Radiochim. Acta 2019; 107(5): 423–429 Parveen Kumar Verma, Rajesh Bhikaji Gujar and Prasanta Kumar Mohapatra* Understanding the recovery of Ruthenium from acidic feeds by oxidative solvent extraction studies https://doi.org/10.1515/ract-2018-3034 Received July 19, 2018; accepted December 31, 2018; published online February 11, 2019 Abstract: Ruthenium ( 106 Ru), a notorious fission product in nuclear reprocessing cycle, which gets partitioned at each step needs to be recovered. The recovery of Ru from acidic high level waste (HLW) is of great importance to the nuclear fuel cycle. Quantitative recovery of Ru was achieved from acidic feeds using oxidative trapping mechanism strategy where NaIO 4 was used as an oxidant to convert different species of Ru in acidic phase to RuO 4 while n-dodecane was used as trapping agent for RuO 4 . Stripping was attempted using NaOH and NaClO mixture. Attempt was made to optimize various parameters for 103 Ru extraction and stripping. 103 Ru tracer spiked simu- lated high level waste was used to understand the 103 Ru behaviour in actual waste. The composition of stripping solution (alkaline hypochlorite) was also optimized to have >95 % Ru into the aqueous phase in ca. 180 min. Keywords: Ruthenium, solvent extraction, high level waste, kinetics. 1 Introduction Ruthenium (mainly 106 Ru) is one of the most troublesome fission products produced in nuclear reactors running with MOX fuels [1–3]. The associated disadvantages of Ru is mainly due to volatile natures of its tetraoxide (RuO 4 ) [1, 4]. This has been one of the prime culprits in the recently reported ‘radioactive cloud’ spread across many parts of Europe. The RuO 4 , once formed at any stage of nuclear fuel reprocessing cycle, will evaporates and get deposit on the stainless steel lines on various streams as black precipitate of RuO 2 . Long time operations will cause more such depositions resulting in high amounts of γ ray dose in the close vicinity [3–5]. The estimated yield of various isotopes of Ru in a given nuclear reactor varies with the type of fuel, burn up and cooling time (Table 1). 106 Ru (and to some extent, 103 Ru) contributes to ∼9–10 % of total β-γ activity of the radioactive wastes [1]. The Ru complexes are also known for their unpredictable nature under the PUREX (Plutonium Uranium Redox Extraction) process conditions owing to the existence of the metal ions in variable oxidation states and existence of multi- ple species in aged nitric acid solution. The separation of Ru [major isotope: 106 Ru (t 1/2 : 368 d) which subsequenly disintegrates by beta decay to 106 Rh (t 1/2 : 29.8 s)] from the high level waste (HLW) will not only solve the problems associated with the reprocessing but also have some soci- etal applications, e.g. for eye cancer treatment [6]. Fur- thermore, the presence of Ru and other palladium group metals can cause problems in vitrification of the waste oxide by forming a separate phase [7]. The speciation of Ru in nitric acid medium is complex and studied in the past by several authors, the complexity in the speciation arises because of its variable oxidation states ranging from 0 to +8 in the waste streams [8–14]. In the nitric acid solu- tions, Ru primarily exists as the Ruthenium(III) nitrosyl species (RuNO 3+ ) which forms nitro and nitrato complexes as well. Boswell and Soentono have identified >9 different RuNO 3+ species in nitric acid solutions aged for a period of over 1 year [9]. Several authors have also reported the presence of Ru(IV) as polynuclear hydroxyl species such as Ru 4 (OH) 12 4+ [9, 10]. The pseudo octahedral complex of Ru dissolved in nitric acid are reported to have general formula of [RuNO(NO 3 ) a (NO 2 ) b (OH) c (H 2 O) d ] (3-a-b-c)+ (a, b, c, and d coefficients are such that a + b + c + d = 5) which mainly depends on the prevalent chemical conditions such as pH, nitrate/nitrite concentration and also may be on the aging of the solution [15]. The presence of RuNO 3+ as multiple species and its different extraction behavior towards regents used in fuel reprocessing complicates Ru separation in the PUREX and subsequent processes [1]. The extraction and speciation of RuNO 3+ was attempted by several authors and had been a topic of active research since long [1, 16–23]. Ru chemistry in the nuclear fuel cycle, its separation and recovery were dis- cussed by Swain et al. [1]. Lefebvre et al. have studied the Ru speciation in solvent extraction conditions by TBP/TPH system using spectroscopic methods such as FTIR and *Corresponding author: Prasanta Kumar Mohapatra, Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085, India, Fax: +91-22-25505151, E-mail: mpatra@barc.gov.in Parveen Kumar Verma and Rajesh Bhikaji Gujar: Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085, India Brought to you by | University of Western Ontario Authenticated Download Date | 5/8/19 2:05 PM