Iodine-129 in Cigar Lake uranium deposits by Pyrohydrolysis and AMS
Hamed Mozafarishamsi1,3, Ian D. Clark1,3, Liam Kieser2,3, Matthew N. Herod1,3
1Department of Earth and Environmental Sciences, University of Ottawa, 25 Templeton St,, Ottawa, Canada, 2Department of Physics, University of Ottawa, 25 Templeton St, Ottawa, Canada, 3A. E. Lalonde AMS Laboratory, University of Ottawa, 25 Templeton St, Ottawa, Canada
129I is a spontaneous fission product in the Cigar Lake uranium deposits, northern Saskatchewan, Canada, can be measured using accelerator mass spectrometry (AMS). It can provide useful information on its mobility in the natural geosphere as an analogue to a nuclear waste repository. The iodine was extracted from rock samples and then measure by AMS. Extracting iodine from rock samples for inductively coupled plasma mass spectrometry (ICP-MS) and AMS analysis can be done using several methods. Of these, pyrohydrolysis is the most widely used.
Here we use pyrohydrolysis with an alkaline solution to trap the iodine released from the sample during combustion. We used two different trap solutions with varying concentrations. These solutions were NaOH (0.4M, 0.3M, 0.2M, 0.1M, and 0.05M) and Tetramethylammonium hydroxide TMAH (1.5%, 1% and 0.5%). Also, we used NaHSO3 (0.02M) as a reducing agent. Iodine recovery is quantified using iodine-125 as a yield tracer. We observed that TMAH 1.5% has the highest Recovery (84%) and TMAH 1%, and 0.5% had slightly lower recoveries (83% and 81%). On the other hand, for NaOH solutions, NaOH 0.4M, NaOH 0.3M, and NaOH 0.2M had the highest recoveries (80%), but NaOH 0.1M and NaOH 0.05M had lower recoveries of 76% and 68%. The rock samples were also mixed with vanadium pentoxide (V2O5) as an oxidizing agent to aid in sample combustion. We examined different ratios of V2O5/sample by weight (0, 0.25, 0.5, 1, 2, 3, 4, and 5) and observed increased recoveries with increased V2O5. We obtained the highest Recovery of 88% for a V2O5/sample mass ratio of 5.
By using TMAH 0.5% as a trap solution and using ratio of three for V2O5/sample, the iodine was extracted from rock samples. An iodine recovery of about 85%-95% was obtained through pyrohydrolysis method. 127I was measured with ICP-MS and 129I was measured with AMS. Along a ~450m rock profile at Cigar Lake study site, the 129I concentration ranged from 1.2E+08 atoms/kg in low uranium concentration area to 1.4E+13 atoms/kg in high uranium concentration zone. The 127I concentration varied from 77ppm to 850ppm in the same profile.
Keywords: Iodine-129; Accelerator mass spectrometry (AMS); Uranium; Pyrohydrolysis
Hamed Mozafarishamsi is a Ph.D. candidate at the University of Ottawa studying earth and environmental sciences under the supervision of Dr. Ian Clark and Dr. Matthew Herod. He started his Ph.D. in 2017, focusing on the background level of Iodine-129 in different environments. Currently, he is working on 129I concentration in Cigar Lake uranium deposits as a natural analogue to a nuclear waste repository.