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XXV ARS SEPARATORIA – Toruń, Poland 2010 291 SEPARATION TECHNIQUES FOR PRETREATMENT OF INSTUTIONAL RADIOACTIVE WASTES Marian HARASIMOWICZ, Paweł BIEŁUSZKA, Grażyna ZAKRZEWSKA-TRZNADEL Institute of Nuclear Chemistry and Technology, ul Dorodna 16, 03-195 Warsaw Abstract The pretreatment techniques for radioactive wastes by filtration with various depth filters, precipitation, sorption of radioactive ions on hexaferrocyanides, polyacrylic acid (PAA) and zeolite 4A was investigated. The aim of the experiments was removal of the radioisotopes of 137Cs, 60Co and 85Sr already in the beginning of decontamination process - before sorption or membrane separation. The value of decontamination factor Df was determined for some radioactive model solutions and filters used as the barrier for the impurities which could damage the UF, NF and RO membranes in next stages of decontamination process. 1. INTRODUCTION The nuclear industry and all users of radioactive materials produce a wide range of liquid wastes. Many of these wastes need to reduce the quantities of radioactive and non-radioactive contaminants to levels, which allow them to be safely discharged according to international and national regulations. The conventional technologies for removal of various radioisotopes from wastewater include chemical precipitation, ion exchange, adsorption, membrane processes and evaporation. The wastes from storage tanks are pumped to the filtering unit. Battery of various filters is used to separate fine particulates, that were not settled in sedimentation process – rest of the sludge, sand slurries and solid particles. Radioactive wastes in storage tanks of the Institute of Atomic Energy at Świerk/Otwock near Warsaw can be treated as low-saline solutions with low-level of radioactivity emitted by beta + gamma radioisotopes: their total salt concentration is 0.1-2.0 g/L and specific activity is 103-105 Bq//L. Large part of radioactivity (60-70%) is emitted by 3 radioisotopes: 137 Cs, 60Co and 85Sr - therefore it is favourable to remove them already in the initial stage of decontamination process. 292 XXV ARS SEPARATORIA – Toruń, Poland 2010 2. EXPERIMENTAL The solutions used in the experiments were prepared by use following salts: CsCl, Co(NO3)2·6H2O, SrCl2·6H2O, and CuSO4. The ion concentration in the feed solutions and the filtrates obtained as a result of applied decontamination process, were analysed by measuring of electrical conductivity by means of a conductivity-meter WTW Multiline P4 and spectrophotometer DR/2000 HACH with the filter RC-45/25. The results for 6 type of hollow filters are shown in Table 1. Following agents were applied: zeolite A4 Na12[(AlO2)12(SiO2)12]·xH2O pulver 5 µm, activated (Aldrich, 1301-78-9), and K4[Fe(CN6)]·6H2O. For preparation of radioactive solutions the following radioisotopes were used: 137CsCl in 0.1% HCl solution, 137Cs: T1/2 = 30,2 years, 60 Co(NO3)2·6H2O in water solution, 60Co: T1/2 = 5.24 years 85 SrCl2·6H2O in water solution, 85Sr: T1/2 = 65 days. The radioactivity of the feed and filtrate samples were determined using A-22p STANDARD-70 Analyzer (made by POLON, Warsaw) connected to SSU spectrometric photo-multiplayer with SKG crystal NaJ/Tl activated (Koch Leb. Ltd, England). The calibration of this measuring system using standard-samples of 137Cs, 60Co and 85Sr makes possible the recalculation of the pulses/sec for 20-mL sample to specific activity in Bq/L. The radioactivity of the feed was as follows: 137Cs - 8.6 Bq/L, 60Co -15.2 Bq/L and for 85Sr - 6.2 Bq/L. Activity of the filtrates was 10- to 200-times lower. *) 137Cs removal As the sorbent of Cs+ ions the slurry of Cu-ferrocyanide was used. The slurry can be precipitated in situ from K4[Fe(CN6)]·6H2O and CuSO4 to obtain a optimal conditions for the sorption of radioactive ions. After 24h seasoning the solution was filtrated and a specific activity of the filtrate was measured. The same experiment was carried out with Ti-ferrocyanide slurry. **) 60Co removal First experiment was carried out by use of