## 1 Introduction  
South Korea's first commercial pressurized heavy water reactor (PHWR), Wolsong Unit 1, was permanently shut down in 2019 in the absence of any record of decommissioning heavy water reactors worldwide. Currently, South Korea is exerting efforts in preparation for nuclear power plant (NPP) decommissioning through the establishment of the world's first heavy water reactor decommissioning institute.  
Wolsong Unit 1 is a pressurized heavy-water reactor type (Canada Deuterium Uranium, CANDU) and, owing to the lack of research and results of decommissioning such NPPs, extensive prior research is necessary before the NPP decommissioning. The U.S. Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM) presents detailed standards for designing, planning, sampling, interpreting, and quality assuring for irradiation through consistent standard guidelines for site survey and instructs the inference of the derived concentration guideline level (DCGL) for site reuse [1]. In the U.S., 10 CFR Part 20, Subpart E prescribes that the exposure dose to the receptor considering all possible exposure pathways should not exceed 0.25 mSv\(\cdot\)year\({}^{-1}\) based on the effective dose in case the business operator reuses the site in an unrestricted manner after decommissioning. In the case of South Korea, the Nuclear Safety and Security Commission Notification No. 2016-33 presents the standard of 0.1 mSv\(\cdot\)year\({}^{-1}\).  
To reuse the site after NPP decommissioning, the DCGL must be derived from the evaluation of the exposure dose for the site using an evaluation model [2] based on a computer code such as RESRAD. The RESRAD computational code, developed by the Argonne National Laboratory (ANL) in the U.S., is designed to evaluate the risk and exposure dose of residual radioactive materials and is used for the decommissioning of commercial NPPs in the U.S. under the approval of the U.S. Nuclear Regulatory Commission (NRC). The Rancho Seco [3] and Zion [4] NPPs in the U.S. are typical examples that applied the RESRAD computer code to NPP decommissioning. The Rancho Seco NPP was a pressurized water reactor (PWR) plant in California, USA which started commercial operation in 1975 and has now been fully decommissioned. To determine the list of potential radionuclides in this plant decommissioning case, a list of nuclides reflecting site characteristics was derived by referring to literature such as NUREG/CR-0130 [5], NUREG/CR-3474 [6], and NUREG/CR-4289 [7]. Furthermore, the DCGL criteria for surface soil, building, and materials were established via the application of the RESRAD computer code [3]. Meanwhile, the Zion NPP was another PWR type reactor that started commercial operation in 1973 and was permanently shut down in 1998 after 15 years. The information regarding the NUREG/CR-3474 [6], 4289 [7], WINCO-1191 [8], and site waste specimens needed to derive a list of site-specific nuclides and the DCGL criteria for site decommissioning were derived by using the RESRAD code [4].  
The objective of this study was to obtain a preliminary DCGL for the soil of the Wolsong NPP site, which is a heavy water reactor, using the RESRAD-ONSTTE computational code. The target radionuclides were selected by referring to previous studies on NPP decommissioning, and a geological structure model of the Wolsong NPP site was established. Further, the distribution coefficient reflecting the site characteristics was derived using the JAEA-SDB [9] and the pH information of groundwater around the Wolsong NPP site. The preliminary DCGL, satisfying the limit of the unrestricted reuse criteria, was derived based on the various site-specific input parameters as input data of the RESRAD-ONSTTE code. Furthermore, the uncertainty of the input parameters for the result of exposure dose was quantified using the probabilistic analysis feature of the RESRAD-ONSTTE. In addition, the effects of the variables used in the calculation of the RESRAD code on the result of exposure dose were analyzed by performing a sensitivity analysis.