Abstract: Objective To understand the expected exposure level and change trend of high toxic occupational hazard factors in nuclear power plants,and provide data support and suggestions for risk control and occupational health management of high toxic occupational hazard factors in the nuclear power industry. Methods Occupational health field investigation,occupational health detection and other methods were used to analyze and evaluate the exposure level of highly toxic occupational hazards in M310 unit of a nuclear power plant for 11 consecutive years from 2014 to 2024. Results The workers were mainly exposed to four highly toxic substances,namely ammonia,chlorine gas,hydrazine,and hydrogen sulfide. The ammonia detection results did not exceed the occupational exposure limit,with exposure concentration of short term（C_STE） detection results ranging from 0.13 to 23.76 mg/m³,and exposure concentration of time weighted average（C_TWA） detection results ranging from 0.01 to 1.09 mg/m³. The exposure level of ammonia during the regular island dosing room ammonia operation was the highest,which was level Ⅲ,and the C_STE test result reached 23.76 mg/m³. Except that the maximum exposure concentration（C_ME） of chlorine at inspection site in the generator room of the freshwater plant exceeded the occupational exposure limit due to the unstable state of ventilation facilities in the early stage of power generation,and the test results of hydrazine exceeded the occupational exposure limit due to leakage points in the inspection area at the back of the top of the hydrazine solution tank of the conventional island,the test results of C_STE for hydrazine were basically lower than the minimum quantitative concentration,the test results of C_ME for chlorine gas were all less than 0.20 mg/m³,and the detection results of C_ME for hydrogen sulfide were all less than 0.53 mg/m³. The exposure levels of chlorine,hydrazine and hydrogen sulfide at all relevant operating sites had been basically at level 0-Ⅱ over the years. Conclusion The overall control effect of occupational hazards of high toxic substances in a nuclear power plant is good. In the past five years,the contact levels of chlorine gas,hydrazine and hydrogen sulfide at each relevant operating position were basically at level 0,while the contact levels of ammonia were mostly Ⅰ and Ⅱ. Therefore,ammonia in the chemical dosing room of the conventional island is the key control point of highly toxic occupational hazard factors,especially during dosing or unloading,which needs to be given special attention. Nuclear power plants need to further optimize dosing processes and strengthen the maintenance of chemical dosing equipment and protective facilities.

Key words: Highly toxic substances, Nuclear power industry, Chemical harmful factors, Occupational exposure limits