Analysis and processing of samples for a carbon-14 monitoring program at a radioactive waste storage site

Abstract

A monitoring program was undertaken to determine the levels of airborne radioactive ¹⁴CO₂ in an operating waste management area, and it its vicinity. As a part of this program, alkaline microporous pellets, recently developed, were used to sample the ¹⁴CO₂ in air near waste storage structures and near the waste management area. These pellets were never fully characterized for their ability to capture high levels of ¹⁴CO₂, and the processing and analysis needed to be improved to provide data on recoveries and consistencies, for an eventual method validation, with independent calibrations and standards. The sample analysis scheme also had to accommodate ¹⁴CO₂ levels varying from near the natural background (250 Bq kg⁻¹ C), to potentially three to four orders of magnitude above this value, near the wastes. The porous alkaline solid pellets were used for the passive capture of airborne ¹⁴CO₂ over a period of weeks, to a few months. The pellets were processed to release the captured CO₂ (¹⁴CO₂ and ¹²CO₂) into a NaOH solution, which was subsequently analyzed by liquid scintillation. Processing of the pellets yielded a ¹⁴C recovery of 96.0 ± 4.2% and a lower, but consistent total carbon recovery, i.e., 85.9 ± 2.7 and 86.9 ± 2.6%, for procedural blanks and standards, respectively. The detection limits for the pellet sampling and processing was sufficient to reach environmental levels. For the higher levels of ¹⁴CO₂ and for ‘spot’ sampling, we also used air samples, pumped into a NaOH solution to trap the ¹⁴CO₂. These NaOH solutions were counted directly for ¹⁴C, also by liquid scintillation. The method limits of this latter technique, although much higher than for pellet samples, also achieved the performance objective for detecting airborne ¹⁴CO₂. Both sampling and processing techniques, when used together, provided sufficient flexibility to be used for low (environmental) levels and high levels, near the wastes.