Abstract: After replacement of one radioactive water filter of the primary loop in LHNP unit 1, the workers had been found surface contaminated on their overall suits and helmets. This report introduces the analysis and investigating process of the case and confirms that the radioactive isotopes were ⁸⁷Kr, ⁸⁸Kr and ¹³⁵Xe. Through the estimation of the irradiation dose of the noble gas intake of the workers, the effective dose of the workers is demonstrated far lower than the daily dose limit. The investigating method and thoughts of this leak out case will be propitious for the other same type nuclear power plants in case of the similar events,in rapidly identifying the radiation source terms and adopting the contamination eliminating actions to insure the radiation safety of the workers.

Key words: nuclear power plant, filter replacement, noble gas, unexpected release case, dose limit