GoldSim을 이용한 IAEA 표준 중저준위방사성폐기물 천층처분시설의 해석

Abstract

Radioactive wastes are generated in different activities and these wastes need to be managed in a safe manner including waste disposal. Safety assessment is an important procedure for evaluating the acceptability of the proposed, on-going or past waste disposal practice. Safety assessment can be seen to be an important component of practical decision making about the long-term safety of disposal facilities. It is generally accepted that radionuclides contained in the radioactive wastes will be eventually released and that these radionuclides will be transported to the accessible environment(Near field, far-field, biosphere). Therefore, the long-term safety assessment of near-surface radioactive waste disposal should be described by modeling the potential releases of radionuclides from the repository, far field area, and biosphere, as well as by the consequent health risk to humans. In this study, characteristics of radionuclides behavior are analyzed and radiological doses are evaluated for the reference near-surface radioactive waste disposal facility in Vaalputs, South Africa, which has been selected as a part of the IAEA coordinated research program on improvement of safety assessment methodologies(ISAM). This disposal facility is composed of 20 concrete vaults located above the ground level and has a total of about 750,000 drums. The analysis includes drinking water scenarios from a well. The parameters for the modeling of the near-field, far-field and biosphere are mainly obtained from the site specific data reported in IAEA publications. The release and transport of radionuclides in the vault, unsaturated zone, and saturated zone, and radiological safety assessment are simulated by using GoldSim. This simulation provides the time variation of effective doses over long-term period after closure of disposal facility. The simulation result is compared with the result of IAEA safety assessment conducted with other tools(AMBER and MASCOT) for the samefacility and scenario. The simulation result shows that the release of radionuclides begins 300 years after closure of the facility. The short-lived radionuclides are almost decayed out in the unsaturated zone, and only long-lived radionuclides come into the well. It is found that the annual effective doses are dominated by the radionuclides such as 99Tc, 129I, 234U, 238U, 230Th, 226Ra, 210Pb, 210Po which can be characterized by the half-live and mobility of parents and daughters. The contribution of 129I to total effective dose is dominant among these radionuclides. Dose from the 129I has a peak dose at 5800 yr, and this dose value is 15 μSv/yr. It is concluded that total effective dose caused by drinking water would be far below the general regulatory limit 0.1 mSv/yr for radioactive waste disposal facility. The comparison of both simulation results is performed to identify similarities and differences between our modeling approach and the IAEA approach which has been carried out using AMBER and MASCOT codes It is shown that there is a good agreement in the peak values of the release rates, well radionuclides concentration, and resulting radiation dose between our simulation and the IAEA simulation conducted with AMBER. However, these are substantial differences in the timing of the peak values.