Research reactor can be used for irradiation experiments of new fuels and new materials. For irradiation experiment, the safety evaluation is indispensable, especially the flow blockage accident. In this paper, the temperature distribution of radiation device is discussed using CFD method. Then, the safety characteristics of radiation device under different blockage ratio are analysed using the temperature safety limit and minimum departure from nucleate boiling ratio (MDNBR). The results show that when the irradiation device is located the place with low neutron flux location, the device can still maintain safe even if the blockage ratio is high enough, but the allowable blockage ratio will gradually decrease with the increase of the material heating power. For four times the heating power, the irradiation device MDBNR will reach the allowable safety limit (1.5) when the flow rate is 40% of the rated flow rate. The departure from nucleate boiling will occurs if the blockage ratio decreases continuously, which means the safety of irradiation device will be threatened.