Measurement of carbon-14 released from Yonggwang Unit-4

Since the amount of Carbon-14 released form pressurized water reactor (PWR) is small and its concentration is low, it is not used as a main monitored nuclide for environmental release in PWR in general. However, the dose conversion coefficient of C-14 in CO2 is relatively high, there is a possibility to overestimate public exposure dose with the assumption that all amount of C-14 is resulted from CO2. Furthermore notice of MOST published in 2001 has included C-14 in the evaluation of tissue equivalent dose at site boundary. So Korea Hydro and Nuclear Power (KHNP) made an implementation plan to monitor C-14 concentration in gaseous effluents from Yonggwang unit 4 for two years from 2003. In order to monitor C-14 in PWR, KHNP devised C-14 sampling instrument which can collect CO2 and methane separately. It is composed of three main components, that is, primary CO2 sampler, a methane oxidization assembly and a secondary CO2 sampler. The primary CO2 sampler has one water bubbler and two NaOH bubblers. The water bubbler prevents the accumulation of NaOH at other bubblers. 2M-NaOH bubblers collect all CO2 in the gas to produce sodium carbonate (Na2CO3). Then the methane oxidization assembly convert methane and CO to CO2. The catalyst is composed of equal mixture of Alumina with Palladium coating (1.0%) and Platinum coating (0.5%). The temperature of a furnace is maintained 500 .deg. C approximately, to convert CO and methane into CO2. Retaining time of the gas in catalytic is designed to be about 25 seconds to maximize conversion. The concentration of CO2 and methane at main vent lines in Yonggwang Unit-3 was analyzed using Gas Chromatography in order to evaluate the optimal treatment condition for the sample and estimate the optimal operating time for this device. As a result, the concentration of CO2 was approximately 450 ppm in average and that of methane was 2-13.3 ppm depending on the location. Based on these results, the optimal operating condition was decided as duration of 1 month and quantity of NaOH 800 ml. Four sampling instruments were installed in Yonggwang Unit-4 on Jan. 2003 to collect C-14 sample and one was installed in laboratory to analyze collecting efficiency and conversion efficiency. These sampling instruments will be operated 2 years to produce C-14 data by the point of seasons, operational conditions and chemical structure. Based on the result, environmental effects of C-14 at PWR will be evaluated which provide us an optimal C-14 monitoring method