Analysis of the radioactivity in bio-shield of Kori NPP unit 1 via computational simulation

Abstract

This study derived neutron flux and radioactivity distribution of the bio-shield to furnish fundamental datasets for evaluating its radiation degradation performance within decommissioning-bound nuclear power plants in Korea. Concrete within nuclear power plants degrades due to continuous radiation exposure. Particularly, neutrons exert a preeminent influence on radiation degradation, inciting changes in the crystal structure through multifarious interactions. Such factors as nuclear fuel enrichment, operation history, and concrete constituent elements have a great influence on the neutron flux and its associated radioactivity. The MCNP simulation based on the change of each factor revealed that the neutron flux decreased exponentially as the depth from the reactor core increased. The activity estimation of the major radioactive products of 60Co, 134Cs, 152Eu, and 154Eu across the depth of the bio-shield unveiled the relative difference of 3–22 % between the two scenarios according to operation history, where 60Co had the highest concentration among radioactive products. As a result of checking the neutron flux trend according to changes in the constituent elements of bio-shield concrete, a greater decrease in neutron flux occurred for constituent elements with a high proportion of light elements (carbon and oxygen). The results will serve as a foundation for future assessment of the radiation degradation performance.