Optimization of C-14 concentration measurement in aqueous samples using direct absorption method and liquid scintillation counting

Cernavoda Nuclear Power Station is the only nuclear power plant in Romania and the only CANDU reactor operating in Europe. The Cernavoda Unit 1 is a pressurized heavy water reactor (PHWR) fuelled with natural uranium and moderated and cooled by heavy water. The routine operation of this reactor type and its auxiliary process systems results in the production of a variety of solid, liquid and gaseous radioactive wastes. C-14 is produced mainly via a neutron capture reaction by oxygen-17 in the CANDU reactor. CANDU production and emission rates of C-14 are higher than those of other types of commercial reactors such as light water reactors and gas-cooled reactors. In order to fulfill the exigency of the monitoring program, we propose several studies to optimize C-14 determination in water samples. Radiocarbon determination using the 'direct' absorption method follows similar principles to that using the benzene synthesis method. A known amount of carbon from a sample, standard, or background material is counted as a liquid in a liquid scintillation counter. The only difference between the two methods is the way in which carbon from the sample, standard or background is converted into liquid form before being measured in the scintillation counter. Both methods convert raw material to CO2. The benzene synthesis method then converts the CO2 to benzene with several intermediate steps (production of LiC and C2H2). In the direct absorption method, the CO2 is bubbled through a liquid containing a tertiary amine forming a liquid known as a carbamate. This is usually mixed with scintillation cocktail. Different laboratories have different ways of estimating the amount of carbon present in the sample. We choose to ensure that the carbamate/liquid scintillation solution is always saturated for samples, standards, and background. We also investigate the major steps of this procedure like: preparation of carbamate/liquid scintillation solution, concentrating dissolved inorganic carbon of the sample, acidification of the sample, bubbling CO2 through solution, measurement in liquid scintillation counter.We established the routine procedure, and we measured different type of water: drinking water, surface water, underground water. There are few drawbacks of this procedure such as: the amount of water sample, which is between 20 L-100 L, or the amount of CO2 needed for saturated solution of carbamate/liquid scintillation solution. Even so, the simplicity of the procedure, and the elimination of intermediate steps from benzene procedure, permits its application in monitoring program. (author)