A scram or SCRAM is a quick shutdown of a nuclear reactor. It is done in an emergency (a dangerous situation). A scram is performed by quickly inserting all of the control rods, or, if this fails, injecting boric acid into the reactor. A reactor that has been shut down still requires cooling due to decay heat. If the cooling system fails, the core can still melt down even if the reactor has been shut down. This has happened during several nuclear accidents, including Three Mile Island and Fukushima. The RBMK reactors used in the Soviet Union had graphite-tipped control rods, a design flaw which caused the reactivity to increase for a moment when the reactor was scrammed. This design flaw contributed to the Chernobyl disaster.