This reactive material can be either moderator or fuel. The water leakage into the core has already been considered. An insertion of graphite can happen in pebble-bed reactors where the upper reflector is suspended about 1 m above the core. In this case a part of the reflector can fall on the core producing a reactivity increase. In reactors with prismatic fuel a fuel element can suddenly fall into the core during a reloading operation. The reactivity increase is due to the insertion of the fuel element and to the elimination of neutron streaming out of the hole.