Pebble-bed nuclear reactor capable of being shut down by a fluidized mass of bodies containing neutron-absorbing material, the fluidized mass of bodies being received in a storage container disposed above the pebble bed during normal operation of the reactor, the storage container being formed with a closable discharge opening, including a piston rigidly connected to the storage container and disposed below the discharge opening thereof, the piston being guided in a cylinder stationary relative to the reactor and being open at the top thereof, the storage container being displaceable in vertical direction between upper and lower end positions thereof, the piston, in the upper end position of the storage container, closing off the cylinder at the upper edge located at the open top of the cylinder and, in the lower end position of the storage container, the fluidized mass of bodies discharged from the discharge opening of the storage container being formed on top of the piston into a discharge cone having a height greater than the distance between the piston and the discharge opening and method for its operation.