practically in the actual nuclear reprocessing Management, Aug. 26~29, Jackson Wyoming , process. USA, (1984). (7) "Control of Semivolatile Radionuclides in Gase- ACKNOWLEDGMENT ous Effluents at Nuclear Facilities", IAEA Tech. The author wishes to express here great Rep. Ser. 220, (1982). (8) SATO, T.: Synthesis of disodium hydroxotetra- appreciation to Mr. I. Sakasai, a student of nitro-nitrosylruthenate, J. Radioanal. Nucl. Ibaraki University, for his sincere assistance, Chem., Lett., 104, 151 (1986). Mr. Y. Ohta, President and Mr. J. Toya, (9) MOTOJIMA, K., et al.: Determination of traces Director of Johoku Chemical Co., Ltd. and of ruthenium by addition of cerium (IV) and Mr. K. Tatenuma, Head of Research Institute atomic absorption spectrometry, Anal. Chim. Acta, 183, 217 (1986). of Kaken Co., Ltd., also including their staff, (10) KODA, Y.: Determination of radioruthenium for their helpful supports and Drs. T. Adachi, using a polyethylene film, J. Radioanal. Chem. Z. Yoshida, E. Akatsu and Mr. H. Takeishi, 6, 345 (1970).