Abstract: The invention relates to a blocking device (5) for stopping solid bodies that have been irradiated or that are to be irradiated in a conduit system (2) which is used to transport the solid bodies by means of a propelling fluid into and out from a nuclear reactor (1). The blocking device has at least one main body (52) with a continuous channel (521) for the solid bodies and an actuator (58), which is arranged inside the main body (52) and which is adjustable relative to the main body (52) between an open position and a closed position in such a way that the channel (521) for the solid bodies is open in the open position and is closed in the closed position. The actuator (58) can be brought from the open position into the closed position and from the closed position into the open position by being displaced relative to the at least one main body (52). The blocking device can be used in particular as part of an aeroball measurement system (3) or as part of a nuclide activation system (4).