Determination of carbon-14 levels in heavy water and groundwatersPresented at SAC 99, Dublin, Ireland, July 25–30, 1999.© Copyright Government of Canada.

Abstract

The radionuclide carbon-14 is a by-product of nuclear reactors, produced predominantly by the neutron activation of ¹⁷O of the water molecule. The chemical form of carbon-14 in heavy water is bicarbonate (D¹⁴CO₃⁻), and the total inorganic carbon levels (TIC) are expected to be 20–30 μg L⁻¹. A technique was developed primarily for ¹⁴C determination in reactor heavy water (ultra-pure water containing up to ≡10¹² Bq L⁻¹ of tritiated water), and is also applicable to analysis of contaminated groundwater. The method consists in acidifying the sample to convert the bicarbonate into CO₂, which is swept by a stream of nitrogen to an absorbent NaOH solution. An aliquot of this NaOH solution is measured by liquid scintillation for counting ¹⁴C. Isolation of the ¹⁴C from the reactor water matrix was excellent, as tritium removal factors of up to 10⁹ were obtained. The recoveries of simulated ¹⁴C solutions were 94.2 ± 6% (n = 61) for five analysts of various levels of experience. The variations in the recoveries followed a non-systematic random pattern, and were independent of the analyst. The ¹⁴C concentrations in reactor water sampled during operation were in the range 0.1–2 MBq L⁻¹ with a typical variability of ±5% or better among replicates, while the reactor water samples obtained during shutdown and samples of groundwater near a waste management area had values ranging from ≡100 to 1800 Bq L⁻¹, but with a higher variability. The detection and determination limits were 47 and 342 Bq L⁻¹, respectively.