An assessment of radiation exposure of workers during decommissioning of reactor vessel at Kori #1 unit

This study is for assessment and optimization of workers' radiation exposure for decommissioning Reactor Vessel (RV) at Kori #1 unit in connection with its dismantling process for the purpose of establishing Radiation Safety Management Plan. The basis of radiological conditions of this assessment is supposed to be those of 10 years after the permanent shutdown of Kori #1 unit when dismantling work of Reactor Vessel would suppose to be started. Dose rates of work areas were evaluated on the basis of spatial dose rate derived from activation level calculated by MCNP(Monte Carlo N-Particle Transport) and ORIGEN-S code. The dismantling process of Kori #1 RV was developed based on in-situ execution in atmospheric environment as the following steps of Preparation, Dismantling of Peripheral Structures, Dismantling of RV and Finishing Work. For assessment of exposure of RV dismantling work, those processes of each steps are correlated with spatial dose rates of each work areas where the jobs being done. Results of the assessment show that workers' collective dose for RV dismantling work would be in the range of 536 man-mSv -778 man-mSv. The most critical process would be dismantling of upper connecting parts of RV with 170 man-mSv -256 man-mSv while among the working groups, the expert group performing dismantling of ICI (In-core instrumentation) nozzles and handling & packaging of cut-off pieces is evaluated as the most significantly affected group with 37.5 man-mSv -39.4 man-mSv. Based on the assessment, improvement plan for better working conditions of the most critical process and the most affected workers in terms of radiation safety were suggested.