Abstract: As the core of high-voltage direct-current transmission technology, the DC converter valve and its key power device thyristor face a high risk of failure caused by atmospheric neutron when operating in high-altitude areas. The atmospheric neutron failure rate of thyristors used at high altitudes can be quantitatively evaluated by conducting accelerated irradiation tests. The accelerated irradiation experiments of 8.5 kV/5 kA thyristor for DC transmission were carried out at different temperatures by using the China spallation neutron source. The atmospheric neutron-induced single-event burnout (SEB) for thyristor was verified by experiments. It is manifested as a sudden surge in reverse bias leakage current during irradiation, and the device loses its high voltage blocking ability. The failure rates of thyristor caused by atmospheric neutrons were also evaluated based on the accelerated irradiation experimental results. The reverse bias voltages and junction temperatures of thyristor are the key factors affecting the failure rates. The atmospheric neutron-induced failure rates increase exponentially with the reverse bias voltage. The atmospheric neutron failure rate of the 8 500 V thyristor operating at 4 350 V will reach 673 FIT at room temperature at sea level; when the bias voltage drops to 4 100 V (a decrease of about 5.7%), the atmospheric neutron failure rate can be reduced to 11 FIT. In addition, the failure rate increases with the decrease of temperature. The failure rate at 5℃ is nearly 6 times higher than that at 25℃ when the device is biased at 50% of the rated voltage. Based on TCAD simulations, the failure mechanism of thyristor caused by irradiation is further verified. The simulation results show that the SEB failure is directly related to the avalanche breakdown effect-induced by the incident radiation particles. For radiation-induced SEB failure of thyristors, the sensitive region is the N-drift/P-substrate junction depletion region (corresponding to the electric field concentration region under reverse bias of thyristors). When radiation particles are incident into the sensitive region, it can cause the concentration of electric field near the incident location, resulting in additional electric field spikes. If the peak of the electric field exceeds the critical electric field E_c (about 1.64×10⁵ V/cm), it will induce avalanche breakdown. The high voltage reverse bias high current state caused by avalanche breakdown ultimately leads to the failure of the thyristor. The avalanche breakdown effect is positively correlated with the reverse bias voltage of thyristor and negatively correlated with the junction temperature. This is consistent with the variation of atmospheric neutron failure rates with bias voltages and junction temperatures.