Radiochemical Studies of 99Tc and 14C in Environmental Samples

Ihsanullah. (1991) Radiochemical Studies of 99Tc and 14C in Environmental Samples. PhD thesis, University of Glasgow.

Abstract

99Tc, the most crucial technetium isotope enviromentally, is a pure, weak beta-emitter with maximum energy 0.292 Mev and specific activity of 630 kBq mg-1. 99Tc is mainly released to the environment as waste discharge from nuclear reprocessing facilities and from previous nuclear atmospheric detonation tests and nuclear accidents. Because of its long half-life and current interest in the collective long-term dose, it is necessary to establish a knowledge of the behaviour of 99Tc in the environment for complete assessment of the impact of nuclear industry. Effort has been made to develop a radiochemical method which can be applied to environmental samples in order to gain information by obtaining more accurate and precise data to assess the impact of Tc releases on man. Attention has been focussed for this purpose on inductively coupled plasma-mass spectrometry (ICP-MS), a new powerful technique for elemental and isotopic analysis which combines the remarkable characteristics of the ICP for atomizing and ionizing injected samples with the sensitivity and selectivity of mass spectrometry. Hence, with the advent of ICP-MS as a highly sensitive method of detection of long-lived radionuclides, this has been investigated and successfully applied to 99Tc analysis. Technetium from large volume water samples was concentrated by anion exchange (Dowex 1-X8) resin and recovered by elution with 12M HNO3. Biological (algae) samples were dissolved in 9M HNO3 by refluxing, followed by filtration, and, as with the water sample eluate, evaporated and redissolved in 2M H2SO4. Soil (silt) and sediment samples were ashed at 600C, treated with 2M H2SO4 containing 2 cm3 H2O2 added to the ash, and finally the solution was filtered to provide a clear solution. Since ICP-MS is based on the measurement of the mass of an element, isobaric interferences at mass 99 must be eliminated as far as possible. For 99Tc, 99Mo is not a problem as this radionuclide decays with a half life of 67 hrs and after few days will have gone. The other isobaric nuclide is 99Ru. 99Ru is stable and 12.7% abundant in natural ruthenium which occurs in the environment and is a key problem. Two methods were used for the decontamination from Ru. The first method consisted of a series of steps, involving valency control with H2O2 and NaCIO, boiling from H2SO4 solution and solvent extractions with CCl4, cyclohexanone and triisooctylamine(TIOA)-xylene. The second method included: the boiling of H2SO4 media, NaOH elution and TIOA-xylene extraction. After treatment to remove Ru, the resulting solution was adjusted to pH 7 and passed through Dowex 1-X8 resin in order to get a clear solution in 2% HNO3 with a salt concentration of <0.1%. 99mTc and 95mTc tracers were used to determine radiochemical yields; 99Tc for method calibration, Ru standard solution to check decontamination and Rh for monitoring the response of the ICP-MS instrument. Two reference materials i.e. IAEA marine algae and BNFL seaweed (Fucus vesiculosus) were analysed for intercomparison studies. The % chemical yield for water samples was found to be 65+/-15, for biological material (algae) 70+/-11 and for soil (silt) samples 65+/-15. The limit of detection of 99Tc by ICP-MS was 0.004 ppb. Various types of samples were analysed in order to verify the new ICP-MS procedures and to determine environmental 99Tc levels as follows: in order to check the liquid discharges and dilution of 99Tc from the Sellafield Reprocessing Plant, Fucus vesiculosus was collected from many relevant coastal sites as it is a well known bioindicator for various radionuclides including Tc; the Ascophyllum/Fucus ratio values were measured and were in good agreement with the literature values; the levels of 99Tc in Porphyra and water samples were below the detection limit; moss and lichen samples were collected from an undisturbed area 1 km east of the Chernobyl site, but the 99Tc concentrations were below the limit of detection. Another problem addressed in the present work was the measurement of environmental 14C contamination in the air from a low power (300 kW) training reactor namely, that at the Scottish Universities Research and Reactor Centre, East Kilbride near Glasgow. For analysis, air from the reactor stack, the reactor hall and two sites in the immediate vicinity (downwind on the east side of the reactor stack) was passed through 0. 1M barium carbonate to collect CO2 as precipitated barium carbonate and the carbon converted to benzene. 14C was measured in this form by liquid scintillation counting using a Tricarb 2260XL instrument (Packard Instrument Company). All the four sites showed enhancement when the reactor was running. The reactor stack and the reactor hall showed some enhanced activity even when the reactor was shut down. The 14C released from the reactor corresponded to = 12 MBq y-1.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Nuclear chemistry, Nuclear physics and radiation, Nuclear engineering, Environmental health
• Date of Award: 1991
• Depositing User: Enlighten Team
• Unique ID: glathesis:1991-78277
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 28 Feb 2020 12:09
• Last Modified: 28 Feb 2020 12:09
• URI: https://theses.gla.ac.uk/id/eprint/78277