Determination of low b -activity of 99-technetium in the bentonite leachate

1. Institute of Chemical Technology Prague, Department of Analytical Chenistry, Technická 5, 166 28 Prague 6, Czech Republic LiquiScint 2004 Determination of low b-activity of 99-technetium in the bentonite leachate H. Vinšová, P.Večerník, V. Jedináková-Křížová

2. LiquiScint 2004 • Radionuclide of 99Tc (T1/2 = 2.1·105 years) fission product of spent nuclear fuel • Aerobic conditions TcO4- • Anaerobiccondtions TcO2 or TcO(OH)2 Fig 1.: Eh-pH diagram of Tc

3. 2FeS + 5H2O + 1/2O2 2Fe(OH)3 + 2H2S (1) 2TcO4- + 7H2S Tc2S7 + 8H2O (2) LiquiScint 2004 Suitable modificators for the uptake of 99TcO4- FeS reduction of TcO4- to insoluble TcO2.nH2O anaerobic conditions aerobic conditions • Fe • reductive conversion of Tc(VII) to the lower oxidation state Tc(IV) • sorption of the reduced species onto hydroxide surfaces • precipitation of Tc with iron hydroxides on surfaces • Activated carbon => sorption

4. GOAL LiquiScint 2004 Liquid scintillation method Sorption of technetium on the bentonite mixtures

5. LiquiScint 2004 Sorption of 99TcO4- on the bentonite mixtures Solid phase: bentonite R (0.315 - 0.8 mm) + Fe (powder) bentonite R (0.315 - 0.8 mm) + FeS (powder) Liquid phase: bentonite water (14 days leaching of bentonite in deionized water on the s:l phase 1:10 ratio) • Sorption experiment: • bentonite R 0.7 g + modificator (Fe, FeS) + 7 mL bentonite water + 50 mL 99TcO4- (c=0.0111 mol/L, A= 740 kBq/mL) • shake (70 period/min.) • centrifugation (20 min./2000rpm) • bentonite leachate

6. LiquiScint 2004 • Bentonite leachate: • Eh, pH,specific conductivity • b-activity 99Tc • b-scintillation detector NaI (Tl), SPF 35(evaporation residue - 1 mL of bentonite leachate) • liquid scintillation counter • ( 2 mL of bentonite leachate) Fig. 2.:b-scintillation detector NaI (Tl), SPF 35 Fig. 3.: Triathler

7. Liquid scintillation method LiquiScint 2004 Scintillation cocktail study Scintillation volume and vials study Determination of uncertainty of sample dilution

8. Scintillation cocktail study Aqua Light Ultima GOLD XR Table 1.: Comparison of selected scintillation cocktails; reaction mixture: 2 mL of scintillation cocktail + 2 mL of bentonite leachate labelled by 99Tc • Next experiments => scintillation cocktail Aqua Light LiquiScint 2004 ? Scintillation cocktail

9. Scintillation volume study Table 2.: The values of b-activity 99Tc measured in the different vials, Sample: b-activity of 99Tc in the aqueous solution 8 Bq/mL. • Optimum reaction mixture => 2mL of scintillation cocktail + 2 mL of bentonite leachate labelled by 99Tc LiquiScint 2004 • Suitable vials and amount of scintillation cocktail ? Fig. 4.: Tested vials for the measurement of studied samples

10. Determination of uncertainty of sample dilution (A0-A).V KD= A.m LiquiScint 2004 Initial value of b-activity 37 kBq/7mL => sample diluted 122.5 times A0 - total activity, A - activity of liquid phase, V - volume of liquid phase (mL), m - weight of solid phase(g) Expanded uncertainty => 16.7 mL Liquid scintillation counter expanded uncertainty of measurement =>2% b-scintillation detector NaI (Tl) expanded uncertainty of measurement =>5%

11. Liquid scintillation method LiquiScint 2004 Figure 5.: Example of liquid scintillation b-spectrum of 99Tc, reaction mixture: 2mL of Aqua Light + 2 mL of aqueous solution labelled by 99Tc

12. Kinetics of sorption 99TcO4- • 2 days of uptake => sorption yield over 99% • low value of b-activity 99Tc => LS method LiquiScint 2004 Figure 6.: Sorption kinetics of 99TcO4- on the different bentonite mixtures A) solid phase: bentonite mixture with Fe; mass ratio bentonite:Fe 10:1 B) solid phase: bentonite mixture with FeS; mass ratio bentonite:FeS 10:1.

13. Kinetics of sorption 99TcO4- LiquiScint 2004 Table 3.: Comparison of sorption yield and KD value on the different solid phases obtained by b-scintillation detector NaI (Tl) and LS cocktail Aqua Light for the sorption kinetics of 99TcO4- A) solid phase: bentonite mixture with Fe; mass ratio bentonite:Fe 10:1, B) solid phase: bentonite mixture with FeS; mass ratio bentonite:FeS 10:1. A) B)

14. Effect of ratio bentonite:additive on 99TcO4- sorption LiquiScint 2004 Figure 7.: Dependence of sorption yield of 99TcO4- for the different bentonite:additive ratios A) Solid phase - bentonite:Fe ratios 4:1 - 150:1, B)Solid phase - bentonite:FeS ratios 5:1 - 100:1, contact of both phases 8 days • bentonite:Fe 4:1 – 150:1 and bentonite:FeS 5:1-20:1 ratios => sorption yield of 99TcO4-> 90%

15. Effect of ratio bentonite:additive on sorption 99TcO4- LiquiScint 2004 Table 4.: Comparison of sorption ylield and KD value for the different solid phases obtained by b-scintillation detector NaI (Tl) and LS cocktail Aqua Light - study of the effect of different bentonite:additive ratios A) solid phase: bentonite mixture with Fe; B) solid phase: bentonite mixture with FeS. A) B)

16. CONCLUSIONS LiquiScint 2004 Liquid scintillation method Scintillation cocktail Aqua Light Optimum compositon of reaction mixture 2 mL of scintillation cocktail + 2 mL of bentonite leachate labelled by 99Tc Sorption experiments Suitable additives for the uptake of 99TcO4- are FeS and Fe Optimum ratios bentonite:FeS and bentonite:Fe 10:1, contact of both phases 1 day => sorption yield > 90%

17. LiquiScint 2004 Thank you for your attention !