In the event of an imbalance in operations, the sophisticated safety systems of U.S. nuclear power plants are designed to shut down automatically well before any safety margins are exceeded. Operators can also shut down the plants manually. Unplanned plant shutdowns, known as "scrams" or "trips" in the industry, are very infrequent. This graph summarizes unplanned automatic and manual scrams in the U.S. nuclear industry between 2000 and 2011.