Determination of 126 Sn in nuclear wastes by using TEVA resin

Transcription

1 Determination of 126 Sn in nuclear wastes by using TEVA resin Ján Bilohuščin, Silvia Dulanská, Veronika Gardoňová Univerzita Komenského, Prírodovedecká fakulta, Katedra jadrovej chémie, Mlynská dolina, Bratislava, Slovenská republika; Abstrakt This paper describes the application of the test results for 126 Sn determination in radioactive concentrate by using TEVA Resin. The behaviour of 113 Sn on TEVA Resin column was studied. Before loading on TEVA Resin column, tin was precipitated as yellow sulfide precipitate SnS 2, by adding (NH 4 ) 2 S in mildly acidic solution employing following reaction [SnCl 6 ] 2- (aq) + 2S 2- (aq) <==> SnS 2 (s) + 4H + (aq) + 6Cl - (aq). The influence of hydrochloric acid on the separation and chemical yield of tin was tested. Evaporated concentrates from NPP Mochovce were used for testing of tin separation. Key words: Sn-126 separation, TEVA Resin, nuclear waste Introduction and the formula of the main aim Evaluation and efficient separation of radioactive fission and activation products from nuclear reactor operations are crucial in order to isolate harmful radiation from living environment and human population. While the most dangerous high-yield fission products such as 137 Cs and 90 Sr have relatively short half-lives, we can not forget on the other, longlived elements, created in reactor s active zone like 239 Pu, 94 Nb, 129 I, 93 Zr, 126 Sn and many others. In our previous works we were determining 239 Pu [1], 93 Zr [2], 94 Nb [3] and 90 Sr [4] using various nuclear wastes which originated from Slovak NPPs. In our new experiments we focused on determining 126 Sn, that is a long-lived beta emitting radionuclide with a half-life of approximately years. As a fission product, the main natural production mechanism of 126 Sn is through spontaneous fission of 238 U and the abundance ratio of the 126 Sn/Sn in the earth s crust is less than Artificially produced 126 Sn has entered our environment through nuclear activities and may also be released from reprocessing plants what may locally lead to strongly enhanced 126 Sn concentrations [5]. Kinard and others [6] attempted to isolate 126 Sn with ion-exchange procedure from specific sludge and to use gamma pulse height analysis to count the 126 Sb (T 1/2 = 12.4 d) daughter that is in secular equilibrium. This procedure required a large decontamination factor from 137 Cs. In the separation procedure, a stable Sn carrier is added, the solution ionexchanged from 9M HCl and the Sn eluted with 0.1M HCI. The separated sample could then be analyzed by direct liquid scintillation counting of 126 Sn or by gama-pha after the 126 Sb has grown in. 901

2 Zhang and Yang [7] used anion exchange method to no-carrier-added separation of 126 Sn from high-level liquid waste (HLLW). In their work, polyvinyl trimethylamine type resin ( mesh) is as anion exchanger to separation 126 Sn. The resin column is made of glass (length L=100 mm, inside diameter R=4 mm), and loaded 1 ml resins. A flow rate of about 0.15 ml per minute is maintained by adjusting the height of a storage container. The column is equilibrated with 6 mol/l HCl solution; the solution containing 126 Sn is adjusted to 0.5 ml 6 mol/l HCl and passed through the column, then the column is washed with 6 ml 6 mol/l HCl. Finally the column is eluated with 10 ml 1 mol/l HCl and the 10 ml 10 mol/l HNO mol/l HCl. The chemical recovery of 126 Sn is more than 90%. During our experiments we employed ion exchange TEVA Resin for capturing Sn 4+ cations in various nuclear wastes obtained from Mochovce NPP. Materials and methods TEVA Resin ( µm or µm) was supplied by Eichrom Industries. 113 Sn (certified solution from Czech Metrology Institute No OL-657/12) was added to every analyzed sample as a chemical yield monitor. All other chemicals used were commonly available analytical grade acids and chemicals. Tin separation method, influence of acid concentration and tin carrier Influence of tin carrier of the 113 Sn separation on sorbent TEVA Resin was tested. 10 ml of 7M HCl was traced with 113 Sn, and from 1.38 to 15 mg of tin carrier was added to the sample. This sample was loaded onto 0.3g of TEVA Resin preconditioned with 10ml of 7M HCl. Flow rate was apptoximately 0.8ml/min. The column was washed with 5ml volumes of 7M HCl and 113 Sn was eluted from the column with 10ml of 2M HNO 3. 10ml of HCl acid of different concentrations (from 0.1 to 11.27M) were traced with 113 Sn and 3mg of stable tin carrier were added. Then the solution was loaded onto 0.3g of TEVA Resin preconditioned with 10 ml of HCl (which was of the same concentration as acid used for each specific sample). Flow rate was apptoximately 0.8ml/min.. The column was washed with two 5ml volumes of HCl, also of the same concentration as sample. 113 Sn was eluted from the column with 10ml of 2M HNO 3. The eluted fraction was counted on a gamma detector at 392 kev for 113 Sn. 902

