Dynamic Radiation Safety Assessment of Workers During the Decommissioning of Radioactive Concrete

Abstract

Concrete is the main building material for nuclear facilities, and some concrete are contaminated or activated during an operation and a maintenance of nuclear power plants. During decommissioning, radioactive concrete undergoes various processes from dismantling to disposal. Proven techniques and equipment are available to dismantle nuclear facilities, but advanced technology is required for safer and more precise decommissioning. The design of the dismantling work is based on the ALARA principle, and the design of the proper work helps to ensure safety and reduce costs. In this study, Safety evaluation for nuclear reactors scheduled to be decommissioned, which has not been previously performed. Dose evaluation that reflects dynamic changes in surrounding requirements such as structures and working hours for the dismantling process and evaluate exposure and the resulting cost reduction through appropriate staffing and training. Reference models used PWR, type reactor, which is the most common reactors in the world and are also a major target from the perspective of decommissioning. Bio-shield has been exposed from neutrons for a long period of time, and its radioactivity varies greatly throughout the position. Activation simulation result using MCNP6 considering axial and radial direction distribution is used.4 stage, Preparation stage, Drilling stage, Cutting stage, Lifting and Cleanup stage is considered for dismantling process. For dose assessment, VISIPLAN 4.0 code is used for external dose assessment, and simulation is performed by reflecting the dismantling process and reflecting the decrease in the source due to the removal of concrete. Internal exposure is evaluated by analyzing dust generation and dispersion. Wright learning forgetting model is used to evaluate the skill level of the worker and calculate the reduction in work time. For the evaluation results, four dose reduction measures were considered. Maximum dose rate after shutdown is 6 mSv/h, but it was reduced to 1.4 mSv after 8.5 years cooling, and 0.82 mSv/h after 13.5 years cooling. The dose of individual workers is estimated to be over 500 mSv per year and is expected to exceed the annual dose limit. When working in high-dose areas, the total worker dose exceeds the annual dose limit of 20 mSv, and it is needed to reduce dose through protective measures. Considering learning effect, initial work time for one cycle decreased from 4.13 hours to 2.85 hours through 6 months of repeated skill. When the graph was shown by averaging the values for 5 days, a constant exponential trend was observed, and it was calculated that the expert would converge to 2.98 hours. By setting various conditions, an algorithm that can derive the dose and working time by considering the worker's experience during work with Pre-training. The optimized training date is derived by comparing the equipment cost due to training and the monetary value by dose.