Analysis of the effect of pavement layer on the evaluation of exposure dose in remediation of decommissioning site

Abstract

As the Kori Unit 1 nuclear power plant has been permanently shut down, interest in its site restoration and reuse is increasing. As of November 2017, 157 nuclear reactors in 19 countries have been permanently shut down. Among them, 102 units are being decommissioned and 21 units have already been decommissioned. However, even the countries with such decommissioning experience perceive that their decommissioning cases are incomplete and are developing technologies to reinforce safety, eco-friendliness, and efficiency. The overall understanding of the decommissioning process, adopting reuse methods, and evaluating site characteristics must be prioritized.

In this study, a methodology for verifying the site contamination degree depending on the decommissioning criteria was analyzed for site restoration after nuclear power plant decommissioning, and site characterization elements were also analyzed through overseas cases. Based on this, the impacts of the area of the contaminated site, depth of the contamination layer, and initial contamination concentration of radioactive elements, which were selected as parameters related to site contamination, on the exposure dose assessment and site reuse method setting, were analyzed using RESRAD, a dose assessment software program. In addition, the exposure dose assessment was conducted using the density and depth of the pavement layer as the parameters under the assumption that the pavement layer will be used when the contaminated site is reused. When the contaminated site is used as an actual road, the durability of the pavement layer was examined according to its components using finite element analysis and the results were applied to the dose assessment.

The impacts of the area of contamination and the thickness of the contamination layer, which are determined depending on the range of contaminants distributed in the decommissioning site, on the exposure dose were found to be insignificant. In particular, when the area of contamination decreased, the contamination concentration of groundwater by Co-60 decreased, but it could not contribute to the reduction in the exposure dose because the absolute contamination concentration was low. Meanwhile, the thickness of the cover zone and the contamination concentration of radionuclides had significant impacts from the initial exposure dose. The thickness of the cover zone significantly decreased the direct exposure dose from soil, which was the medium of contamination, and the contamination concentration affected the exposure dose of the decision group by the reduced rate. The loss of the pavement layer was extremely small even though excessive conditions were provided compared to the reality. However, damages to the auxiliary layer may affect the pavement layer, resulting in an increase in the exposure dose. Therefore, the design must consider the loss when the pavement layer is used.