3 Results and discussion The results are presented in Figure 1. During experiments, 0.3g of sorbent was used. It was found out that Sn separation is quantitative up to 7 mg of tin carrier. With 7-14 mg of stable tin, the 113 Sn recovery decreased from 86.8 % to 72.5%. For further tests, 7 mg of stable tin was chosen as a carrier. Table 1. Influence of tin carrier on recovery. Concentration of Sn 4+ R( 113 Sn) ± U (mg/ml) (%) 1,376 85,8±1,8 2,060 98,3±2,2 3,440 78,6±3,5 4,128 82,3±4,2 6, ,5±2,1 8,940 78,8±3,9 12,000 62,9±2,9 13,800 65,7±3,2 Sn in HCl medium forms a very stable anionic SnCl 6 complex at HCl concentrations. The behaviour of tin in HCl acid is shown in Figure 2. Tetravalent tin shows a maximum uptake in the wide region of 0.1M to 10M hydrochloric acid. When the concentration of HCl acid is higher than 10M, recoveries decrease below 81 %. Table 2. Influence of HCl concentration on recovery Concentration of HCl R( 113 Sn) ± U (mol/dm 3 ) (%) 0,1 87,5±3,1 0,25 103,0±2,2 0,5 109,7±2,1 7 88,1±3,2 8 96,9±2,4 9 89,5±3, ,1±3,6 11,27 80,6±4,2 Evaporated concentrates of various volumes (3 30 ml) from NPP Mochovce were used for testing of tin separation method. Before application on TEVA Resin, samples were adjusted with Sn and S 2- precipitation method. Stannic sulfide can be precipitated quantitavely by addition of H 2 S to an acid solution of stannic salt. This cannot, however, be accomplished 903

4 in strong HCl solutions because of formation of SnCl 2-6 complex. For SnS 2 precipitation we used (NH 4 ) 2 S (>= 20% - < 25%). Concentrate samples were simply adjusted with 11.27M HCl on concentration value under 0.5M required for creation of yellow SnS 2 precipitate. Precipitate was centrifuged and dissolved with warm concentrated HCl or warm mixture of HNO 3 and HCl. In case of dissolution with mentioned mixture, further evaporation and dissolution with HCl is needed. Then, acidity of the sample is lowered by addition of deionized water to 1M HCl concentration level. The sample is ready to be added on TEVA Resin column. The actual column with TEVA Resin was conditioned with 1M HCl, which is the HCl concetration of adjusted sample after dissolving precipitate with warm concentrated HCl or with aqua regia and evaporating. 126 Sn was eluted from the column with 10ml of 2M HNO 3. The activity of 126 Sn was measured by gamma spectrometry gamma rays (666.3 kev) of its daughter nuclides 126m Sb. All measured activities were below minimum detectable activity. Conclusion During recent years it was popular to apply sorbents of extraction chromatography for many reasons including easy handling, reduction of employed reactants and thus reduction of time/cost factor while obtaining similar or even better recoveries of radiochemical analasys. We used TEVA Resin to determine 126 Sn from real nuclear waste samples originating from Slovak NPP Mochovce. Creation of SnS 2 precipitate gave us an elegant way to bypass purifing steps required when analysing such a complex matrices as nuclear wastes are. Atestation of hydrochloric acid and tin carrier concentrations was also successfull. Potential of TEVA resin application determination of 126 Sn in combination with various sorbents could be forseen in employing continuous sequential analysis for other radionuclides present in nuclear wastes. References [1] Dulanská S., Remenec B., Mátel Ľ., Durkot E. (2012) J Radioanal Nucl Chem, 293, p.81 [2] Dulanská S., Remenec B., Gardoňová V., Mátel Ľ. (2012) J Radioanal Nucl Chem, 293, p.635 [3] Dulanská S., Remenec B., Mátel Ľ., Galanda D. (2011) J Radioanal Nucl Chem 288, p. 705 [4] Dulanská S, Remenec B, Gardoňová V, Mátel Ľ. (2013) J Radioanal Nucl Chem 295, 904

5 p.907 [5] Hongtao S., Shan J., Ming H., Kejun D. et al. (2011) Nuclear Instruments and Methods in Physics Research 269, p.392 [6] Kinard WF., Bibler NE., Coleman CJ., Dewberry RA. (1997) J Radioanal Nucl Chem 219, p.197 [7] Zhang S., Yang L., Guo J. et al. (2008) Anal Chim Acta, 391, p