the radioactive methyl iodide by using isotope exchange or salt formation impregnated activated carbon for example with potassium iodide as the impregnation device can be used the particulate filter 26 which is connected downstream of the iodine filter 24 is provided for retaining dust particles from the activated carbon bed downstream of the second filter stage a conveyor fan or in brief the fan 12 is connected to transport the fresh air flow into the air supply line 10 the preferably electrically driven fan 12 has a suction power in the range for example of from 1000 to 6000 m 3 h in order to provide the necessary operating current there is provided an autonomous power supply module 28 which is independent of the normally operational individual power supply and preferably also of the conventional emergency power network across the plant for example on the basis of electrical batteries accumulators and or a diesel generator unit the power supply module 28 becomes activated as required preferably independently in the manner of a non interrupted power supply or is alternatively controlled through an associated control unit 30 further downstream there may optionally be connected to the air supply line 10 an air dryer 32 which is also referred to as a cold trap and with which condensable components can be separated from the fresh air flow this may be for example a passive cold trap with silica gel and or ice as a drying agent the moisture content of the fresh air flow which flows into the downstream functional units see below is thereby reduced an alternatively or additionally present throttle 34 which is disposed in this case in the embodiment when viewed in the direction of flow of the fresh air.