We introduce a new on-line method for measuring radiocarbon (C-14) gaseous emissions using laser spectroscopy. In particular, we will present the results from a measurement campaign at a nuclear power plant, where the method was validated in an industrial setting. Demonstration of the method for the characterisation of outgasses from nuclear waste will also be presented.
In nuclear power plants, radiocarbon is a result of interaction of thermal neutrons with oxygen, nitrogen and carbon, and is one of the main sources of radioactive gas emissions. In nuclear power plants using graphite moderators, irradiated graphite is the main type of solid waste containing C-14. During nuclear power plant operation, C-14 is emitted through power plants stacks normally at levels of 50 - 200 Bq/m3 corresponding to a few ppb’s of 14CO2/CO2 with atmospheric concentrations of CO2. Regulators require power plants to monitor those emissions, but no method is currently capable of providing on-line on-site monitoring. Currently, radiocarbon content is measured using liquid scintillation counting, requiring long collection time and off-line laboratory analysis. In recent years, optical detection of C-14 has been demonstrated using an advanced laser spectroscopy method called cavity ring-down spectroscopy (CRDS). The method has in-situ measurement capabilities, but so far only measurements in the laboratory have been reported. We present here the results of field measurement campaign at a nuclear facility, where on-line detection of C-14 using this new optical method was demonstrated for the first time. In addition, we will show how the method can be applied to the characterisation of nuclear waste with a focus on irradiated graphite waste characterisation.
The radiocarbon detection system consists of two parts: an on-line sampling unit and a cavity ring-down spectroscopy instrument. The sampling unit extracts the carbon dioxide from atmospheric samples, to enable measurements from purified carbon dioxide at low pressure. 14CO2 is then detected using a quantum cascade laser and a CRDS system. Detection of other molecular forms of C-14, such as methane, is possible via catalytic conversion into carbon dioxide. To meet the field measurement requirements, the measurement cell was thermally stabilised and mechanical vibrations were suppressed with the carefully planned instrument structure. The results of the field campaign demonstrate the potential of the method for continuous automated monitoring of radiocarbon stack emissions with hourly data points, thus greatly increasing the temporal resolution of such measurements. The same system can also be used to analyse waste outgasses without the need for complex radiochemistry usually required to separate the different radionuclides.