poly(acrylic acid) PAA 35% sodium salt water solution (Mw 250 000 D). For the second experiment the portion of zeolite A4, activated 4h at 400 0C, was added. The saturation level was reached, when the proportion of zeolite concentration to Co2+ ions was 20 : 1. ***) 85Sr removal The experiments on removal of strontium ions were carried out by use of the same sorbents, as for caesium ions removal (Cu-ferrocyanide) and cobalt ions removal (zeolite A4, Cl3Fe·6H2O). The results of the experiments are collected in Table 2. XXV ARS SEPARATORIA – Toruń, Poland 2010 293 3. RESULTS AND DISCUSSION All cylindrical depth filters used as a first step of radioactive waste pretreatment play very important role: they should be a barrier for the impurities which can damage the next stage elements – the membranes or ion-exchange sorption columns. As shown in Table 1., PS-filters eliminate sand, sludge, rust, small solid particles, coal dust and micro-organisms. They can work at very small operating pressure (< 0.1 bar), therefore by their assembling at the input of the pressure pump delivering the wastes to UF or NF/RO membrane modules is possible. Mostly effective filter battery can be built from 3 types of the filters: PS + (BL, IR or GAC) + CERAM. Table 1. Characteristics of cylindrical depth filters Filter type Pore size Qf [L/min] Material Substances retained PS-50 > 50 µm, 20 Polypropylene foam PS-10 > 10 µm 8 Polypropylene foam BL 10-5 µm 8 Sintered active coal GAC 50-10 µm 8 Granulated activated coal IR 100-50 µm 5 Mixed sorption bed 3 Sintered ceramic pulver Sand, sludges, rust, coal dust, bacteria Sand, rust, coal dust, other slurries Chlorine, organic compounds, Pb, heavy metals Chlorine, organic compounds, heavy metals Ferrous, ferric and manganese hydroxides All solid impurities > 0.3 µm CERAM 0,3 µm Although the role of these filters in decontamination process is small, some of them showed sorption property: specific activity of the solution obtained from BL, GAC and IR filters was lower than for feed stream, because 1-7 % of ions was absorbed in the filtering bed (Df = 1.01-1.7). Filtration of the solutions obtained in precipitation and adsorption processes was carried out by Whatman NY 0.2 µm syringe filters. The best results for 137Cs+ ions removal was obtained using sorption column with polymeric-Ti-ferrocyanide spherical granules: value of Df =173 is about 4-times higher than for experiment with Ti-ferroceanide slurry and 5-times for Cu-ferrocyanide slurry. This last reagent was used for radioactive 294 XXV ARS SEPARATORIA – Toruń, Poland 2010 strontium removal: the value of Df = 23 shows, that the sorption of ions was by 40% lower than 137Cs+ ions. 85 Sr2+ Table 2. Decontamination of model solutions in adsorption/precipitation Solution CsCl Co(NO3)2·6H2O SrCl2·6H2O Feed Filtrate Feed Filtrate Feed Filtrate Reagent Cu-ferrocyanide PAA-Na/35 Cu-ferrocyanide A [kBq/L] 8.68 0.235 15.2 1.05 5.87 0.25 Df 37 14 23 Reagent Ti-ferrocyanide Zeolite A4 Zeolite A4 A 8.68 0.195 15.2 0.20 5.87 0.16 [kBq/L} Df 45 75 37 Reagent *) column with TFC **) Cl3Fe·6H2O **) Cl3Fe·6H2O A 8.68 0.050 15.2 1,15 5.87 0.84 Df 173 13 7 *) polymeric-Ti-ferrocyanide granules for sorption column, **) after 2h, pH=8.5 reRr Zeolite A4 can be used for cobalt ions removal – its sorption capacity is 5-times higher than PAA-Na/35 or Cl3Fe·6H2O. 4. CONCLUSIONS The pretreatment processes tested in the experiments presented above decrease the concentration of radioactive ions of cesium, cobalt and strontium 7 to 75-times. In the beginning of the radioactive waste decontamination system the filter battery can be assembled to prevent the next stages from various damages caused by particulate impurities. Acknowledgement This research has been supported by the National Center for Research and Development (NCBiR). Research Grant No. R05-058 06/2009. REFERENCES [1] R. G. Gutman et al., Active Liquid Treatment by a Combination of Precipitation and Membrane Processes, Rep.AERG-G-3777, UKAEA, Harwell, UK, 1986. [2] IAEA, Technical Report No.337. Chem Precipitation of ARW, Vienna, 1992.