Analysis of Radiation-Induced Demagnetization Influence on the Performance of Permanent Magnet Damper

The magnetic vortex damper, a key unit of the control rod drive mechanism of high-temperature gas-cooled reactor (HTR), is a permanent magnet damper that produces eddy current resistance by rotating the conductor disc in magnetic field, and acts as a speed limit during the rod dropping process. To analyze the influence of neutron irradiation leaking from the core on the damping performance of the permanent magnet damper, the mechanism and the influencing factors of radiation-induced demagnetization of the Nd-Fe-B magnet were summarized through literature investigation and a magnetic vortex damper simulation model was established based on ANSYS Maxwell software and verified by experimental data. Current research shows that the magnet with higher intrinsic coercivity and length diameter ratio results in less demagnetization. The magnet permeance coefficient was simulated by the static magnetic field simulation to calculate its equivalent length diameter ratio. According to the literature experimental results, the equivalent length diameter ratio of the magnet, and its intrinsic coercivity, it was conservatively estimated that the remanence attenuation amplitude of the permanent magnet in this study should not exceed 1%. Based on this simulation model, the damping torque before and after the certain amplitude of magnet remanence attenuation was simulated and the corresponding maximum rod dropping speed was calculated. The simulation results show that the damping torque decrease is within 5%, which can meet the service requirements of high-temperature gas-cooled reactor over its service life.