Unit 1 of Kori nuclear power plant, which had been operated for 40 years in Korea, was permanently shut down on June 2017. After its closing and decommissioning, it is important to determine its site-specific cleanup guidance levels. The most commonly used evaluation for determining the site release condition is RESRAD code developed by Argonne National Laboratory. But, the input values in RESRAD should be defined by applying the characteristics of site, exposure pathways and public members in Kori region. It is also important to determine the critical group by exposure scenario applicable to radiological assessment. Critical group was defined by reviewing three age groups of infant, child, and adult recommended in ICRP 101A. Determining critical group and its radiological assessment have been applied for resident farmer scenario understood to be conservative. Input values have been derived by reviewing FSAR of unit 1 of Kori nuclear power plant, population statistics, and national food and nutrition statistics. Critical group has been determined by calculating maximum dose by age group, the preference of foodstuff, metabolic characteristic and population distribution of Kori region, and is then to be 40~50 age. Doses have been evaluated by probabilistic approach by applying site-specific data, and sensitivity analysis is also performed. Uncertainty analysis is also performed and the representative quantities are finally determined for dose distribution of resident farmer scenario. It is summarized that critical group has been determined by applying site-specific data of Kori region, and so has dose assessment. From overall results, it is concluded that the optimized site release condition can be determined in decommissioning of nuclear facilities in Korea and the radiological protection for workers, public members and environment be improved.