Accurate quantification of tritium at low levels in water streams is both a regulatory and environmental necessity for the nuclear industry. The most widely used current method is liquid scintillation counting (LSC), which measures the beta-radiation emitted from tritium decay and can therefore be used to indirectly quantify the concentration of tritium. This is a relatively laborious off-line technique requiring separate sampling, preparation and analysis steps. An on-line or in-line monitoring system which directly measured the tritiated water concentration would likely have a variety of benefits, being both cheaper to operate and possibly affording instantaneous access to results.
The technology to achieve such a system already exists in the form of in-situ infrared spectroscopy equipment designed to follow the kinetics of chemical reactions in real-time (ReactIR). The infrared absorbances of H2O, deuterated water (HDO) and tritiated water (HTO) are distinct enough to quantify separately, and therefore a program of work to investigate the feasibility of this approach has been carried out. Using HDO as a proxy for HTO, off-the-shelf ReactIR equipment and software has been employed to create and methodically test a system for obtaining real-time data on the deuterium atom concentration in water samples.
The development and subsequent validation of the method showed that it was indeed possible to quantify deuterium content in this way, with a standard deviation for repeated determinations of 42.6 ppm deuterium (δD of 273.53 ‰) and a limit of detection (LOD) of 137 ppm D (1212.00 ‰), with the method proving to be linear up to ca. 10000 ppm D and beyond. In addition, the method demonstrated robustness to changes in pH, ionic strength and ambient atmosphere. A further development of the method using protic ethanol to chemically amplify the signal afforded a much improved standard deviation of 247 ppb D (1.60 ‰) and an LOD of 5.30 ‰. See Table 1 for a comparison with currently available techniques.
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