The radioactivity analysis of ¹⁴C and ³H in graphite from the dismantled korea research reactor and its dose estimation

Abstract
A high temperature combustion method was used to analyze the ¹⁴C and ³H activities in graphite and the dose assessment was carried out to determine the clearance in the conservative way. By this method, the ³H and ¹⁴C were simultaneously trapped in the nitric acid and carbosorb, respectively. Accordingly, the sample preparation time for the measurement was reduced to the half. The combustion temperature was more than 800 degrees in centigrade for obtaining total tritium and ¹⁴C in the sample. The combustion ratio was about 99% on the graphite sample with the weight of 0.1 g. Minimum detectable activity was 0.05 Bq/g for the ¹⁴C and 0.15 Bq/g for the ³H at the same background counting time. The recoveries from the combustion furnace were around 100% and 90% in ¹⁴C and ³H, respectively. The radioactivity were 2,530 ~ 3,160 Bq/g in ¹⁴C and 1,700 ~2,040 Bq/g in ³H at this experiment. The experimental uncertainty was less than 6% in both radionuclides where the furnace recovery was dominant factor. An individual effective dose from beta and gamma radionuclides was estimated by consideration of the scenario of inhalation, ingestion and external exposure. ⁶⁰Co, the radioactivity of which was measured by using HPGe detector, had a predominant effect in estimating the effective dose. The estimation showed that the graphite wastes from the dismantled research reactor should be disposed of as a low level radioactive waste rather than clearance.