An improvised method for carbon-14 measurement in gaseous effluents

A set of two 125ml bubblers containing 50 ml of 1M ultra-pure NaOH solution was used for sampling ¹⁴CO₂ from gaseous effluents from the common stack of two units of PHWR (each of 220 MWe) at Kaiga, south India. Before passing through the NaOH solution the gaseous effluent sample was bubbled through 0.1M HNO₃ in order to remove ³H. The flow rate was maintained at 1 L/min and sampling duration was 24 h. An aliquot (3 ml) of CO₂ dissolved NaOH was mixed with Hionic-Fluor scintillator (Perkin-Elmer, Inc, USA) in a glass vial and analyzed for ¹⁴C activity in a LSS (Quantulus1220, Perkin-Elmer, Inc.m USA) following standard method. Sample-scintillator ratio was optimised by mixing different combinations (1 ml+19 ml, 2 ml+18 ml, 3 ml+17 ml etc,) of sample (NaOH) and scintillator (Hionic-Fluor) by keeping the total volume fixed (20 ml). The miscibility of the sample + scintillator combination was checked. It was observed that a maximum 3 ml of sample is miscible with 17 ml of scintillator without any phase separation and turbidity formation. Hence, 3 ml of sample with 17 ml of scintillator was used for all experiments. The quench curve was generated by preparing a set of ¹⁴C standards in which the activity (DPM) per vial was constant but quench level varied by addition of external quench agent nitromethane. Upon standardization of the method, its suitability was tested for stack monitoring program of the nuclear power plant at Kaiga, south India. (P.A.)