Irradiation damage and activation analysis of a diagnostic port plug for BEST

This study presents a neutronics and radiation analysis of a diagnostic port plug in the Burning plasma Experimental Superconducting Tokamak (BEST). Neutron and gamma flux distributions, displacement damage, neutron-induced hydrogen and helium production, operating and shutdown dose rates, and the evolution of radioactive nuclides inside the diagnostic port plug are systematically evaluated. The results show that neutron irradiation within the port plug is strongly attenuated along the structural direction, with the displacement per atom (DPA) reaching a maximum of approximately 3 DPA per full power year (FPY) on the plasma-facing side and decreasing to about 10⁻³ DPA/FPY in the rear region, demonstrating the effectiveness of the structural design and shielding. The diagnostic mirrors experience more localized irradiation, with the highest DPA of about 0.643 DPA/FPY occurring on the M1 and M2 mirrors of the CO₂ dispersion interferometer. When extrapolated to an operational lifetime of 5–10 years, the accumulated DPA and neutron-induced He/H production in the mirror materials remain within ranges considered acceptable based on ITER first mirror studies. Dose rate analysis indicates that the operating dose rate (ODR) at the rear of the port plug is approximately 1×10⁴ Sv/h. After shutdown, the radiation field is initially dominated by short-lived activation nuclides such as N-16 and Mn-56, and gradually transitions with the increasing cooling time to a residual radiation field governed by medium- and long-lived nuclides, including Fe-55, Co-60, and Ni-63. The results indicate that neutron-induced irradiation is not the dominant lifetime-limiting factor for the diagnostic mirrors; however, the dose rates during operation and the early shutdown phase impose important constraints on the diagnostic layout and maintenance strategies.