Assessment of Radiation Dose during the Installation and Removal of Steam Generator Primary Nozzle Dam in Overhaul Period

Background: The installation and removal of nozzle dam work is one of the representative maintenance works in the steam generator primary side and can result in high levels of exposure due to the Chalk River unidentified deposit deposited inside the steam generator. Therefore, it is necessary to assess the radiation dose to workers installing and removing the nozzle dam to determine the exposure level for each task. This study assessed the radiation dose to workers installing and removing the nozzle dam during the overhaul period.Materials and Methods: The work scenarios for installation and removal of the nozzle dam were analyzed based on Advanced Power Reactor 1400 (APR1400) mock-up operation video and worker questionnaire. Then, based on the steam generator design data presented in the APR1400 design criteria document (DCD), the steam generator was simulated using Monte Carlo N-Particle (MCNP) code. Based on analyzed work scenarios and simulated steam generator, the radiation doses to workers were assessed.Results and Discussion: The work scenarios were analyzed for the four sub-works and the steam generator was simulated using MCNP6. The simulated steam generator was used to derive dose rates for each work location. For each sub-work, the assessment results of radiation doses were in the range of 2.13–4.51 man·mSv. For each worker, the radiation doses were in the range of 1.01–2.70 mSv. The highest radiation dose to workers was about 5.4% of the annual dose limit for workers (based on a maximum of 50 mSv per year). In this study, the source term was set using the specific activity of each radionuclide provided as conservative values in the APR1400 DCD. Consequently, the results of the dose assessment are likely to be somewhat conservative.Conclusion: The results of this study can be used for optimizing radiation exposure to workers installing and removing nozzle dam during the overhaul period.